whisky-dave wrote:

what makes yuo think it;s 40 amps will you answer this or not.

I do not think your heaters are taking 40A. We were discussing why the
MCB didn't trip, because a 32A breaker can supply more than 40A for
hours without tripping.

only 140 amps where or how did you get that figure ?

You just glibly replied to Johns message where he showed you ... he
wasn't saying your heaters are taking 140A, rather that in the even of a
short circuit (where ideally you'd want many hundreds of amps available
to ensure the MCB trips in under half a second) your 202V supply could
only supply 140A which would take it 20 seconds to trip.

140A at 202V is 22kW heating up your cabling, your lab should be toasty
until the fire service arrive.

Andy Burns wrote:

140A at 202V is 22kW heating up your cabling

Sorry 28kW.

On 17/11/2017 09:59, whisky-dave wrote:
On Thursday, 16 November 2017 20:05:21 UTC, Roger Hayter wrote:

In reality the 2kW will be at a particular
voltage, usually 240V in the case of electric showers which have a
similar rating system.

Because 240V is the maxuim continous voltage suplied by the gird to households.

Its the target, not the maximum. 253V is the official maximum.

BTW, I am quite sure they would not claim the 2kW was precise to the
nearest watt at any voltage. +/- 5% would be a reasonable achievement,
say 1.9 to 2.1kW. Or it could be a maximum, then 1.8 to 2.0kW. In the
latter case the 220V power might be as low as 1500W

So I was suprised to find that I was getting just 700W from a full on 2KW heater.

All that tells you is that either it was not full on (regardless of how
you had the controls set), or you have a serious undervolt problem.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 17/11/2017 10:06, whisky-dave wrote:
On Thursday, 16 November 2017 08:58:00 UTC, John Rumm wrote:
On 15/11/2017 17:28, Andy Burns wrote:
whisky-dave wrote:

John Rumm wrote:

If the voltage sags too far, then you get lower PSCC so can expect
slower operation of protective devices under fault conditions, so
increase electrocution risk, and catastrophic cable failure risk.

But not in under 3 hours.

The 3 hours at 40+ Amps is the "slow" part of the MCB curve, in the
event of a short circuit you want sufficient current to ensure it trips
within the "fast" part of the curve ...

Indeed...

You need 160A to make sure a B type MCB will trip in the magnetic part
of its response curve and that implies a maximum loop impedance at the
point of the fault of 230 / 160 = 1.44 ohms [1]

So why did it trip out ?

It didn't. At least not in the magnetic part of the response. Neither
would you expect it to in the absence of a *fault current*.

Even if the circuit is in spec and meets that requirement at all
sockets, running at only 202 volt gives a reduced potential worst case
PSSC of 202 / 1.44 = 140A, which could leave you on the thermal portion
of the response curve and *20 seconds* away from disconnection. (and
that ignores the effect of elevated conductor temperature on the loop
impedance)

140A I don't think we were drawing anywhere near that.

Go nail through a cable and measure it again!

Is that the sort of current you expect from 5 2KW heaters running on 202V and a soldering re-work station of about 160W max. ?

No, you are confusing overload current with fault current.

A fault current is what you will see when something bad happens in a big
way; say a cable or flex is damaged and a short circuit between live
conductors or live and earth occurs.

In these situations the current that flows will be limited only by the
round trip resistance of the circuit's cables. Faults of this nature
will result in rapid adiabatic heating of the cable's conductors. Unless
the current is interrupted rapidly then cable damage will occur, and
this could include it melting, charring, or bursting into flames.

MCBs are designed with a separate trip mechanism. One of which is
intended to handle this situation *quickly* (typically under 0.1 secs)
using a solenoid to trip the mechanism (this is independent of the bi
metal strip that provides the normal *overload current* protection).

For this to work, the MCB needs to see enough fault current. For a type
B device this could be up to 5x its nominal rating - so 160A for a B32
device.

[1] I am assuming that the wiring was done before 17th edition amendment
3 and the 5% reduction in Cmin.

I'm not sure that is relevant to this.

It is, and I will explain why.

Real world results[1] demonstrated that the maximum loop impedances
allowed in the standard circuit designs presented in BS7671 were
slightly too high, since when circuits designed to these specs were
operated in combination with supply voltages toward the bottom of the
allowable supply range, you could not always rely on the protective
devices operating quickly enough to clear faults.

Hence amendment 3 of the 17th edition applied a 5% reduction in the
maximum allowable loop impedances. e.g. that specified for a B32 device
was reduced from 1.44 ohms to 1.37 ohms.

That means that a circuit installed today that meets the new
requirements will be more likely to clear faults even when the supply
voltage is as low as 216V.

Since your circuits are likely to pre-date this change, they will at
best only be compliant with the previously specified maximum impedance,
and hence you will see a progressively increasing risk that a fault will
not clear promptly at supply voltages under 230V.

Note: for the avoidance of doubt this is an issue that will be of most
concern on longer circuits - i.e. those approaching the maximum
permitted cable length. It will also be dependent on the MCB in
question, since the permitted range of fault currents detected go from
3x to 5x In for a type B device so will be a problem only for less
sensitive examples.



[1] Documented in: CENELEC: "Technical Report PD CLC/TR 50480:2011
Determination of cross sectional area of conductors and selection of
protective devices"

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 17/11/2017 09:53, whisky-dave wrote:
On Thursday, 16 November 2017 14:22:01 UTC, Andy Burns wrote:
whisky-dave wrote:

Andy Burns wrote:

The 3 hours at 40+ Amps

what makes you think it's 40 amps + ?

I thought that's where we started weeks ago, that you expected to be
able to trip a 32A breaker with the heaters, and I gave you the 2.7
hours figure?

what makes yuo think it;s 40 amps will you answer this or not.

Its not relevant anyway since you have seized on the wrong bit. The
takeaway message was in the second bit of Andy's sentance: "...is the
'slow' part of the MCB curve, in the event of a short circuit you want
sufficient current to ensure it trips within the 'fast' part of the curve"


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Friday, 17 November 2017 11:05:38 UTC, Andy Burns wrote:
whisky-dave wrote:

what makes yuo think it;s 40 amps will you answer this or not.

I do not think your heaters are taking 40A. We were discussing why the
MCB didn't trip, because a 32A breaker can supply more than 40A for
hours without tripping.

only 140 amps where or how did you get that figure ?

You just glibly replied to Johns message where he showed you ... he
wasn't saying your heaters are taking 140A, rather that in the even of a
short circuit (where ideally you'd want many hundreds of amps available
to ensure the MCB trips in under half a second) your 202V supply could
only supply 140A which would take it 20 seconds to trip.

What makes you think it could only supply 140A ?. Isn't that the case with all 32A MCB anywhere in the country so if that is a fault of the MCBs maybe they need redesigning.
Suppose we were drawing 32 amps (4 heaters) what would be the calculations then ?




140A at 202V is 22kW heating up your cabling, your lab should be toasty
until the fire service arrive.

What makes you think that

but it wouldn't be 140A at 202V would it. If we can only get 40A at 202V what makes yuo think we could get 140A at 202V surely as the current increased the voltage between L-N would drop.
It droped from ~215V at 0A to ~202V at 40A, I'm pretty sure that once it got to 50 Amps the voltage would drop below 200V.

On Fri, 17 Nov 2017 11:34:40 +0000, John Rumm
wrote:

On 17/11/2017 09:59, whisky-dave wrote:

snip

So I was suprised to find that I was getting just 700W from a full on 2KW heater.

All that tells you is that either it was not full on (regardless of how
you had the controls set),

Quite, but the thing that still seems to elude him is the *why* he is
seeing this lower value?

He seems to be looking at it all like a basic consumer where no
'technical answers' seem to actually mean anything ... and so he just
repeats the same 'consumer' level observation / complaint, like any of
us sold him the rads! ;-(

'Why won't you let me on the plane'
'Because you have missed the last boarding time sir'
'Yes, but why won't you let me on the plane ...?'

He isn't 'just getting 700W from a 2kW heater' in any case, he's
getting the total output from a 700 + 1300W heater for a period, then
getting 700W + the average over the cycle time of the 1300W element
thereafter.

It's not difficult or complicated but in spite of plenty of advice and
answers from people who obviously know what they are talking about
(and me g) he still seems to think he understands it all better, or
that we don't understand it at all? ;-(

or you have a serious undervolt problem.

'And' by the sounds of it (but not enough to justify his 'only 700W
output' issue though). ;-)

Cheers, T i m

On Friday, 17 November 2017 12:28:04 UTC, John Rumm wrote:
On 17/11/2017 10:06, whisky-dave wrote:
On Thursday, 16 November 2017 08:58:00 UTC, John Rumm wrote:
On 15/11/2017 17:28, Andy Burns wrote:
whisky-dave wrote:

John Rumm wrote:

If the voltage sags too far, then you get lower PSCC so can expect
slower operation of protective devices under fault conditions, so
increase electrocution risk, and catastrophic cable failure risk.

But not in under 3 hours.

The 3 hours at 40+ Amps is the "slow" part of the MCB curve, in the
event of a short circuit you want sufficient current to ensure it trips
within the "fast" part of the curve ...

Indeed...

You need 160A to make sure a B type MCB will trip in the magnetic part
of its response curve and that implies a maximum loop impedance at the
point of the fault of 230 / 160 = 1.44 ohms [1]

So why did it trip out ?

It didn't. At least not in the magnetic part of the response. Neither
would you expect it to in the absence of a *fault current*.

So what happened ? a student was soldering and he said my soldering station has stopped working.... this was on the same trip as the heaters who's LEDs had also just gone out. When we looked in the riser cupboard where the CU is the MCB has tripped or cut out or whatever you prefer to call it. The heaters (3 of them) were removed the other were switchd off, and the MBC switch put to the ON position and those things that went off came back on again so how did that happen ?



Even if the circuit is in spec and meets that requirement at all
sockets, running at only 202 volt gives a reduced potential worst case
PSSC of 202 / 1.44 = 140A, which could leave you on the thermal portion
of the response curve and *20 seconds* away from disconnection. (and
that ignores the effect of elevated conductor temperature on the loop
impedance)

140A I don't think we were drawing anywhere near that.

Go nail through a cable and measure it again!

Measure what ?


Is that the sort of current you expect from 5 2KW heaters running on 202V and a soldering re-work station of about 160W max. ?

No, you are confusing overload current with fault current.

I'm not as current is current there is NO difference, it's just electrons and charge.


A fault current is what you will see when something bad happens in a big
way; say a cable or flex is damaged and a short circuit between live
conductors or live and earth occurs.

Do you think the heaters or the soldering iron produced his fault current. ?



In these situations the current that flows will be limited only by the
round trip resistance of the circuit's cables. Faults of this nature
will result in rapid adiabatic heating of the cable's conductors. Unless
the current is interrupted rapidly then cable damage will occur, and
this could include it melting, charring, or bursting into flames.

Yes and will this happen at 32A 40 A 50 A and how long will it take ?


MCBs are designed with a separate trip mechanism. One of which is
intended to handle this situation *quickly* (typically under 0.1 secs)

what situation ?

using a solenoid to trip the mechanism (this is independent of the bi
metal strip that provides the normal *overload current* protection).

So which tripped out ?


For this to work, the MCB needs to see enough fault current. For a type
B device this could be up to 5x its nominal rating - so 160A for a B32
device.

So are you saying that the professionals or the company employed to install these MCBs installed cable that wasn't up to carrying the 32A 40A or 140A or 160A you say the MCB is designed for ?




[1] I am assuming that the wiring was done before 17th edition amendment
3 and the 5% reduction in Cmin.

I'm not sure that is relevant to this.

It is, and I will explain why.

Real world results[1] demonstrated that the maximum loop impedances
allowed in the standard circuit designs presented in BS7671 were
slightly too high, since when circuits designed to these specs were
operated in combination with supply voltages toward the bottom of the
allowable supply range, you could not always rely on the protective
devices operating quickly enough to clear faults.

Hence amendment 3 of the 17th edition applied a 5% reduction in the
maximum allowable loop impedances. e.g. that specified for a B32 device
was reduced from 1.44 ohms to 1.37 ohms.

That means that a circuit installed today that meets the new
requirements will be more likely to clear faults even when the supply
voltage is as low as 216V.

Since your circuits are likely to pre-date this change, they will at
best only be compliant with the previously specified maximum impedance,
and hence you will see a progressively increasing risk that a fault will
not clear promptly at supply voltages under 230V.

Note: for the avoidance of doubt this is an issue that will be of most
concern on longer circuits - i.e. those approaching the maximum
permitted cable length. It will also be dependent on the MCB in
question, since the permitted range of fault currents detected go from
3x to 5x In for a type B device so will be a problem only for less
sensitive examples.


The inspection panel has a date of 16th june 2013.

whisky-dave wrote:

What makes you think it could only supply 140A ?. Isn't that the
case with all 32A MCB anywhere in the country

The MCB doesn't really limit how much current you can draw, it just cuts
off if you happen to take too much for too long, it's the length and
cross-section of your cables that limit what you really can draw
140A at 202V is 22kW heating up your cabling, your lab should be toasty
until the fire service arrive.

What makes you think that

stick a 6" nail through the submain coming into your lab and find out.

but it wouldn't be 140A at 202V would it. If we can only get 40A at 202V

bzzzt, you've got to be trolling ...

On 17/11/2017 13:55, whisky-dave wrote:
On Friday, 17 November 2017 12:28:04 UTC, John Rumm wrote:
On 17/11/2017 10:06, whisky-dave wrote:
On Thursday, 16 November 2017 08:58:00 UTC, John Rumm wrote:
On 15/11/2017 17:28, Andy Burns wrote:
whisky-dave wrote:

John Rumm wrote:

If the voltage sags too far, then you get lower PSCC so
can expect slower operation of protective devices under
fault conditions, so increase electrocution risk, and
catastrophic cable failure risk.

But not in under 3 hours.

The 3 hours at 40+ Amps is the "slow" part of the MCB curve,
in the event of a short circuit you want sufficient current
to ensure it trips within the "fast" part of the curve ...

Indeed...

You need 160A to make sure a B type MCB will trip in the
magnetic part of its response curve and that implies a maximum
loop impedance at the point of the fault of 230 / 160 = 1.44
ohms [1]

So why did it trip out ?

It didn't. At least not in the magnetic part of the response.
Neither would you expect it to in the absence of a *fault
current*.

So what happened ?

The MCB tripped on its thermal response because the circuit was overloaded.

You need to understand that there are two different classes of over
current: overload current, and fault current.

Overloads are caused when a circuit is operating normally, but the total
load from the appliances connected exceeds the nominal rating of the
circuits protective device. Say for example, drawing 40A from a circuit
with a B32 MCB. The effects of this are to cause gradual heating of the
circuit wires and accessories. If left unchecked it could result in
cable damage or reduced life expectancy of the cables. The heating will
take time to reach a damaging level. So MCBs include a trip mechanism
based on a bi-metal strip, that heats in a way analogous to that of the
rest of the circuit. It will tolerate small overloads for a very long
time, and then ever reducing durations as the overload increases. So for
some overloads they may run for minutes or hours before tripping.

Fault currents however are a different class of fault. Here you have
something causing a short circuit, and the current that flows can be
100s or 1000s of amps. This results in very rapid heating of the circuit
wires (in some cases even explosive heating) - there is also no time for
this heat to be dissipated to the surroundings (i.e. its adiabatic
heating). This class of fault needs the kind of immediate response that
the bi-metal strip of the MCB can't provide. Hence it includes the
magnetic response that will react with the speed you would expect from a
fuse.

a student was soldering and he said my soldering
station has stopped working....
this was on the same trip as the
heaters who's LEDs had also just gone out. When we looked in the
riser cupboard where the CU is the MCB has tripped or cut out or
whatever you prefer to call it. The heaters (3 of them) were removed
the other were switchd off, and the MBC switch put to the ON position
and those things that went off came back on again so how did that
happen ?

As you would expect

Even if the circuit is in spec and meets that requirement at
all sockets, running at only 202 volt gives a reduced potential
worst case PSSC of 202 / 1.44 = 140A, which could leave you on
the thermal portion of the response curve and *20 seconds* away
from disconnection. (and that ignores the effect of elevated
conductor temperature on the loop impedance)

140A I don't think we were drawing anywhere near that.

Go nail through a cable and measure it again!

Measure what ?

You made the claim that you were not drawing anything close to 140A. I
agree with you, you weren't. I suggest that if however you were to drive
a nail through one of the circuit cables (i.e. to introduce a fault) and
then measure the current draw, you will see a *significantly* larger
current - hopefully only briefly.

Is that the sort of current you expect from 5 2KW heaters running
on 202V and a soldering re-work station of about 160W max. ?

No, you are confusing overload current with fault current.

I'm not as current is current there is NO difference, it's just
electrons and charge.

Its may just be electrons, but that does not mean you can handle
situations where you are drawing 5A too much in the same way you handle
those where you are drawing 500A too much.

This is why circuit designers consider both scenarios, and the equipment
manufacturers design kit that behaves in an appropriate way to cope with
both.

A fault current is what you will see when something bad happens in
a big way; say a cable or flex is damaged and a short circuit
between live conductors or live and earth occurs.

Do you think the heaters or the soldering iron produced his fault
current. ?

No.

They produced an overload. However the voltage drop you witnessed during
this episode does cast doubt on the ability of the circuit to correctly
deal with faults.

In these situations the current that flows will be limited only by
the round trip resistance of the circuit's cables. Faults of this
nature will result in rapid adiabatic heating of the cable's
conductors. Unless the current is interrupted rapidly then cable
damage will occur, and this could include it melting, charring, or
bursting into flames.

Yes and will this happen at 32A 40 A 50 A and how long will it take
?

No it won't happen at 40A or 50A. Those are not fault currents, those
are overloads.

You can get an estimate of the time to trip by looking at the response
curve:

http://wiki.diyfaq.org.uk/images/d/d...e-MCBTypeB.png

The vertical bits of the curves represent the magnetic trip - that for
dealing with fault currents. The curved bits are the thermal response.
If you find 50A on the 32A curve you will see it intersects the time
axis at around 1000 secs. Real world conditions may mean you don't see
this - but you can be reasonably confident it will ultimately trip, but
it will likely take tens of mins to do so. If you are running on a
circuit also suffering an undervolt then the times will be longer.


MCBs are designed with a separate trip mechanism. One of which is
intended to handle this situation *quickly* (typically under 0.1
secs)

what situation ?

The occurrence of a fault current.

using a solenoid to trip the mechanism (this is independent of the
bi metal strip that provides the normal *overload current*
protection).

So which tripped out ?

The thermal one.

For this to work, the MCB needs to see enough fault current. For a
type B device this could be up to 5x its nominal rating - so 160A
for a B32 device.

So are you saying that the professionals or the company employed to
install these MCBs installed cable that wasn't up to carrying the 32A
40A or 140A or 160A you say the MCB is designed for ?

I can't say with any certainty what you have installed. I have never
seen it, or tested it. I can offer only educated guess work. I can say I
am suspicious that all is not well based on what you have told us, if
what you have told us is correct.

The fact that you have a voltage reduction device in place will make the
case more borderline. The installers *should* have checked that the
impedances of the circuits already installed were low enough for this to
be safely installed. I would not be surprised if this was not done.

There is a limit to the maximum length of cable that can be installed
for a circuit. This may have been observed as originally installed.
However it is not uncommon for extensions to be made later.



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 17/11/2017 12:59, whisky-dave wrote:
On Friday, 17 November 2017 11:05:38 UTC, Andy Burns wrote:
whisky-dave wrote:

what makes yuo think it;s 40 amps will you answer this or not.

I do not think your heaters are taking 40A. We were discussing why
the MCB didn't trip, because a 32A breaker can supply more than 40A
for hours without tripping.

only 140 amps where or how did you get that figure ?

You just glibly replied to Johns message where he showed you ...
he wasn't saying your heaters are taking 140A, rather that in the
even of a short circuit (where ideally you'd want many hundreds of
amps available to ensure the MCB trips in under half a second) your
202V supply could only supply 140A which would take it 20 seconds
to trip.

What makes you think it could only supply 140A ?. Isn't that the case
with all 32A MCB anywhere in the country so if that is a fault of the
MCBs maybe they need redesigning.

This has nothing to do with the MCBs or the heaters - this is asking the
question what happens when there is an actual fault. Under fault
conditions its loop impedance that matters. That will be made up of your
supply impedance, your external earth impedance, and the total
impedance of all the submain and circuit cables between your supply head
end and the location of the fault, added together.

Suppose we were drawing 32 amps (4
heaters) what would be the calculations then ?

Not relevant

140A at 202V is 22kW heating up your cabling, your lab should be
toasty until the fire service arrive.

What makes you think that

but it wouldn't be 140A at 202V would it. If we can only get 40A at
202V

That is through the parallel load of the heaters. Not through a direct
short.

what makes yuo think we could get 140A at 202V surely as the
current increased the voltage between L-N would drop. It droped from
~215V at 0A to ~202V at 40A, I'm pretty sure that once it got to 50
Amps the voltage would drop below 200V.

Which is what makes the situation more dangerous. Should your circuit
experience a fault, it may not be able to pass adequate current to trip
the MCB quickly. Which is what we have been trying to explain all along!

Unless you have a clear understanding of the differences between
overload currents and fault currents, not much of this will make sense
to you.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

In article ,
whisky-dave wrote:
On Wednesday, 15 November 2017 15:12:17 UTC, T i m wrote:
On Wed, 15 Nov 2017 05:41:26 -0800 (PST), whisky-dave
wrote:

On Wednesday, 15 November 2017 11:55:42 UTC, T i m wrote:
On Wed, 15 Nov 2017 03:30:12 -0800 (PST), whisky-dave


It would depend on where it bridged itself (if it does). It might
already have done that, in which case the measured voltage is
even more worrying!

doesn't worry me in the least because I expected it.

I don't think you can be sure exactly where the voltage drop is
coming from though can (even) you?

No but againn I donlt care.

I'm guessing you do care or this thread wouldn't be as long as it is?

It's not me that's making the thread long.

Mine was a simple question regarding what power would yuo expect a 2KW
heater to use. I wanslt expecting pin point accuracy when I knew the
jeaters specs was 2KW 220-240V there;s a 10% change without the power
altering.


A great many years ago, I lived for a time in a part of the country where
the mains was only 200v. My soldering iron didn't get hot, so I had to
buy a 200v element for it. When I moved back into the "real world" the iron
got much hotter than intended and eventually burned out.

--
from KT24 in Surrey, England

On Mon, 20 Nov 2017 10:03:23 +0000 (GMT), charles
wrote:

In article ,
whisky-dave wrote:
On Wednesday, 15 November 2017 15:12:17 UTC, T i m wrote:
On Wed, 15 Nov 2017 05:41:26 -0800 (PST), whisky-dave
wrote:

On Wednesday, 15 November 2017 11:55:42 UTC, T i m wrote:
On Wed, 15 Nov 2017 03:30:12 -0800 (PST), whisky-dave


It would depend on where it bridged itself (if it does). It might
already have done that, in which case the measured voltage is
even more worrying!

doesn't worry me in the least because I expected it.

I don't think you can be sure exactly where the voltage drop is
coming from though can (even) you?

No but againn I donlt care.

I'm guessing you do care or this thread wouldn't be as long as it is?

It's not me that's making the thread long.

Mine was a simple question regarding what power would yuo expect a 2KW
heater to use. I wanslt expecting pin point accuracy when I knew the
jeaters specs was 2KW 220-240V there;s a 10% change without the power
altering.


A great many years ago, I lived for a time in a part of the country where
the mains was only 200v. My soldering iron didn't get hot, so I had to
buy a 200v element for it. When I moved back into the "real world" the iron
got much hotter than intended and eventually burned out.

But you understood why that happened from the beginning right? weg

Cheers, T i m

On Friday, 17 November 2017 18:42:48 UTC, John Rumm wrote:
On 17/11/2017 13:55, whisky-dave wrote:
On Friday, 17 November 2017 12:28:04 UTC, John Rumm wrote:
On 17/11/2017 10:06, whisky-dave wrote:
On Thursday, 16 November 2017 08:58:00 UTC, John Rumm wrote:
On 15/11/2017 17:28, Andy Burns wrote:
whisky-dave wrote:

John Rumm wrote:

If the voltage sags too far, then you get lower PSCC so
can expect slower operation of protective devices under
fault conditions, so increase electrocution risk, and
catastrophic cable failure risk.

But not in under 3 hours.

The 3 hours at 40+ Amps is the "slow" part of the MCB curve,
in the event of a short circuit you want sufficient current
to ensure it trips within the "fast" part of the curve ...

Indeed...

You need 160A to make sure a B type MCB will trip in the
magnetic part of its response curve and that implies a maximum
loop impedance at the point of the fault of 230 / 160 = 1.44
ohms [1]

So why did it trip out ?

It didn't. At least not in the magnetic part of the response.
Neither would you expect it to in the absence of a *fault
current*.

So what happened ?

The MCB tripped on its thermal response because the circuit was overloaded.

Yes I assume that is what tripped it put.


You need to understand that there are two different classes of over
current: overload current, and fault current.

Yes but knowing which tripped it isn't certain. It was running happily for over two hours and only tripped after someone started using the soldering iron station.




Overloads are caused when a circuit is operating normally, but the total
load from the appliances connected exceeds the nominal rating of the
circuits protective device. Say for example, drawing 40A from a circuit
with a B32 MCB. The effects of this are to cause gradual heating of the
circuit wires and accessories.

I doon;t believe circuit wire blow MCBs

If left unchecked it could result in
cable damage or reduced life expectancy of the cables.

I wpould expect the cables to be able to cope with such a thing the MCB lioke a fuse is meant to be the weakest link in the chain NOT the strongest.

The heating will
take time to reach a damaging level. So MCBs include a trip mechanism
based on a bi-metal strip, that heats in a way analogous to that of the
rest of the circuit.

Exactly and I would expect that to trip BEFORE cable damage is likely.
In a simialr way that a basic fuse is meant to protect the cable NOT the equipment.



It will tolerate small overloads for a very long
time, and then ever reducing durations as the overload increases. So for
some overloads they may run for minutes or hours before tripping.

Yes just like a fuse would.


Fault currents however are a different class of fault. Here you have
something causing a short circuit, and the current that flows can be
100s or 1000s of amps. This results in very rapid heating of the circuit
wires (in some cases even explosive heating)

called a fuse yes.

- there is also no time for
this heat to be dissipated to the surroundings (i.e. its adiabatic
heating). This class of fault needs the kind of immediate response that
the bi-metal strip of the MCB can't provide. Hence it includes the
magnetic response that will react with the speed you would expect from a
fuse.

I thought they were meant to be faster than fuses.
Although there are differnt fuses and different MCBs I guess and then there's the Bussmann fuse curves .....




a student was soldering and he said my soldering
station has stopped working....
this was on the same trip as the
heaters who's LEDs had also just gone out. When we looked in the
riser cupboard where the CU is the MCB has tripped or cut out or
whatever you prefer to call it. The heaters (3 of them) were removed
the other were switchd off, and the MBC switch put to the ON position
and those things that went off came back on again so how did that
happen ?

As you would expect.

Trouble was that the heaters while claiming they were going to draw about 8amps for their 2KW capacity at about the 2+ hours mark they switched to 700W so about so about 3 amps rather than 8 amps. So with just ONE heater stwiching down that is the 4 heaters running at 8 amps will reach the limit of 32 amps, the 5th heater running at 3 amps or was it two or 3 heaters switching up of down from 3-8 amps that tripped the MCB or the constant 40 amps..
Youy see trips and fuses both blow with a rap[id change or a significant change in current so we don;t know why it tripped other than it's rating was eventuallky surpassed snd you can;t say for sure what caused it.
Was it because of the soldering iron switching on or a heater switching....


Even if the circuit is in spec and meets that requirement at
all sockets, running at only 202 volt gives a reduced potential
worst case PSSC of 202 / 1.44 = 140A, which could leave you on
the thermal portion of the response curve and *20 seconds* away
from disconnection. (and that ignores the effect of elevated
conductor temperature on the loop impedance)

140A I don't think we were drawing anywhere near that.

Go nail through a cable and measure it again!

Measure what ?

You made the claim that you were not drawing anything close to 140A. I
agree with you, you weren't. I suggest that if however you were to drive
a nail through one of the circuit cables (i.e. to introduce a fault) and
then measure the current draw, you will see a *significantly* larger
current - hopefully only briefly.

along with sonme sparks perhaps, but I don't see how this would make a differnce because if we had ZERO currunt draw and then put the nail through to make a short circuit the MCB would have most likely tripped irrespective of the current already flowing. If anyhting it;s make tripping slightly faster NOT slower.



Is that the sort of current you expect from 5 2KW heaters running
on 202V and a soldering re-work station of about 160W max. ?

No, you are confusing overload current with fault current.

I'm not as current is current there is NO difference, it's just
electrons and charge.

Its may just be electrons, but that does not mean you can handle
situations where you are drawing 5A too much in the same way you handle
those where you are drawing 500A too much.

This is why circuit designers consider both scenarios, and the equipment
manufacturers design kit that behaves in an appropriate way to cope with
both.

And I believe that if a MCB 32 amp is installed the wiring in that ciruits would be designed to take the current that a MCB of 32a could pass.




A fault current is what you will see when something bad happens in
a big way; say a cable or flex is damaged and a short circuit
between live conductors or live and earth occurs.

Do you think the heaters or the soldering iron produced his fault
current. ?

No.

They produced an overload. However the voltage drop you witnessed during
this episode does cast doubt on the ability of the circuit to correctly
deal with faults.

I'm not sure where you get that idea from.


In these situations the current that flows will be limited only by
the round trip resistance of the circuit's cables. Faults of this
nature will result in rapid adiabatic heating of the cable's
conductors. Unless the current is interrupted rapidly then cable
damage will occur, and this could include it melting, charring, or
bursting into flames.

Yes and will this happen at 32A 40 A 50 A and how long will it take
?

No it won't happen at 40A or 50A. Those are not fault currents, those
are overloads.

I never said they were fault currents, the whole idea behind this excercise was to find the overload current NOT the fault current which is pretty much irrelivant to us.


You can get an estimate of the time to trip by looking at the response
curve:

http://wiki.diyfaq.org.uk/images/d/d...e-MCBTypeB.png

The vertical bits of the curves represent the magnetic trip - that for
dealing with fault currents. The curved bits are the thermal response.
If you find 50A on the 32A curve you will see it intersects the time
axis at around 1000 secs. Real world conditions may mean you don't see
this - but you can be reasonably confident it will ultimately trip,

Which is what happend and NOT at 50 amps but a littel over 40 if it was over 40.


but
it will likely take tens of mins to do so. If you are running on a
circuit also suffering an undervolt then the times will be longer.

and so will the heating efect on the cables.


MCBs are designed with a separate trip mechanism. One of which is
intended to handle this situation *quickly* (typically under 0.1
secs)

what situation ?

The occurrence of a fault current.

Which we are NOT interested in.


using a solenoid to trip the mechanism (this is independent of the
bi metal strip that provides the normal *overload current*
protection).

So which tripped out ?

The thermal one.

Which is the thing we were intrested in which is why we set it up in this way
and left it.



For this to work, the MCB needs to see enough fault current. For a
type B device this could be up to 5x its nominal rating - so 160A
for a B32 device.

So are you saying that the professionals or the company employed to
install these MCBs installed cable that wasn't up to carrying the 32A
40A or 140A or 160A you say the MCB is designed for ?

I can't say with any certainty what you have installed. I have never
seen it, or tested it. I can offer only educated guess work. I can say I
am suspicious that all is not well based on what you have told us, if
what you have told us is correct.

Then that is down to those that installed it.

If a 32 amp MCB can rally passs 140 or 160 amps for 5 second what will the state of the cabling be after this event ?
Seems silly to istall such a MCB doens't it if the cable can't handle the fault condition or an overload condition.
I would assujme the overload is there to protect the cable and the short circuit trip was there to protect equipment and possible lives when there is a fault .



The fact that you have a voltage reduction device in place will make the
case more borderline. The installers *should* have checked that the
impedances of the circuits already installed were low enough for this to
be safely installed. I would not be surprised if this was not done.

Off load the voltage is about 223V, although I can;t turn off everything just the heaters I can't turn off the router/switch unit (not 240V as some expect).


There is a limit to the maximum length of cable that can be installed
for a circuit. This may have been observed as originally installed.
However it is not uncommon for extensions to be made later.

It has been upgraded a few times since the late 50s.

On Friday, 17 November 2017 18:07:55 UTC, Andy Burns wrote:
whisky-dave wrote:

What makes you think it could only supply 140A ?. Isn't that the
case with all 32A MCB anywhere in the country

The MCB doesn't really limit how much current you can draw, it just cuts
off if you happen to take too much for too long, it's the length and
cross-section of your cables that limit what you really can draw

But I don't see how that is relivant anymore than it is in your home.
Do you consider the CSA of you're home cabling ?


140A at 202V is 22kW heating up your cabling, your lab should be toasty
until the fire service arrive.

What makes you think that

stick a 6" nail through the submain coming into your lab and find out.

Why the submain ?

whisky-dave wrote:

On Friday, 17 November 2017 18:07:55 UTC, Andy Burns wrote:
whisky-dave wrote:

What makes you think it could only supply 140A ?. Isn't that the
case with all 32A MCB anywhere in the country

The MCB doesn't really limit how much current you can draw, it just cuts
off if you happen to take too much for too long, it's the length and
cross-section of your cables that limit what you really can draw

But I don't see how that is relivant anymore than it is in your home.
Do you consider the CSA of you're home cabling ?

The person who installs it *must* do so. Fortunately, if they just do
what everyone else does they may well be not far wrong.



140A at 202V is 22kW heating up your cabling, your lab should be toasty
until the fire service arrive.

What makes you think that

stick a 6" nail through the submain coming into your lab and find out.

Why the submain ?


--

Roger Hayter

On Monday, 20 November 2017 13:55:26 UTC, Roger Hayter wrote:
whisky-dave wrote:

On Friday, 17 November 2017 18:07:55 UTC, Andy Burns wrote:
whisky-dave wrote:

What makes you think it could only supply 140A ?. Isn't that the
case with all 32A MCB anywhere in the country

The MCB doesn't really limit how much current you can draw, it just cuts
off if you happen to take too much for too long, it's the length and
cross-section of your cables that limit what you really can draw

But I don't see how that is relivant anymore than it is in your home.
Do you consider the CSA of you're home cabling ?

The person who installs it *must* do so.

That's what I assumed from the last £30k upgrade to the electrics of the lab.
But I've no idea what sort of current carrying capacity the cable is to our 13amp 3 pin sockets. It could be that they've used 3 amp flex, I don't know or care.
If they have used 32 amp MCBs I assume there's a reason for 32 amp and not any other value.

And it's far better to check this out during the day when we are here than leaving it to happen over night, it has been suggested because it gets so cold that we should leave these heaters on overnight despite the manaual that comes with them says they must not be left on unattended.
But that is the sort of thing I;d expect from someone in charge of this sort of things that has their managment job because they have a degree in history or geography.



Fortunately, if they just do
what everyone else does they may well be not far wrong.



140A at 202V is 22kW heating up your cabling, your lab should be toasty
until the fire service arrive.

What makes you think that

stick a 6" nail through the submain coming into your lab and find out..

Why the submain ?


--

Roger Hayter

whisky-dave wrote:

On Monday, 20 November 2017 13:55:26 UTC, Roger Hayter wrote:
whisky-dave wrote:

On Friday, 17 November 2017 18:07:55 UTC, Andy Burns wrote:
whisky-dave wrote:

What makes you think it could only supply 140A ?. Isn't that the
case with all 32A MCB anywhere in the country

The MCB doesn't really limit how much current you can draw, it just cuts
off if you happen to take too much for too long, it's the length and
cross-section of your cables that limit what you really can draw

But I don't see how that is relivant anymore than it is in your home.
Do you consider the CSA of you're home cabling ?

The person who installs it *must* do so.

That's what I assumed from the last £30k upgrade to the electrics of the
lab. But I've no idea what sort of current carrying capacity the cable is
to our 13amp 3 pin sockets. It could be that they've used 3 amp flex, I
don't know or care. If they have used 32 amp MCBs I assume there's a
reason for 32 amp and not any other value.

And it's far better to check this out during the day when we are here than
leaving it to happen over night, it has been suggested because it gets so
cold that we should leave these heaters on overnight despite the manaual
that comes with them says they must not be left on unattended. But that is
the sort of thing I;d expect from someone in charge of this sort of things
that has their managment job because they have a degree in history or
geography.

If they cannot be left unattended (possibly after screwing to the wall)
then someone has bought the wrong item. Leaving unattended would seem to
be a rather common use case for an oil-filled electric heater.







Fortunately, if they just do
what everyone else does they may well be not far wrong.



140A at 202V is 22kW heating up your cabling, your lab should be
toasty until the fire service arrive.

What makes you think that

stick a 6" nail through the submain coming into your lab and find
out.

Why the submain ?


--

Roger Hayter


--

Roger Hayter

On Monday, 20 November 2017 14:38:43 UTC, Roger Hayter wrote:
whisky-dave wrote:

On Monday, 20 November 2017 13:55:26 UTC, Roger Hayter wrote:
whisky-dave wrote:

On Friday, 17 November 2017 18:07:55 UTC, Andy Burns wrote:
whisky-dave wrote:

What makes you think it could only supply 140A ?. Isn't that the
case with all 32A MCB anywhere in the country

The MCB doesn't really limit how much current you can draw, it just cuts
off if you happen to take too much for too long, it's the length and
cross-section of your cables that limit what you really can draw

But I don't see how that is relivant anymore than it is in your home.
Do you consider the CSA of you're home cabling ?

The person who installs it *must* do so.

That's what I assumed from the last £30k upgrade to the electrics of the
lab. But I've no idea what sort of current carrying capacity the cable is
to our 13amp 3 pin sockets. It could be that they've used 3 amp flex, I
don't know or care. If they have used 32 amp MCBs I assume there's a
reason for 32 amp and not any other value.

And it's far better to check this out during the day when we are here than
leaving it to happen over night, it has been suggested because it gets so
cold that we should leave these heaters on overnight despite the manaual
that comes with them says they must not be left on unattended. But that is
the sort of thing I;d expect from someone in charge of this sort of things
that has their managment job because they have a degree in history or
geography.

If they cannot be left unattended (possibly after screwing to the wall)

There is no facility to screw them to the wall, they are provided with wheels so I doubt they designed to mount on walls.

then someone has bought the wrong item. Leaving unattended would seem to
be a rather common use case for an oil-filled electric heater.

Perhaps managment have brought the worng heater, tehy certainly brought the wrongs ones on their first attempt.

Of course I;m not sure if the student lab is classed as it says.

"This appliance is intended for domestic use only. It should not be used for
commercial purposes"


http://cpc.farnell.com/pro-elec/pel0...ack/dp/HG00575

On 20/11/2017 12:51, whisky-dave wrote:
On Friday, 17 November 2017 18:42:48 UTC, John Rumm wrote:

You need to understand that there are two different classes of
over current: overload current, and fault current.

Yes but knowing which tripped it isn't certain.

It looks pretty cut'n'dried in this case...

It was running happily for over two hours and only tripped after
someone started using the soldering iron station.

If its been running for hours and then trips, its an overload.

If it trips the moment you attempt to energise the circuit (quite
possibly with a "pop" from the MCB), then that *might* be a fault
current. If you plug your soldering iron it, and it goes bang, and
immediately trips, and you can't reset it while the iron is still
connected, then that would likely be a fault.

Overloads are caused when a circuit is operating normally, but the
total load from the appliances connected exceeds the nominal rating
of the circuits protective device. Say for example, drawing 40A
from a circuit with a B32 MCB. The effects of this are to cause
gradual heating of the circuit wires and accessories.

I doon;t believe circuit wire blow MCBs

?

If left unchecked it could result in cable damage or reduced life
expectancy of the cables.

I wpould expect the cables to be able to cope with such a thing the
MCB lioke a fuse is meant to be the weakest link in the chain NOT the
strongest.

Its much the same situation with a fuse. Both will permit small
overloads for a long duration. In some cases (much depending on the
installation method used for the cable) even that may result in cable
damage, or at the very least premature ageing.

The heating will take time to reach a damaging level. So MCBs
include a trip mechanism based on a bi-metal strip, that heats in a
way analogous to that of the rest of the circuit.

Exactly and I would expect that to trip BEFORE cable damage is
likely.

Generally it will, although there is a slightly grey area for small
magnitude overloads. Say running a 32A circuit at the low 40s. The MCB
will permit that pretty much indefinitely. For a ring circuit with all
the cable run in masonry, or clipped to the surface, there is unlikely
to be a problem. However where a circuit has cables running in less
thermally favourable environments you can get close to exceeding the
maximum conductor temperatures.

In a simialr way that a basic fuse is meant to protect the cable NOT
the equipment.

For the one at the origin of a circuit, or in a plug yes. (equipment may
have additional internal fuses for self protection though)

It will tolerate small overloads for a very long time, and then
ever reducing durations as the overload increases. So for some
overloads they may run for minutes or hours before tripping.

Yes just like a fuse would.

Indeed.

Fault currents however are a different class of fault. Here you
have something causing a short circuit, and the current that flows
can be 100s or 1000s of amps. This results in very rapid heating of
the circuit wires (in some cases even explosive heating)

called a fuse yes.

I think its generally accepted that if you circuit wires vaporise in the
event of a fault you can consider your circuit protective devices were
inadequate (or at least the operating characteristics of the circuit was
so far from ideal, that the CPDs are were operating out of spec)

- there is also no time for this heat to be dissipated to the
surroundings (i.e. its adiabatic heating). This class of fault
needs the kind of immediate response that the bi-metal strip of the
MCB can't provide. Hence it includes the magnetic response that
will react with the speed you would expect from a fuse.

I thought they were meant to be faster than fuses.

For fault currents they are comparable for practical purposes, but quite
often a fuse will have a higher energy let through (I^2t) during its
pre-arc time. (which is sometimes you design cascaded systems with fuses
upstream of MCBs since they will usually discriminate)

Although there are differnt fuses and different MCBs I guess and then
there's the Bussmann fuse curves .....

Indeed, you can get "time delayed" and anti surge fuses, that allow more
inrush (and hence require larger fault currents to open quickly)

a student was soldering and he said my soldering station has
stopped working.... this was on the same trip as the heaters
who's LEDs had also just gone out. When we looked in the riser
cupboard where the CU is the MCB has tripped or cut out or
whatever you prefer to call it. The heaters (3 of them) were
removed the other were switchd off, and the MBC switch put to the
ON position and those things that went off came back on again so
how did that happen ?

As you would expect.

Trouble was that the heaters while claiming they were going to draw
about 8amps for their 2KW capacity at about the 2+ hours mark they
switched to 700W so about so about 3 amps rather than 8 amps. So
with just ONE heater stwiching down that is the 4 heaters running at
8 amps will reach the limit of 32 amps, the 5th heater running at 3
amps or was it two or 3 heaters switching up of down from 3-8 amps
that tripped the MCB or the constant 40 amps. Youy see trips and
fuses both blow with a rap[id change or a significant change in
current so we don;t know why it tripped other than it's rating was
eventuallky surpassed snd you can;t say for sure what caused it. Was
it because of the soldering iron switching on or a heater
switching....

Is there any likelihood that your combination of loads will have
exceeded 100A?

If the answer is no, then you did not trip the fault current detection
mechanism of the MCB - since that is the minimum required current for
that to happen (and indeed it would still be in spec if it required 160A
to trip using its fault current mechanism)

Even if the circuit is in spec and meets that requirement
at all sockets, running at only 202 volt gives a reduced
potential worst case PSSC of 202 / 1.44 = 140A, which could
leave you on the thermal portion of the response curve and
*20 seconds* away from disconnection. (and that ignores the
effect of elevated conductor temperature on the loop
impedance)

140A I don't think we were drawing anywhere near that.

Go nail through a cable and measure it again!

Measure what ?

You made the claim that you were not drawing anything close to
140A. I agree with you, you weren't. I suggest that if however you
were to drive a nail through one of the circuit cables (i.e. to
introduce a fault) and then measure the current draw, you will see
a *significantly* larger current - hopefully only briefly.

along with sonme sparks perhaps, but I don't see how this would make
a differnce because if we had ZERO currunt draw and then put the nail
through to make a short circuit the MCB would have most likely
tripped irrespective of the current already flowing. If anyhting it;s
make tripping slightly faster NOT slower.

If your fault current is not high enough to trip the magnetic response
of the MCB, then it will still trip, but it will have to do so using the
thermal mechanism, and this may react *significantly* more slowly -
especially if it was not already right on the boundary of tripping
(which you can't assume - a fault can happen at any time).

One can use the adiabatic equation to assess the effects on the cables.

Let's say you have a circuit wired in 2.5mm^2 T&E - that means your
smallest conductor is the pair of CPCs, totalling 3mm^2.

Let's say you have a fault current of 200A, and we can assume the MCB
will open the circuit within 0.1 secs. We have a minimum conductor CSA of:

MinCSA = sqrt( I^2 x t ) / k

(K will be 115 for PVC insulated cable)

So you get sqrt( 200^2 x 0.1 ) / 115 which means you need a conductor
CSA of at least 1.1mm^2 to survive the fault and not be damaged.

Now compare with a case when you can only muster say 130A of fault
current. That may take 25 secs to open the MCB. So run the sum again:

MinCSA = sqrt( 130^2 x 25 ) / 115

and you now need circuit conductors of at least 5.7 mm^2 to survive
without damage.

Is that the sort of current you expect from 5 2KW heaters
running on 202V and a soldering re-work station of about
160W max. ?

No, you are confusing overload current with fault current.

I'm not as current is current there is NO difference, it's just
electrons and charge.

Its may just be electrons, but that does not mean you can handle
situations where you are drawing 5A too much in the same way you
handle those where you are drawing 500A too much.

This is why circuit designers consider both scenarios, and the
equipment manufacturers design kit that behaves in an appropriate
way to cope with both.

And I believe that if a MCB 32 amp is installed the wiring in that
ciruits would be designed to take the current that a MCB of 32a could
pass.

You are misunderstanding what a MCB (or fuse) does.

MCBs have absolutely *no ability* to limit the current that passes
though them (save for a tiny internal resistance). Under fault
conditions, the current limitation is mostly[1] down the the fault loop
impedance. If you have (say) a loop impedance of 0.05 ohms, then you
could see a 4600A fault current irrespective of the MCB's nominal trip
current.

All a MCB can do is limit the time during which the fault current is
allowed to pass.

[1] The inductance of the supply transformer at the sub station will
slow the rise time somewhat for very high fault currents.

A fault current is what you will see when something bad happens
in a big way; say a cable or flex is damaged and a short
circuit between live conductors or live and earth occurs.

Do you think the heaters or the soldering iron produced his
fault current. ?

No.

They produced an overload. However the voltage drop you witnessed
during this episode does cast doubt on the ability of the circuit
to correctly deal with faults.

I'm not sure where you get that idea from.

By doing sums with the data you provided.

In these situations the current that flows will be limited only
by the round trip resistance of the circuit's cables. Faults of
this nature will result in rapid adiabatic heating of the
cable's conductors. Unless the current is interrupted rapidly
then cable damage will occur, and this could include it
melting, charring, or bursting into flames.

Yes and will this happen at 32A 40 A 50 A and how long will it
take ?

No it won't happen at 40A or 50A. Those are not fault currents,
those are overloads.

I never said they were fault currents, the whole idea behind this
excercise was to find the overload current NOT the fault current
which is pretty much irrelivant to us.

Your exercise highlighted to Adam and I that there may be problem with
the circuit. We have simply tried to explain to you, why this *could* be
dangerous in some circumstances. Should our fears be correct, then a
genuine fault may not be cleared in time to prevent bad things happening.

Hopefully you are now aware of this and may choose to do with this
information whatever you like.

You can get an estimate of the time to trip by looking at the
response curve:

http://wiki.diyfaq.org.uk/images/d/d...e-MCBTypeB.png

The vertical bits of the curves represent the magnetic trip - that
for dealing with fault currents. The curved bits are the thermal
response. If you find 50A on the 32A curve you will see it
intersects the time axis at around 1000 secs. Real world conditions
may mean you don't see this - but you can be reasonably confident
it will ultimately trip,

Which is what happend and NOT at 50 amps but a littel over 40 if it
was over 40.

Whatever


but it will likely take tens of mins to do so. If you are running
on a circuit also suffering an undervolt then the times will be
longer.

and so will the heating efect on the cables.

And?

MCBs are designed with a separate trip mechanism. One of which
is intended to handle this situation *quickly* (typically under
0.1 secs)

what situation ?

The occurrence of a fault current.

Which we are NOT interested in.

Ignorance is bliss huh?

I can't say with any certainty what you have installed. I have
never seen it, or tested it. I can offer only educated guess work.
I can say I am suspicious that all is not well based on what you
have told us, if what you have told us is correct.

Then that is down to those that installed it.

They have probably gone home by now. I guess its now someone else's
problem.

If a 32 amp MCB can rally passs 140 or 160 amps for 5 second what
will the state of the cabling be after this event ?

Slightly shagged perhaps.

Seems silly toistall such a MCB doens't it if the cable can't handle the fault
condition or an overload condition. I would assujme the overload is
there to protect the cable and the short circuit trip was there to
protect equipment and possible lives when there is a fault .

This is why when designing circuits you pay attention to things like
loop impedance. That way you can ensure that fault currents are large
enough to be disconnected quickly before damage occurs).

The fact that you have a voltage reduction device in place will
make the case more borderline. The installers *should* have checked
that the impedances of the circuits already installed were low
enough for this to be safely installed. I would not be surprised if
this was not done.

Off load the voltage is about 223V, although I can;t turn off
everything just the heaters I can't turn off the router/switch unit
(not 240V as some expect).



There is a limit to the maximum length of cable that can be
installed for a circuit. This may have been observed as originally
installed. However it is not uncommon for extensions to be made
later.

It has been upgraded a few times since the late 50s.

Do you mean upgraded, or simply extended?

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

John Rumm wrote:

On 20/11/2017 12:51, whisky-dave wrote:
On Friday, 17 November 2017 18:42:48 UTC, John Rumm wrote:

You need to understand that there are two different classes of over
current: overload current, and fault current.

Yes but knowing which tripped it isn't certain.

It looks pretty cut'n'dried in this case...

It was running happily for over two hours and only tripped after someone
started using the soldering iron station.

If its been running for hours and then trips, its an overload.

If it trips the moment you attempt to energise the circuit (quite possibly
with a "pop" from the MCB), then that *might* be a fault current. If you
plug your soldering iron it, and it goes bang, and immediately trips, and
you can't reset it while the iron is still connected, then that would
likely be a fault.

Overloads are caused when a circuit is operating normally, but the
total load from the appliances connected exceeds the nominal rating of
the circuits protective device. Say for example, drawing 40A from a
circuit with a B32 MCB. The effects of this are to cause gradual
heating of the circuit wires and accessories.

I doon;t believe circuit wire blow MCBs

?

If left unchecked it could result in cable damage or reduced life
expectancy of the cables.

I wpould expect the cables to be able to cope with such a thing the MCB
lioke a fuse is meant to be the weakest link in the chain NOT the
strongest.

Its much the same situation with a fuse. Both will permit small overloads
for a long duration. In some cases (much depending on the installation
method used for the cable) even that may result in cable damage, or at the
very least premature ageing.

The heating will take time to reach a damaging level. So MCBs include a
trip mechanism based on a bi-metal strip, that heats in a way analogous
to that of the rest of the circuit.

Exactly and I would expect that to trip BEFORE cable damage is likely.

Generally it will, although there is a slightly grey area for small
magnitude overloads. Say running a 32A circuit at the low 40s. The MCB
will permit that pretty much indefinitely. For a ring circuit with all the
cable run in masonry, or clipped to the surface, there is unlikely to be a
problem. However where a circuit has cables running in less thermally
favourable environments you can get close to exceeding the maximum
conductor temperatures.

In a simialr way that a basic fuse is meant to protect the cable NOT the
equipment.

For the one at the origin of a circuit, or in a plug yes. (equipment may
have additional internal fuses for self protection though)

It will tolerate small overloads for a very long time, and then ever
reducing durations as the overload increases. So for some overloads
they may run for minutes or hours before tripping.

Yes just like a fuse would.

Indeed.

Fault currents however are a different class of fault. Here you have
something causing a short circuit, and the current that flows can be
100s or 1000s of amps. This results in very rapid heating of the
circuit wires (in some cases even explosive heating)

called a fuse yes.

I think its generally accepted that if you circuit wires vaporise in the
event of a fault you can consider your circuit protective devices were
inadequate (or at least the operating characteristics of the circuit was
so far from ideal, that the CPDs are were operating out of spec)

- there is also no time for this heat to be dissipated to the
surroundings (i.e. its adiabatic heating). This class of fault needs
the kind of immediate response that the bi-metal strip of the MCB can't
provide. Hence it includes the magnetic response that will react with
the speed you would expect from a fuse.

I thought they were meant to be faster than fuses.

For fault currents they are comparable for practical purposes, but quite
often a fuse will have a higher energy let through (I^2t) during its
pre-arc time. (which is sometimes you design cascaded systems with fuses
upstream of MCBs since they will usually discriminate)

Although there are differnt fuses and different MCBs I guess and then
there's the Bussmann fuse curves .....

Indeed, you can get "time delayed" and anti surge fuses, that allow more
inrush (and hence require larger fault currents to open quickly)

a student was soldering and he said my soldering station has stopped
working.... this was on the same trip as the heaters who's LEDs had
also just gone out. When we looked in the riser cupboard where the CU
is the MCB has tripped or cut out or whatever you prefer to call it.
The heaters (3 of them) were removed the other were switchd off, and
the MBC switch put to the ON position and those things that went off
came back on again so how did that happen ?

As you would expect.

Trouble was that the heaters while claiming they were going to draw
about 8amps for their 2KW capacity at about the 2+ hours mark they
switched to 700W so about so about 3 amps rather than 8 amps. So with
just ONE heater stwiching down that is the 4 heaters running at 8 amps
will reach the limit of 32 amps, the 5th heater running at 3 amps or was
it two or 3 heaters switching up of down from 3-8 amps that tripped the
MCB or the constant 40 amps. Youy see trips and fuses both blow with a
rap[id change or a significant change in current so we don;t know why it
tripped other than it's rating was eventuallky surpassed snd you can;t
say for sure what caused it. Was it because of the soldering iron
switching on or a heater switching....

Is there any likelihood that your combination of loads will have exceeded
100A?

If the answer is no, then you did not trip the fault current detection
mechanism of the MCB - since that is the minimum required current for that
to happen (and indeed it would still be in spec if it required 160A to
trip using its fault current mechanism)

Even if the circuit is in spec and meets that requirement at all
sockets, running at only 202 volt gives a reduced potential worst
case PSSC of 202 / 1.44 = 140A, which could leave you on the
thermal portion of the response curve and *20 seconds* away from
disconnection. (and that ignores the effect of elevated conductor
temperature on the loop impedance)

140A I don't think we were drawing anywhere near that.

Go nail through a cable and measure it again!

Measure what ?

You made the claim that you were not drawing anything close to 140A. I
agree with you, you weren't. I suggest that if however you were to
drive a nail through one of the circuit cables (i.e. to introduce a
fault) and then measure the current draw, you will see a
*significantly* larger current - hopefully only briefly.

along with sonme sparks perhaps, but I don't see how this would make a
differnce because if we had ZERO currunt draw and then put the nail
through to make a short circuit the MCB would have most likely tripped
irrespective of the current already flowing. If anyhting it;s make
tripping slightly faster NOT slower.

If your fault current is not high enough to trip the magnetic response of
the MCB, then it will still trip, but it will have to do so using the
thermal mechanism, and this may react *significantly* more slowly -
especially if it was not already right on the boundary of tripping (which
you can't assume - a fault can happen at any time).

One can use the adiabatic equation to assess the effects on the cables.

Let's say you have a circuit wired in 2.5mm^2 T&E - that means your
smallest conductor is the pair of CPCs, totalling 3mm^2.

Let's say you have a fault current of 200A, and we can assume the MCB will
open the circuit within 0.1 secs. We have a minimum conductor CSA of:

MinCSA = sqrt( I^2 x t ) / k

(K will be 115 for PVC insulated cable)

So you get sqrt( 200^2 x 0.1 ) / 115 which means you need a conductor CSA
of at least 1.1mm^2 to survive the fault and not be damaged.

Now compare with a case when you can only muster say 130A of fault
current. That may take 25 secs to open the MCB. So run the sum again:

MinCSA = sqrt( 130^2 x 25 ) / 115

and you now need circuit conductors of at least 5.7 mm^2 to survive
without damage.

Is that the sort of current you expect from 5 2KW heaters running on
202V and a soldering re-work station of about 160W max. ?

No, you are confusing overload current with fault current.

I'm not as current is current there is NO difference, it's just
electrons and charge.

Its may just be electrons, but that does not mean you can handle
situations where you are drawing 5A too much in the same way you handle
those where you are drawing 500A too much.

This is why circuit designers consider both scenarios, and the
equipment manufacturers design kit that behaves in an appropriate way
to cope with both.

And I believe that if a MCB 32 amp is installed the wiring in that
ciruits would be designed to take the current that a MCB of 32a could
pass.

You are misunderstanding what a MCB (or fuse) does.

MCBs have absolutely *no ability* to limit the current that passes though
them (save for a tiny internal resistance). Under fault conditions, the
current limitation is mostly[1] down the the fault loop impedance. If you
have (say) a loop impedance of 0.05 ohms, then you could see a 4600A fault
current irrespective of the MCB's nominal trip current.

All a MCB can do is limit the time during which the fault current is
allowed to pass.

[1] The inductance of the supply transformer at the sub station will slow
the rise time somewhat for very high fault currents.

A fault current is what you will see when something bad happens in a
big way; say a cable or flex is damaged and a short circuit between
live conductors or live and earth occurs.

Do you think the heaters or the soldering iron produced his fault
current. ?

No.

They produced an overload. However the voltage drop you witnessed
during this episode does cast doubt on the ability of the circuit to
correctly deal with faults.

I'm not sure where you get that idea from.

By doing sums with the data you provided.

In these situations the current that flows will be limited only by
the round trip resistance of the circuit's cables. Faults of this
nature will result in rapid adiabatic heating of the cable's
conductors. Unless the current is interrupted rapidly then cable
damage will occur, and this could include it melting, charring, or
bursting into flames.

Yes and will this happen at 32A 40 A 50 A and how long will it take ?

No it won't happen at 40A or 50A. Those are not fault currents, those
are overloads.

I never said they were fault currents, the whole idea behind this
excercise was to find the overload current NOT the fault current which
is pretty much irrelivant to us.

Your exercise highlighted to Adam and I that there may be problem with the
circuit. We have simply tried to explain to you, why this *could* be
dangerous in some circumstances. Should our fears be correct, then a
genuine fault may not be cleared in time to prevent bad things happening.

Hopefully you are now aware of this and may choose to do with this
information whatever you like.

You can get an estimate of the time to trip by looking at the response
curve:

http://wiki.diyfaq.org.uk/images/d/d...e-MCBTypeB.png

The vertical bits of the curves represent the magnetic trip - that for
dealing with fault currents. The curved bits are the thermal response.
If you find 50A on the 32A curve you will see it intersects the time
axis at around 1000 secs. Real world conditions may mean you don't see
this - but you can be reasonably confident it will ultimately trip,

Which is what happend and NOT at 50 amps but a littel over 40 if it was
over 40.

Whatever


but it will likely take tens of mins to do so. If you are running on a
circuit also suffering an undervolt then the times will be longer.

and so will the heating efect on the cables.

And?

MCBs are designed with a separate trip mechanism. One of which is
intended to handle this situation *quickly* (typically under 0.1
secs)

what situation ?

The occurrence of a fault current.

Which we are NOT interested in.

Ignorance is bliss huh?

I can't say with any certainty what you have installed. I have never
seen it, or tested it. I can offer only educated guess work. I can say
I am suspicious that all is not well based on what you have told us, if
what you have told us is correct.

Then that is down to those that installed it.

They have probably gone home by now. I guess its now someone else's
problem.

If a 32 amp MCB can rally passs 140 or 160 amps for 5 second what will
the state of the cabling be after this event ?

Slightly shagged perhaps.

Seems silly toistall such a MCB doens't it if the cable can't handle the
fault condition or an overload condition. I would assujme the overload
is there to protect the cable and the short circuit trip was there to
protect equipment and possible lives when there is a fault .

This is why when designing circuits you pay attention to things like loop
impedance. That way you can ensure that fault currents are large enough to
be disconnected quickly before damage occurs).

The fact that you have a voltage reduction device in place will make
the case more borderline. The installers *should* have checked that the
impedances of the circuits already installed were low enough for this
to be safely installed. I would not be surprised if this was not done.

Off load the voltage is about 223V, although I can;t turn off everything
just the heaters I can't turn off the router/switch unit (not 240V as
some expect).



There is a limit to the maximum length of cable that can be installed
for a circuit. This may have been observed as originally installed.
However it is not uncommon for extensions to be made later.

It has been upgraded a few times since the late 50s.

Do you mean upgraded, or simply extended?

I am fascinated by the autotransformer the OP says is lowering the
supply voltage to his lab. Is it supplying the whole building or one
ring main? Given that its voltage output apparently drops 10% with a
40A load, are the assumptions about the supply under which fault
protection for the socket circuits was designed still true? Should
somewhat smaller current MCBs be installed? Should the whole
installation be condemned?

--

Roger Hayter

On Monday, 20 November 2017 19:03:54 UTC, John Rumm wrote:
On 20/11/2017 12:51, whisky-dave wrote:
On Friday, 17 November 2017 18:42:48 UTC, John Rumm wrote:

You need to understand that there are two different classes of
over current: overload current, and fault current.

Yes but knowing which tripped it isn't certain.

It looks pretty cut'n'dried in this case...

Seems reasonable but this trip has tripped before for no obvious reason like 10KW heaters hanging off it, and as that is the test we were giving it.



It was running happily for over two hours and only tripped after
someone started using the soldering iron station.

If its been running for hours and then trips, its an overload.

Seems that whay as the calculated time is 2.7 hours. (10k seconds)
but the heaters were cutting back to 700W after about 2 hours.



If it trips the moment you attempt to energise the circuit (quite
possibly with a "pop" from the MCB), then that *might* be a fault
current. If you plug your soldering iron it, and it goes bang, and
immediately trips, and you can't reset it while the iron is still
connected, then that would likely be a fault.

When the trip goes we always diconnect everything that was connected before resetting the trip and it reset immediatly.


Overloads are caused when a circuit is operating normally, but the
total load from the appliances connected exceeds the nominal rating
of the circuits protective device. Say for example, drawing 40A
from a circuit with a B32 MCB. The effects of this are to cause
gradual heating of the circuit wires and accessories.

I doon;t believe circuit wire blow MCBs

?

If left unchecked it could result in cable damage or reduced life
expectancy of the cables.

I wpould expect the cables to be able to cope with such a thing the
MCB lioke a fuse is meant to be the weakest link in the chain NOT the
strongest.

Its much the same situation with a fuse. Both will permit small
overloads for a long duration. In some cases (much depending on the
installation method used for the cable) even that may result in cable
damage, or at the very least premature ageing.

Yes I know, but what I don't know is whether a 32A MCB would be used with cable that the MCB is not up to protecting.
Isn't this why you shouldn't but a 13amp fuse in a lead that has 3amp flex ?




The heating will take time to reach a damaging level. So MCBs
include a trip mechanism based on a bi-metal strip, that heats in a
way analogous to that of the rest of the circuit.

Exactly and I would expect that to trip BEFORE cable damage is
likely.

Generally it will, although there is a slightly grey area for small
magnitude overloads. Say running a 32A circuit at the low 40s. The MCB
will permit that pretty much indefinitely. For a ring circuit with all
the cable run in masonry, or clipped to the surface, there is unlikely
to be a problem. However where a circuit has cables running in less
thermally favourable environments you can get close to exceeding the
maximum conductor temperatures.

Probabely is but as I don't know the specs of the cable used.



the how but I assumed the teaching lab (being a lab) had the triple rated 70C cable as standard

In a simialr way that a basic fuse is meant to protect the cable NOT
the equipment.

For the one at the origin of a circuit, or in a plug yes. (equipment may
have additional internal fuses for self protection though)

for the equipment not the cable.
MCBs aren't there to protect the equipment, so what are they for.
well a short circuit is the most likely reason for a trip or rather a 'faulty'
mains adapted which has tripped it in the past.




Fault currents however are a different class of fault. Here you
have something causing a short circuit, and the current that flows
can be 100s or 1000s of amps. This results in very rapid heating of
the circuit wires (in some cases even explosive heating)

called a fuse yes.

I think its generally accepted that if you circuit wires vaporise in the
event of a fault you can consider your circuit protective devices were
inadequate (or at least the operating characteristics of the circuit was
so far from ideal, that the CPDs are were operating out of spec)

Yes that is what I would have thought, if the wires get damaged then the MCB isn't doing it's job maybe that's why they come in differnt amp ratings.
So an even better reason for testing this out.
Maybe it would have been good to see the wires ignite the wooden benches while we were present and not when no ones here to see it again a bit like the forest and trees.....



- there is also no time for this heat to be dissipated to the
surroundings (i.e. its adiabatic heating). This class of fault
needs the kind of immediate response that the bi-metal strip of the
MCB can't provide. Hence it includes the magnetic response that
will react with the speed you would expect from a fuse.

I thought they were meant to be faster than fuses.

For fault currents they are comparable for practical purposes, but quite
often a fuse will have a higher energy let through (I^2t) during its
pre-arc time. (which is sometimes you design cascaded systems with fuses
upstream of MCBs since they will usually discriminate)

As long as the professionals but in what was required.


Although there are differnt fuses and different MCBs I guess and then
there's the Bussmann fuse curves .....

Indeed, you can get "time delayed" and anti surge fuses, that allow more
inrush (and hence require larger fault currents to open quickly)

Yes I know I buy then, Quick blow, anti surge, time delay, semi-delay, 'normal'
I'm just glad I don't have to worry about male and female and LGBTQ versions.
Don't seem to nhave those options with MCBs



Is there any likelihood that your combination of loads will have
exceeded 100A?

No, we no longer have a power lab, labs here. well they are all done on the ELVIS system now or which we have about 30 in use.
http://www.ni.com/en-gb/shop/select/...ab-workstation





If the answer is no, then you did not trip the fault current detection
mechanism of the MCB - since that is the minimum required current for
that to happen (and indeed it would still be in spec if it required 160A
to trip using its fault current mechanism)

I don't know what tripped it, I think switching tripped it as the '2KW' heaters went from 2KW down to 700W then back up to 1.6KW there were 5 of them..
Perhaps it was the 60W soldering iron that was the 'last straw for the camel'
But at least we know the MCB actually tripped.
Previously we had RCD and I used a 10K resistor between earth and live in a plug and used to go around testing the ciruits once a month. The H&S got involved and I had to stop testing and if the trips did trip, we had to call maintaince who would then come and inspect & corect a few hours later, so we don't tell them now, unless something really weird starts happening.


Measure what ?

You made the claim that you were not drawing anything close to
140A. I agree with you, you weren't. I suggest that if however you
were to drive a nail through one of the circuit cables (i.e. to
introduce a fault) and then measure the current draw, you will see
a *significantly* larger current - hopefully only briefly.

along with sonme sparks perhaps, but I don't see how this would make
a differnce because if we had ZERO currunt draw and then put the nail
through to make a short circuit the MCB would have most likely
tripped irrespective of the current already flowing. If anyhting it;s
make tripping slightly faster NOT slower.

If your fault current is not high enough to trip the magnetic response
of the MCB,

What current is that then ?
Is 32 amp OK and 33 a fault. ?



then it will still trip, but it will have to do so using the
thermal mechanism, and this may react *significantly* more slowly -
especially if it was not already right on the boundary of tripping
(which you can't assume - a fault can happen at any time).

One can use the adiabatic equation to assess the effects on the cables.

Only if you know the cables used.


Let's say you have a circuit wired in 2.5mm^2 T&E - that means your
smallest conductor is the pair of CPCs, totalling 3mm^2.

Let's say you have a fault current of 200A, and we can assume the MCB
will open the circuit within 0.1 secs. We have a minimum conductor CSA of:

MinCSA = sqrt( I^2 x t ) / k

(K will be 115 for PVC insulated cable)

So you get sqrt( 200^2 x 0.1 ) / 115 which means you need a conductor
CSA of at least 1.1mm^2 to survive the fault and not be damaged.

Now compare with a case when you can only muster say 130A of fault
current. That may take 25 secs to open the MCB. So run the sum again:

MinCSA = sqrt( 130^2 x 25 ) / 115

and you now need circuit conductors of at least 5.7 mm^2 to survive
without damage.

Is this how say the wiring of a home is wired ?
i.e that the cable has to survive the fault current ?


And I believe that if a MCB 32 amp is installed the wiring in that
ciruits would be designed to take the current that a MCB of 32a could
pass.

You are misunderstanding what a MCB (or fuse) does.

MCBs have absolutely *no ability* to limit the current that passes
though them (save for a tiny internal resistance). Under fault
conditions, the current limitation is mostly[1] down the the fault loop
impedance. If you have (say) a loop impedance of 0.05 ohms, then you
could see a 4600A fault current irrespective of the MCB's nominal trip
current.

All a MCB can do is limit the time during which the fault current is
allowed to pass.

Very similar to most fuses then.


[1] The inductance of the supply transformer at the sub station will
slow the rise time somewhat for very high fault currents.

But the fault curretn is a bit obsure because you;re factoring in time.
What is the fault current of a 32amp MCB.





They produced an overload. However the voltage drop you witnessed
during this episode does cast doubt on the ability of the circuit
to correctly deal with faults.

I'm not sure where you get that idea from.

By doing sums with the data you provided.

Then perhaps we have a faulty instalation.


In these situations the current that flows will be limited only
by the round trip resistance of the circuit's cables. Faults of
this nature will result in rapid adiabatic heating of the
cable's conductors. Unless the current is interrupted rapidly
then cable damage will occur, and this could include it
melting, charring, or bursting into flames.

Yes and will this happen at 32A 40 A 50 A and how long will it
take ?

No it won't happen at 40A or 50A. Those are not fault currents,
those are overloads.

I never said they were fault currents, the whole idea behind this
excercise was to find the overload current NOT the fault current
which is pretty much irrelivant to us.

Your exercise highlighted to Adam and I that there may be problem with
the circuit. We have simply tried to explain to you, why this *could* be
dangerous in some circumstances. Should our fears be correct, then a
genuine fault may not be cleared in time to prevent bad things happening.

True but I've no idea what a genuine fault might be.

Hopefully you are now aware of this and may choose to do with this
information whatever you like.

Nothing I can do with it.



You can get an estimate of the time to trip by looking at the
response curve:

http://wiki.diyfaq.org.uk/images/d/d...e-MCBTypeB.png

The vertical bits of the curves represent the magnetic trip - that
for dealing with fault currents. The curved bits are the thermal
response. If you find 50A on the 32A curve you will see it
intersects the time axis at around 1000 secs. Real world conditions
may mean you don't see this - but you can be reasonably confident
it will ultimately trip,

Which is what happend and NOT at 50 amps but a littel over 40 if it
was over 40.

Whatever

So for me the MCB is pretty much working as expected.

I also expect or assume that if we are pulling 40A through a 32a MCB that it will trip just as fast if we shorted L-N with a 6 inch nail as it would if we just had one 60W soldering iron on the circuit.



but it will likely take tens of mins to do so. If you are running
on a circuit also suffering an undervolt then the times will be
longer.

and so will the heating efect on the cables.

And?

cables runnig at 200V at 40A might get less hot than those running at 240V at 40A. for same CSA.



MCBs are designed with a separate trip mechanism. One of which
is intended to handle this situation *quickly* (typically under
0.1 secs)

what situation ?

The occurrence of a fault current.

Which we are NOT interested in.

Ignorance is bliss huh?

What is the fault current then ?





If a 32 amp MCB can rally passs 140 or 160 amps for 5 second what
will the state of the cabling be after this event ?

Slightly shagged perhaps.

So why have a 32amp why not a 20A ?


Seems silly toistall such a MCB doens't it if the cable can't handle the fault
condition or an overload condition. I would assujme the overload is
there to protect the cable and the short circuit trip was there to
protect equipment and possible lives when there is a fault .

This is why when designing circuits you pay attention to things like
loop impedance. That way you can ensure that fault currents are large
enough to be disconnected quickly before damage occurs).

This is why we pay qualified electricical companies to come in and install the cabling and do all woiring previously it was up to teh technicains to do it all on a strict budget but that;s all changed I am NOT meant to even 'untrip' a MCB let along decide what cable sizes we should have.
And I would expect a MCB of 32 amps to have cable that could support such an thing.

In the same why if you look at the title/subject I was asking what is expected of a 2KW heater it now seems that a 2KW heater is only expected to give 2KW for about 2 hours then it switches down to 700W.
So if I were to run the heaters over night they wouldn;t be consuming 1.6KW each as most of the time they's be consuming 700W, so 5 of these is 3.5KW rather than 10KW , I would have expected both the MCB and the lab cabling to support this at least all night if not forever.


It has been upgraded a few times since the late 50s.

Do you mean upgraded, or simply extended?

I think both, while the lab has stayed the same physical size we have had more socket points installed. IN the old days we had power meters and ran variacs connected to rehostates and experimants like yuo;d expect in the 1960s.
Now we have soldering stations, LCD oscilloscopes and we run ardunios etc and PCs, laptops so need more sockets. But I really don;t know if the POWER requirement s are higher or lower.

On Monday, 20 November 2017 20:02:32 UTC, Roger Hayter wrote:
John Rumm wrote:

On 20/11/2017 12:51, whisky-dave wrote:
On Friday, 17 November 2017 18:42:48 UTC, John Rumm wrote:

You need to understand that there are two different classes of over
current: overload current, and fault current.

Yes but knowing which tripped it isn't certain.

It looks pretty cut'n'dried in this case...

It was running happily for over two hours and only tripped after someone
started using the soldering iron station.

If its been running for hours and then trips, its an overload.

If it trips the moment you attempt to energise the circuit (quite possibly
with a "pop" from the MCB), then that *might* be a fault current. If you
plug your soldering iron it, and it goes bang, and immediately trips, and
you can't reset it while the iron is still connected, then that would
likely be a fault.

Overloads are caused when a circuit is operating normally, but the
total load from the appliances connected exceeds the nominal rating of
the circuits protective device. Say for example, drawing 40A from a
circuit with a B32 MCB. The effects of this are to cause gradual
heating of the circuit wires and accessories.

I doon;t believe circuit wire blow MCBs

?

If left unchecked it could result in cable damage or reduced life
expectancy of the cables.

I wpould expect the cables to be able to cope with such a thing the MCB
lioke a fuse is meant to be the weakest link in the chain NOT the
strongest.

Its much the same situation with a fuse. Both will permit small overloads
for a long duration. In some cases (much depending on the installation
method used for the cable) even that may result in cable damage, or at the
very least premature ageing.

The heating will take time to reach a damaging level. So MCBs include a
trip mechanism based on a bi-metal strip, that heats in a way analogous
to that of the rest of the circuit.

Exactly and I would expect that to trip BEFORE cable damage is likely..

Generally it will, although there is a slightly grey area for small
magnitude overloads. Say running a 32A circuit at the low 40s. The MCB
will permit that pretty much indefinitely. For a ring circuit with all the
cable run in masonry, or clipped to the surface, there is unlikely to be a
problem. However where a circuit has cables running in less thermally
favourable environments you can get close to exceeding the maximum
conductor temperatures.

In a simialr way that a basic fuse is meant to protect the cable NOT the
equipment.

For the one at the origin of a circuit, or in a plug yes. (equipment may
have additional internal fuses for self protection though)

It will tolerate small overloads for a very long time, and then ever
reducing durations as the overload increases. So for some overloads
they may run for minutes or hours before tripping.

Yes just like a fuse would.

Indeed.

Fault currents however are a different class of fault. Here you have
something causing a short circuit, and the current that flows can be
100s or 1000s of amps. This results in very rapid heating of the
circuit wires (in some cases even explosive heating)

called a fuse yes.

I think its generally accepted that if you circuit wires vaporise in the
event of a fault you can consider your circuit protective devices were
inadequate (or at least the operating characteristics of the circuit was
so far from ideal, that the CPDs are were operating out of spec)

- there is also no time for this heat to be dissipated to the
surroundings (i.e. its adiabatic heating). This class of fault needs
the kind of immediate response that the bi-metal strip of the MCB can't
provide. Hence it includes the magnetic response that will react with
the speed you would expect from a fuse.

I thought they were meant to be faster than fuses.

For fault currents they are comparable for practical purposes, but quite
often a fuse will have a higher energy let through (I^2t) during its
pre-arc time. (which is sometimes you design cascaded systems with fuses
upstream of MCBs since they will usually discriminate)

Although there are differnt fuses and different MCBs I guess and then
there's the Bussmann fuse curves .....

Indeed, you can get "time delayed" and anti surge fuses, that allow more
inrush (and hence require larger fault currents to open quickly)

a student was soldering and he said my soldering station has stopped
working.... this was on the same trip as the heaters who's LEDs had
also just gone out. When we looked in the riser cupboard where the CU
is the MCB has tripped or cut out or whatever you prefer to call it..
The heaters (3 of them) were removed the other were switchd off, and
the MBC switch put to the ON position and those things that went off
came back on again so how did that happen ?

As you would expect.

Trouble was that the heaters while claiming they were going to draw
about 8amps for their 2KW capacity at about the 2+ hours mark they
switched to 700W so about so about 3 amps rather than 8 amps. So with
just ONE heater stwiching down that is the 4 heaters running at 8 amps
will reach the limit of 32 amps, the 5th heater running at 3 amps or was
it two or 3 heaters switching up of down from 3-8 amps that tripped the
MCB or the constant 40 amps. Youy see trips and fuses both blow with a
rap[id change or a significant change in current so we don;t know why it
tripped other than it's rating was eventuallky surpassed snd you can;t
say for sure what caused it. Was it because of the soldering iron
switching on or a heater switching....

Is there any likelihood that your combination of loads will have exceeded
100A?

If the answer is no, then you did not trip the fault current detection
mechanism of the MCB - since that is the minimum required current for that
to happen (and indeed it would still be in spec if it required 160A to
trip using its fault current mechanism)

Even if the circuit is in spec and meets that requirement at all
sockets, running at only 202 volt gives a reduced potential worst
case PSSC of 202 / 1.44 = 140A, which could leave you on the
thermal portion of the response curve and *20 seconds* away from
disconnection. (and that ignores the effect of elevated conductor
temperature on the loop impedance)

140A I don't think we were drawing anywhere near that.

Go nail through a cable and measure it again!

Measure what ?

You made the claim that you were not drawing anything close to 140A. I
agree with you, you weren't. I suggest that if however you were to
drive a nail through one of the circuit cables (i.e. to introduce a
fault) and then measure the current draw, you will see a
*significantly* larger current - hopefully only briefly.

along with sonme sparks perhaps, but I don't see how this would make a
differnce because if we had ZERO currunt draw and then put the nail
through to make a short circuit the MCB would have most likely tripped
irrespective of the current already flowing. If anyhting it;s make
tripping slightly faster NOT slower.

If your fault current is not high enough to trip the magnetic response of
the MCB, then it will still trip, but it will have to do so using the
thermal mechanism, and this may react *significantly* more slowly -
especially if it was not already right on the boundary of tripping (which
you can't assume - a fault can happen at any time).

One can use the adiabatic equation to assess the effects on the cables.

Let's say you have a circuit wired in 2.5mm^2 T&E - that means your
smallest conductor is the pair of CPCs, totalling 3mm^2.

Let's say you have a fault current of 200A, and we can assume the MCB will
open the circuit within 0.1 secs. We have a minimum conductor CSA of:

MinCSA = sqrt( I^2 x t ) / k

(K will be 115 for PVC insulated cable)

So you get sqrt( 200^2 x 0.1 ) / 115 which means you need a conductor CSA
of at least 1.1mm^2 to survive the fault and not be damaged.

Now compare with a case when you can only muster say 130A of fault
current. That may take 25 secs to open the MCB. So run the sum again:

MinCSA = sqrt( 130^2 x 25 ) / 115

and you now need circuit conductors of at least 5.7 mm^2 to survive
without damage.

Is that the sort of current you expect from 5 2KW heaters running on
202V and a soldering re-work station of about 160W max. ?

No, you are confusing overload current with fault current.

I'm not as current is current there is NO difference, it's just
electrons and charge.

Its may just be electrons, but that does not mean you can handle
situations where you are drawing 5A too much in the same way you handle
those where you are drawing 500A too much.

This is why circuit designers consider both scenarios, and the
equipment manufacturers design kit that behaves in an appropriate way
to cope with both.

And I believe that if a MCB 32 amp is installed the wiring in that
ciruits would be designed to take the current that a MCB of 32a could
pass.

You are misunderstanding what a MCB (or fuse) does.

MCBs have absolutely *no ability* to limit the current that passes though
them (save for a tiny internal resistance). Under fault conditions, the
current limitation is mostly[1] down the the fault loop impedance. If you
have (say) a loop impedance of 0.05 ohms, then you could see a 4600A fault
current irrespective of the MCB's nominal trip current.

All a MCB can do is limit the time during which the fault current is
allowed to pass.

[1] The inductance of the supply transformer at the sub station will slow
the rise time somewhat for very high fault currents.

A fault current is what you will see when something bad happens in a
big way; say a cable or flex is damaged and a short circuit between
live conductors or live and earth occurs.

Do you think the heaters or the soldering iron produced his fault
current. ?

No.

They produced an overload. However the voltage drop you witnessed
during this episode does cast doubt on the ability of the circuit to
correctly deal with faults.

I'm not sure where you get that idea from.

By doing sums with the data you provided.

In these situations the current that flows will be limited only by
the round trip resistance of the circuit's cables. Faults of this
nature will result in rapid adiabatic heating of the cable's
conductors. Unless the current is interrupted rapidly then cable
damage will occur, and this could include it melting, charring, or
bursting into flames.

Yes and will this happen at 32A 40 A 50 A and how long will it take ?

No it won't happen at 40A or 50A. Those are not fault currents, those
are overloads.

I never said they were fault currents, the whole idea behind this
excercise was to find the overload current NOT the fault current which
is pretty much irrelivant to us.

Your exercise highlighted to Adam and I that there may be problem with the
circuit. We have simply tried to explain to you, why this *could* be
dangerous in some circumstances. Should our fears be correct, then a
genuine fault may not be cleared in time to prevent bad things happening.

Hopefully you are now aware of this and may choose to do with this
information whatever you like.

You can get an estimate of the time to trip by looking at the response
curve:

http://wiki.diyfaq.org.uk/images/d/d...e-MCBTypeB.png

The vertical bits of the curves represent the magnetic trip - that for
dealing with fault currents. The curved bits are the thermal response.
If you find 50A on the 32A curve you will see it intersects the time
axis at around 1000 secs. Real world conditions may mean you don't see
this - but you can be reasonably confident it will ultimately trip,

Which is what happend and NOT at 50 amps but a littel over 40 if it was
over 40.

Whatever


but it will likely take tens of mins to do so. If you are running on a
circuit also suffering an undervolt then the times will be longer.

and so will the heating efect on the cables.

And?

MCBs are designed with a separate trip mechanism. One of which is
intended to handle this situation *quickly* (typically under 0.1
secs)

what situation ?

The occurrence of a fault current.

Which we are NOT interested in.

Ignorance is bliss huh?

I can't say with any certainty what you have installed. I have never
seen it, or tested it. I can offer only educated guess work. I can say
I am suspicious that all is not well based on what you have told us, if
what you have told us is correct.

Then that is down to those that installed it.

They have probably gone home by now. I guess its now someone else's
problem.

If a 32 amp MCB can rally passs 140 or 160 amps for 5 second what will
the state of the cabling be after this event ?

Slightly shagged perhaps.

Seems silly toistall such a MCB doens't it if the cable can't handle the
fault condition or an overload condition. I would assujme the overload
is there to protect the cable and the short circuit trip was there to
protect equipment and possible lives when there is a fault .

This is why when designing circuits you pay attention to things like loop
impedance. That way you can ensure that fault currents are large enough to
be disconnected quickly before damage occurs).

The fact that you have a voltage reduction device in place will make
the case more borderline. The installers *should* have checked that the
impedances of the circuits already installed were low enough for this
to be safely installed. I would not be surprised if this was not done.

Off load the voltage is about 223V, although I can;t turn off everything
just the heaters I can't turn off the router/switch unit (not 240V as
some expect).



There is a limit to the maximum length of cable that can be installed
for a circuit. This may have been observed as originally installed.
However it is not uncommon for extensions to be made later.

It has been upgraded a few times since the late 50s.

Do you mean upgraded, or simply extended?

I am fascinated by the autotransformer the OP says is lowering the
supply voltage to his lab. Is it supplying the whole building or one
ring main?

The whole of the engineering building I think or perhaps just our part of teh building.
Building 15 bottom mid right
http://www.qmul.ac.uk/docs/about/26065.pdf


Given that its voltage output apparently drops 10% with a
40A load,

Just for one circuit in the lab, we have two cicuits in the two labs on 2 phases.
This is one reason we tried 5 heaters on 1 circuit ring of one phase becase we were told we were being given 20 2KW heaters.


are the assumptions about the supply under which fault
protection for the socket circuits was designed still true?

No idea.

Should
somewhat smaller current MCBs be installed? Should the whole
installation be condemned?

The whole lab should be updated, yesterday we finally had some of the new triple glazed windows installed and with the warmer weather the heaters are getting the temerature up to 20C by midday so we don't have to leave the heaters (5x2KW + 4x1.6KW + a 2.5KW) on overnight on full I leave 2 or 3 on the 1 element setting with the thermostat at about 70%.

This all should have happened before the end of august, we should have all our windows and the 'central' heating system in operation.







--

Roger Hayter

On Wed, 15 Nov 2017 05:38:07 -0800 (PST), whisky-dave
wrote:

snip

I know you don't, in spite of me explaining it several times and
suggesting you check one yourself (wiring) and contact the supplier /
manufacturer.

Not interested when 3 of the 5 all do the same just like it says in the supplied manual.

snip

'Sent: 15 November 2017 13:42
To: CPC Sales
Subject: PEL00489, Functionality?

Hi,

I am trying to assist someone determine if they have a problem with
one of the:

Portable Black 2kW Oil Filled Radiator With 9 Fins - PEL00489

When they have both elements / heats on and the thermostat on full, it
appears that after a while the overtemp stat kicks in *but*, the
heater only drops back to the low power (~700W) setting, not off
completely?

Can you confirm that this the way the overtemp protection function
should work please?

All the best, T i m'

Their reply:

'Hello,

Many thanks for your email and my apologies for this error.
Our technical team have advised that they believed this to be faulty.

If you can advise of the order details for this order this can be
logged for you.

Kind regards,

CPC Sales team
03447 88 00 88'

I don't expect any thanks btw.

Cheers, T i m

On Tuesday, 21 November 2017 15:45:06 UTC, T i m wrote:
On Wed, 15 Nov 2017 05:38:07 -0800 (PST), whisky-dave
wrote:

snip

I know you don't, in spite of me explaining it several times and
suggesting you check one yourself (wiring) and contact the supplier /
manufacturer.

Not interested when 3 of the 5 all do the same just like it says in the supplied manual.

snip

'Sent: 15 November 2017 13:42
To: CPC Sales
Subject: PEL00489, Functionality?

Hi,

I am trying to assist someone determine if they have a problem with
one of the:

Portable Black 2kW Oil Filled Radiator With 9 Fins - PEL0048

When they have both elements / heats on and the thermostat on full, it
appears that after a while the overtemp stat kicks in *but*, the
heater only drops back to the low power (~700W) setting, not off
completely?

Can you confirm that this the way the overtemp protection function
should work please?

All the best, T i m'

Their reply:

'Hello,

Many thanks for your email and my apologies for this error.
Our technical team have advised that they believed this to be faulty.

If you can advise of the order details for this order this can be
logged for you.

Kind regards,

CPC Sales team
03447 88 00 88'

I don't expect any thanks btw.

That's up to those that ordered them, and if I should tell those that ordered them they ordered faulty equipment they won't like it.
There's already an offical complaint about me complaining that they were breaking the 1992 factories act so they have put a complaint of sexist attitude on my record and I can't do anything about it and I don't really care.
If you look in the news you'll find the Russel group of prestigous universites and not exactly doing what you'd expect and I'd prefer to wait until I retire until I really hit out at them.

https://www.theguardian.com/uk-news/...-lecturers-pay

But beofore the heaters get returned I'll check them again then tell estates and they can tell financne and they can tell themselves and in a few years time they might do something about it.

Afterall it only took a few months to get the 'emergency' phone to be repaired.
And about 7 years for my lab phone to be replaced after the batteries leaked.

Once we've finsihed with the heaters then and only then will I tell them to send them back otherwise it'll end up being my job and I really don't want to repackage and send back 10 or more heaters.

On Tue, 21 Nov 2017 08:55:50 -0800 (PST), whisky-dave
wrote:

On Tuesday, 21 November 2017 15:45:06 UTC, T i m wrote:
On Wed, 15 Nov 2017 05:38:07 -0800 (PST), whisky-dave
wrote:

snip

I know you don't, in spite of me explaining it several times and
suggesting you check one yourself (wiring) and contact the supplier /
manufacturer.

Not interested when 3 of the 5 all do the same just like it says in the supplied manual.

snip

'Sent: 15 November 2017 13:42
To: CPC Sales
Subject: PEL00489, Functionality?

Hi,

I am trying to assist someone determine if they have a problem with
one of the:

Portable Black 2kW Oil Filled Radiator With 9 Fins - PEL0048

When they have both elements / heats on and the thermostat on full, it
appears that after a while the overtemp stat kicks in *but*, the
heater only drops back to the low power (~700W) setting, not off
completely?

Can you confirm that this the way the overtemp protection function
should work please?

All the best, T i m'

Their reply:

'Hello,

Many thanks for your email and my apologies for this error.
Our technical team have advised that they believed this to be faulty.

If you can advise of the order details for this order this can be
logged for you.

Kind regards,

CPC Sales team
03447 88 00 88'

I don't expect any thanks btw.

That's up to those that ordered them, and if I should tell those that ordered them they ordered faulty equipment they won't like it.

I don't see why?

There's already an offical complaint about me complaining that they were breaking the 1992 factories act

Ok.

so they have put a complaint of sexist attitude on my record and I can't do anything about it and I don't really care.

Ok.

If you look in the news you'll find the Russel group of prestigous universites and not exactly doing what you'd expect and I'd prefer to wait until I retire until I really hit out at them.

Ok.

https://www.theguardian.com/uk-news/...-lecturers-pay

But beofore the heaters get returned I'll check them again then tell estates and they can tell financne and they can tell themselves and in a few years time they might do something about it.

;-)


Afterall it only took a few months to get the 'emergency' phone to be repaired.
And about 7 years for my lab phone to be replaced after the batteries leaked.

Sounds like a nice forward-thinking place to work. ;-(

Once we've finsihed with the heaters then and only then will I tell them to send them back otherwise it'll end up being my job and I really don't want to repackage and send back 10 or more heaters.

Well, what might be a good idea (at some point) for someone to follow
up CPC's suggestion that they could be faulty in case something goes
wrong somewhere and someone gets hurt (even if not at your place)?

Cheers, T i m

On 21/11/2017 11:29, whisky-dave wrote:
On Monday, 20 November 2017 19:03:54 UTC, John Rumm wrote:

Its much the same situation with a fuse. Both will permit small
overloads for a long duration. In some cases (much depending on
the installation method used for the cable) even that may result in
cable damage, or at the very least premature ageing.

Yes I know, but what I don't know is whether a 32A MCB would be used
with cable that the MCB is not up to protecting.

For a general purpose ring circuit, the short answer is it should offer
both fault and overload protection. That means the cable in each leg (as
installed) has at least 21A of current carrying capacity, that loads are
not disproportionately bundled right at the end of the ring, and that
the overall loop impedance at the furthest reach of the circuit is low
enough to ensure "instant" disconnection in the event of a fault.

Isn't this why you
shouldn't but a 13amp fuse in a lead that has 3amp flex ?

If all the fuse is doing is providing fault protection (i.e. any
portable appliance made in the last few decades) then it will usually
remain "safe" even with the wrong fuse. (flexes on modern appliances are
usually sized / length limited to ensure they are fault protected on
your typical Euro style 16A circuit with no additional fusing).

In a simialr way that a basic fuse is meant to protect the cable
NOT the equipment.

For the one at the origin of a circuit, or in a plug yes.
(equipment may have additional internal fuses for self protection
though)

for the equipment not the cable. MCBs aren't there to protect the
equipment, so what are they for. well a short circuit is the most
likely reason for a trip or rather a 'faulty' mains adapted which has
tripped it in the past.

For a general purpose socket circuit, then the MCB will serve two
purposes. Protecting against faults, and protecting against excessive
overload.

The one it *must* provide is the fault protection. In some circumstances
the overload protection can be provided elsewhere and/or by other means
(the most obvious example of which is a spur from a ring - where the
maximum current carrying capacity of the spur cable is less than the
nominal trip of the MCB - the overload protection then comes from the
limitation to only a single socket (unfused spur). It will still
adequately offer fault protection though)

Fault currents however are a different class of fault. Here
you have something causing a short circuit, and the current
that flows can be 100s or 1000s of amps. This results in very
rapid heating of the circuit wires (in some cases even
explosive heating)

called a fuse yes.

I think its generally accepted that if you circuit wires vaporise
in the event of a fault you can consider your circuit protective
devices were inadequate (or at least the operating characteristics
of the circuit was so far from ideal, that the CPDs are were
operating out of spec)

Yes that is what I would have thought, if the wires get damaged then
the MCB isn't doing it's job maybe that's why they come in differnt
amp ratings.

Indeed.

Yes I know I buy then, Quick blow, anti surge, time delay,
semi-delay, 'normal' I'm just glad I don't have to worry about male
and female and LGBTQ versions. Don't seem to nhave those options with
MCBs

You have a similar choice (at least for the larger loads): Common
nominal ratings of 3, 6, 10, 16, 20, 32, 40, 45, 50, 63 (and possibly
others depending on range and brand)

Three different fault / inrush characteristics: Types B, C, & D

And often a range of maximum breaking currents, typically 6kA, and 10KA,
but again there are others. (those plug in wylex 3036 rewireable
replacements often only do 3kA)

Is there any likelihood that your combination of loads will have
exceeded 100A?

No, we no longer have a power lab, labs here. well they are all done
on the ELVIS system now or which we have about 30 in use.
http://www.ni.com/en-gb/shop/select/...ab-workstation

Yup, cute, but not really "power electrics" is it? ;-)

(I recall an ex Marconi college engineer lamenting the lack of exposure
to things over 5V by most of the current generation of new engineers -
he used to like demoing drawing an arc a couple of metres long from the
output of a high power transmitter!)

If the answer is no, then you did not trip the fault current
detection mechanism of the MCB - since that is the minimum required
current for that to happen (and indeed it would still be in spec if
it required 160A to trip using its fault current mechanism)

I don't know what tripped it, I think switching tripped it as the
'2KW' heaters went from 2KW down to 700W then back up to 1.6KW there
were 5 of them. Perhaps it was the 60W soldering iron that was the
'last straw for the camel' But at least we know the MCB actually
tripped. Previously we had RCD and I used a 10K resistor between
earth and live in a plug and used to go around testing the ciruits
once a month. The H&S got involved and I had to stop testing and if
the trips did trip, we had to call maintaince who would then come and
inspect & corect a few hours later, so we don't tell them now, unless
something really weird starts happening.


Measure what ?

You made the claim that you were not drawing anything close to
140A. I agree with you, you weren't. I suggest that if however
you were to drive a nail through one of the circuit cables
(i.e. to introduce a fault) and then measure the current draw,
you will see a *significantly* larger current - hopefully only
briefly.

along with sonme sparks perhaps, but I don't see how this would
make a differnce because if we had ZERO currunt draw and then put
the nail through to make a short circuit the MCB would have most
likely tripped irrespective of the current already flowing. If
anyhting it;s make tripping slightly faster NOT slower.

If your fault current is not high enough to trip the magnetic
response of the MCB,

What current is that then ? Is 32 amp OK and 33 a fault. ?

For a B32 MCB the minimum fault current to be *sure* of getting an
instant trip would be the 5x In rating, or 160A

Its the one in the table on the RHS of the graph:

http://wiki.diyfaq.org.uk/images/d/d...e-MCBTypeB.png

then it will still trip, but it will have to do so using the
thermal mechanism, and this may react *significantly* more slowly
- especially if it was not already right on the boundary of
tripping (which you can't assume - a fault can happen at any
time).

One can use the adiabatic equation to assess the effects on the
cables.

Only if you know the cables used.

You can use the equation to indicate the minimum size of cable required.
If those used are equal or greater, then you are happy. If they are
smaller, you have a problem.

(I would be very surprised if anyone would have installed undersized
cables initially - generally if you install one of the standard circuits
(say 2.5mm^2 ring for a 32A protected ring) all will be fine unless you
have excessive de-rating factors to take into account.

Let's say you have a circuit wired in 2.5mm^2 T&E - that means
your smallest conductor is the pair of CPCs, totalling 3mm^2.

Let's say you have a fault current of 200A, and we can assume the
MCB will open the circuit within 0.1 secs. We have a minimum
conductor CSA of:

MinCSA = sqrt( I^2 x t ) / k

(K will be 115 for PVC insulated cable)

So you get sqrt( 200^2 x 0.1 ) / 115 which means you need a
conductor CSA of at least 1.1mm^2 to survive the fault and not be
damaged.

Now compare with a case when you can only muster say 130A of fault
current. That may take 25 secs to open the MCB. So run the sum
again:

MinCSA = sqrt( 130^2 x 25 ) / 115

and you now need circuit conductors of at least 5.7 mm^2 to
survive without damage.

Is this how say the wiring of a home is wired ? i.e that the cable
has to survive the fault current ?

Yes. The "On Site guide" has a table that gives a maximum length of
cable permitted for each of the standard circuit types to save needing
to do the sums. In some cases the limitation is that of voltage drop,
and in others its maximum earth loop impedance. Where the limitation is
the latter, the level is set so that the cable should always have proper
fault protection.

And I believe that if a MCB 32 amp is installed the wiring in
that ciruits would be designed to take the current that a MCB of
32a could pass.

You are misunderstanding what a MCB (or fuse) does.

MCBs have absolutely *no ability* to limit the current that passes
though them (save for a tiny internal resistance). Under fault
conditions, the current limitation is mostly[1] down the the fault
loop impedance. If you have (say) a loop impedance of 0.05 ohms,
then you could see a 4600A fault current irrespective of the MCB's
nominal trip current.

All a MCB can do is limit the time during which the fault current
is allowed to pass.

Very similar to most fuses then.

Yup.

[1] The inductance of the supply transformer at the sub station
will slow the rise time somewhat for very high fault currents.

But the fault curretn is a bit obsure because you;re factoring in
time. What is the fault current of a 32amp MCB.

Nominally 160A for a type B device. 320A for a type C, and 640A for a D
type D.

They produced an overload. However the voltage drop you
witnessed during this episode does cast doubt on the ability of
the circuit to correctly deal with faults.

I'm not sure where you get that idea from.

By doing sums with the data you provided.

Then perhaps we have a faulty instalation.

Possibly.

Yes and will this happen at 32A 40 A 50 A and how long will
it take ?

No it won't happen at 40A or 50A. Those are not fault
currents, those are overloads.

I never said they were fault currents, the whole idea behind
this excercise was to find the overload current NOT the fault
current which is pretty much irrelivant to us.

Your exercise highlighted to Adam and I that there may be problem
with the circuit. We have simply tried to explain to you, why this
*could* be dangerous in some circumstances. Should our fears be
correct, then a genuine fault may not be cleared in time to prevent
bad things happening.

True but I've no idea what a genuine fault might be.

The most likely case is when a cable gets physically damaged - say
penetrated, crushed, burnt etc.

Hopefully you are now aware of this and may choose to do with this
information whatever you like.

Nothing I can do with it.

Probably nothing you can do directly...

For those wishing to don the tin knickers, a memo to "the powers that
be" would be about all you could do.

i.e. When we did this, we observed that. It has been drawn to my
attention this may indicate a problem, but I am no expert and can't say
for certain. You might want to consider having the wiring checked etc.

If it then blows up later, or someone gets electrocuted, you can put on
the "I told you so" tee shirt and wash your hands of it.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Tuesday, 21 November 2017 18:53:46 UTC, T i m wrote:
On Tue, 21 Nov 2017 08:55:50 -0800 (PST), whisky-dave
wrote:

On Tuesday, 21 November 2017 15:45:06 UTC, T i m wrote:
On Wed, 15 Nov 2017 05:38:07 -0800 (PST), whisky-dave
wrote:

snip

I know you don't, in spite of me explaining it several times and
suggesting you check one yourself (wiring) and contact the supplier /
manufacturer.

Not interested when 3 of the 5 all do the same just like it says in the supplied manual.

snip

'Sent: 15 November 2017 13:42
To: CPC Sales
Subject: PEL00489, Functionality?

Hi,

I am trying to assist someone determine if they have a problem with
one of the:

Portable Black 2kW Oil Filled Radiator With 9 Fins - PEL0048

When they have both elements / heats on and the thermostat on full, it
appears that after a while the overtemp stat kicks in *but*, the
heater only drops back to the low power (~700W) setting, not off
completely?

Can you confirm that this the way the overtemp protection function
should work please?

All the best, T i m'

Their reply:

'Hello,

Many thanks for your email and my apologies for this error.
Our technical team have advised that they believed this to be faulty.

If you can advise of the order details for this order this can be
logged for you.

Kind regards,

CPC Sales team
03447 88 00 88'

I don't expect any thanks btw.

That's up to those that ordered them, and if I should tell those that ordered them they ordered faulty equipment they won't like it.

I don't see why?

CPC apparently have asked for :
Quote 'If you can advise of the order details for this order this can be logged for you."

As I have No idea of such 'order details' and we didn't order through CPC anyway we have to go through onecall who in the past have sent me a returns number and the address to send the product too and if it is faulty it would include free postage back to them.
Not that I've found CPC or onecall very clued up when I've contacted them in the past, so I wouldn't necessarily think the call centre know what should be done, they always say send it back if you believe there is a problem.






There's already an offical complaint about me complaining that they were breaking the 1992 factories act

Ok.

so they have put a complaint of sexist attitude on my record and I can't do anything about it and I don't really care.

Ok.

If you look in the news you'll find the Russel group of prestigous universites and not exactly doing what you'd expect and I'd prefer to wait until I retire until I really hit out at them.

Ok.

https://www.theguardian.com/uk-news/...-lecturers-pay

But beofore the heaters get returned I'll check them again then tell estates and they can tell financne and they can tell themselves and in a few years time they might do something about it.

;-)

Had 6 emails from estates helpdesk today regarding broken windows, of course it has nothing to do with me I've never reorted a broken window but as there are TWO people with the same name working here for soem reason they have an automated reply that has contacted me.
You;d have throught they could scrape the original senders email address and use that as the reply address but then agauin this isn't the first time .....

If I email them I'll get a ticket and that get send to someone who then deals with teh problem thenb someone else will get back to me asking if I want the ticket closed.


Afterall it only took a few months to get the 'emergency' phone to be repaired.
And about 7 years for my lab phone to be replaced after the batteries leaked.

Sounds like a nice forward-thinking place to work. ;-(

well you have to go through managment who like employing mamagers rather than those that can get things done.


Once we've finsihed with the heaters then and only then will I tell them to send them back otherwise it'll end up being my job and I really don't want to repackage and send back 10 or more heaters.

Well, what might be a good idea (at some point) for someone to follow
up CPC's suggestion that they could be faulty in case something goes
wrong somewhere and someone gets hurt (even if not at your place)?

True, I might take one out of action at a time and test it in my office although if I find them all behaving the same the receptionist at CPC might then call it a feature.



Cheers, T i m

On Wed, 22 Nov 2017 03:39:37 -0800 (PST), whisky-dave
wrote:

snip

Once we've finsihed with the heaters then and only then will I tell them to send them back otherwise it'll end up being my job and I really don't want to repackage and send back 10 or more heaters.

Well, what might be a good idea (at some point) for someone to follow
up CPC's suggestion that they could be faulty in case something goes
wrong somewhere and someone gets hurt (even if not at your place)?

True, I might take one out of action at a time and test it in my office although if I find them all behaving the same the receptionist at CPC might then call it a feature.

I'm not sure what the CPC receptionist has to do with it as the
contact I made was in Sales and they had referenced the Tech dept?

"Our technical team have advised that they believed this to be faulty"

The bigger issue is if we are talking a one off or the whole batch
here.

Cheers, T i m

On Tuesday, 21 November 2017 20:13:50 UTC, John Rumm wrote:
On 21/11/2017 11:29, whisky-dave wrote:
On Monday, 20 November 2017 19:03:54 UTC, John Rumm wrote:

Its much the same situation with a fuse. Both will permit small
overloads for a long duration. In some cases (much depending on
the installation method used for the cable) even that may result in
cable damage, or at the very least premature ageing.

Yes I know, but what I don't know is whether a 32A MCB would be used
with cable that the MCB is not up to protecting.

For a general purpose ring circuit,


I'm not sure if that is the case, we are an electronics teaching lab.


the short answer is it should offer
both fault and overload protection. That means the cable in each leg (as
installed) has at least 21A of current carrying capacity, that loads are
not disproportionately bundled right at the end of the ring, and that
the overall loop impedance at the furthest reach of the circuit is low
enough to ensure "instant" disconnection in the event of a fault.

I would assume this had been checked and done and was even safer than a home system as we are dealing with students.




Isn't this why you
shouldn't but a 13amp fuse in a lead that has 3amp flex ?

If all the fuse is doing is providing fault protection (i.e. any
portable appliance made in the last few decades) then it will usually
remain "safe" even with the wrong fuse. (flexes on modern appliances are
usually sized / length limited to ensure they are fault protected on
your typical Euro style 16A circuit with no additional fusing).



In a simialr way that a basic fuse is meant to protect the cable
NOT the equipment.

For the one at the origin of a circuit, or in a plug yes.
(equipment may have additional internal fuses for self protection
though)

for the equipment not the cable. MCBs aren't there to protect the
equipment, so what are they for. well a short circuit is the most
likely reason for a trip or rather a 'faulty' mains adapted which has
tripped it in the past.

For a general purpose socket circuit, then the MCB will serve two
purposes. Protecting against faults, and protecting against excessive
overload.

and in our case ?


The one it *must* provide is the fault protection. In some circumstances
the overload protection can be provided elsewhere and/or by other means
(the most obvious example of which is a spur from a ring - where the
maximum current carrying capacity of the spur cable is less than the
nominal trip of the MCB - the overload protection then comes from the
limitation to only a single socket (unfused spur). It will still
adequately offer fault protection though)

Well here;s a link to our riser.
https://www.dropbox.com/s/u1rnoxretw..._0844.JPG?dl=0

the box opened is the one in the centre of the riser

https://www.dropbox.com/s/jr39axmtv0..._0845.JPG?dl=0

If that tells you anything.



Fault currents however are a different class of fault. Here
you have something causing a short circuit, and the current
that flows can be 100s or 1000s of amps. This results in very
rapid heating of the circuit wires (in some cases even
explosive heating)

called a fuse yes.

I think its generally accepted that if you circuit wires vaporise
in the event of a fault you can consider your circuit protective
devices were inadequate (or at least the operating characteristics
of the circuit was so far from ideal, that the CPDs are were
operating out of spec)

Yes that is what I would have thought, if the wires get damaged then
the MCB isn't doing it's job maybe that's why they come in differnt
amp ratings.

Indeed.

Yes I know I buy then, Quick blow, anti surge, time delay,
semi-delay, 'normal' I'm just glad I don't have to worry about male
and female and LGBTQ versions. Don't seem to nhave those options with
MCBs

You have a similar choice (at least for the larger loads): Common
nominal ratings of 3, 6, 10, 16, 20, 32, 40, 45, 50, 63 (and possibly
others depending on range and brand)

Three different fault / inrush characteristics: Types B, C, & D

And often a range of maximum breaking currents, typically 6kA, and 10KA,
but again there are others. (those plug in wylex 3036 rewireable
replacements often only do 3kA)

I would assume that the installers that charged us £30k last renovation had the sorted.


Is there any likelihood that your combination of loads will have
exceeded 100A?

No, we no longer have a power lab, labs here. well they are all done
on the ELVIS system now or which we have about 30 in use.
http://www.ni.com/en-gb/shop/select/...ab-workstation

Yup, cute, but not really "power electrics" is it? ;-)

Yes it is for teaching purposes we run a course on it and it's the standard kit for teaching power at degree level anyway.


(I recall an ex Marconi college engineer lamenting the lack of exposure
to things over 5V by most of the current generation of new engineers -
he used to like demoing drawing an arc a couple of metres long from the
output of a high power transmitter!)

5V ! we're trying to keep them to 3.3V. ;-)




If the answer is no, then you did not trip the fault current
detection mechanism of the MCB - since that is the minimum required
current for that to happen (and indeed it would still be in spec if
it required 160A to trip using its fault current mechanism)

I don't know what tripped it, I think switching tripped it as the
'2KW' heaters went from 2KW down to 700W then back up to 1.6KW there
were 5 of them. Perhaps it was the 60W soldering iron that was the
'last straw for the camel' But at least we know the MCB actually
tripped. Previously we had RCD and I used a 10K resistor between
earth and live in a plug and used to go around testing the ciruits
once a month. The H&S got involved and I had to stop testing and if
the trips did trip, we had to call maintaince who would then come and
inspect & corect a few hours later, so we don't tell them now, unless
something really weird starts happening.


Measure what ?

You made the claim that you were not drawing anything close to
140A. I agree with you, you weren't. I suggest that if however
you were to drive a nail through one of the circuit cables
(i.e. to introduce a fault) and then measure the current draw,
you will see a *significantly* larger current - hopefully only
briefly.

along with sonme sparks perhaps, but I don't see how this would
make a differnce because if we had ZERO currunt draw and then put
the nail through to make a short circuit the MCB would have most
likely tripped irrespective of the current already flowing. If
anyhting it;s make tripping slightly faster NOT slower.

If your fault current is not high enough to trip the magnetic
response of the MCB,

What current is that then ? Is 32 amp OK and 33 a fault. ?

For a B32 MCB the minimum fault current to be *sure* of getting an
instant trip would be the 5x In rating, or 160A

I've always thought that fault tripping was meant to go at 30ma or less when there was an inbalance between of curretn detected in the earth.


Its the one in the table on the RHS of the graph:

http://wiki.diyfaq.org.uk/images/d/d...e-MCBTypeB.png

I wonder if 160A going through a student would cause any damage or would it even be noticable, woul,d oit wake them up ?

To me it seems a bit late to trigger a fault if it takes 160A.




then it will still trip, but it will have to do so using the
thermal mechanism, and this may react *significantly* more slowly
- especially if it was not already right on the boundary of
tripping (which you can't assume - a fault can happen at any
time).

One can use the adiabatic equation to assess the effects on the
cables.

Only if you know the cables used.

You can use the equation to indicate the minimum size of cable required.
If those used are equal or greater, then you are happy. If they are
smaller, you have a problem.

Not sure I should start measuring things if size is important.


(I would be very surprised if anyone would have installed undersized
cables initially - generally if you install one of the standard circuits
(say 2.5mm^2 ring for a 32A protected ring) all will be fine unless you
have excessive de-rating factors to take into account.

Let's say you have a circuit wired in 2.5mm^2 T&E - that means
your smallest conductor is the pair of CPCs, totalling 3mm^2.

Let's say you have a fault current of 200A, and we can assume the
MCB will open the circuit within 0.1 secs. We have a minimum
conductor CSA of:

MinCSA = sqrt( I^2 x t ) / k

(K will be 115 for PVC insulated cable)

So you get sqrt( 200^2 x 0.1 ) / 115 which means you need a
conductor CSA of at least 1.1mm^2 to survive the fault and not be
damaged.

Now compare with a case when you can only muster say 130A of fault
current. That may take 25 secs to open the MCB. So run the sum
again:

MinCSA = sqrt( 130^2 x 25 ) / 115

and you now need circuit conductors of at least 5.7 mm^2 to
survive without damage.

Is this how say the wiring of a home is wired ? i.e that the cable
has to survive the fault current ?

Yes. The "On Site guide" has a table that gives a maximum length of
cable permitted for each of the standard circuit types to save needing
to do the sums. In some cases the limitation is that of voltage drop,
and in others its maximum earth loop impedance. Where the limitation is
the latter, the level is set so that the cable should always have proper
fault protection.

I would hope a teaching lab would come up to those standards.


And I believe that if a MCB 32 amp is installed the wiring in
that ciruits would be designed to take the current that a MCB of
32a could pass.

You are misunderstanding what a MCB (or fuse) does.

MCBs have absolutely *no ability* to limit the current that passes
though them (save for a tiny internal resistance). Under fault
conditions, the current limitation is mostly[1] down the the fault
loop impedance. If you have (say) a loop impedance of 0.05 ohms,
then you could see a 4600A fault current irrespective of the MCB's
nominal trip current.

All a MCB can do is limit the time during which the fault current
is allowed to pass.

Very similar to most fuses then.

Yup.

[1] The inductance of the supply transformer at the sub station
will slow the rise time somewhat for very high fault currents.

But the fault curretn is a bit obsure because you;re factoring in
time. What is the fault current of a 32amp MCB.

Nominally 160A for a type B device. 320A for a type C, and 640A for a D
type D.

I would assume that each MCB would be used with the appropriate cables in place. Threre must be some reason why a B C or D would be installed.
what happened to A ?


They produced an overload. However the voltage drop you
witnessed during this episode does cast doubt on the ability of
the circuit to correctly deal with faults.

I'm not sure where you get that idea from.

By doing sums with the data you provided.

Then perhaps we have a faulty instalation.

Possibly.

Just as well we've been promised a new lab then for the past 10+ years.


Yes and will this happen at 32A 40 A 50 A and how long will
it take ?

No it won't happen at 40A or 50A. Those are not fault
currents, those are overloads.

I never said they were fault currents, the whole idea behind
this excercise was to find the overload current NOT the fault
current which is pretty much irrelivant to us.

Your exercise highlighted to Adam and I that there may be problem
with the circuit. We have simply tried to explain to you, why this
*could* be dangerous in some circumstances. Should our fears be
correct, then a genuine fault may not be cleared in time to prevent
bad things happening.

True but I've no idea what a genuine fault might be.

The most likely case is when a cable gets physically damaged - say
penetrated, crushed, burnt etc.

Unlikely in this lab.


Hopefully you are now aware of this and may choose to do with this
information whatever you like.

Nothing I can do with it.

Probably nothing you can do directly...

For those wishing to don the tin knickers, a memo to "the powers that
be" would be about all you could do.

'Powers' interesting phrase.
I wonder if these are the powers that women lack and that is why they don't reprot sexual misconduct.


i.e. When we did this, we observed that. It has been drawn to my
attention this may indicate a problem, but I am no expert and can't say
for certain. You might want to consider having the wiring checked etc.



If it then blows up later, or someone gets electrocuted, you can put on
the "I told you so" tee shirt and wash your hands of it.

That's OK in theory but not in practice.

On Wednesday, 22 November 2017 11:48:23 UTC, T i m wrote:
On Wed, 22 Nov 2017 03:39:37 -0800 (PST), whisky-dave
wrote:

snip

Once we've finsihed with the heaters then and only then will I tell them to send them back otherwise it'll end up being my job and I really don't want to repackage and send back 10 or more heaters.

Well, what might be a good idea (at some point) for someone to follow
up CPC's suggestion that they could be faulty in case something goes
wrong somewhere and someone gets hurt (even if not at your place)?

True, I might take one out of action at a time and test it in my office although if I find them all behaving the same the receptionist at CPC might then call it a feature.

I'm not sure what the CPC receptionist has to do with it as the
contact I made was in Sales and they had referenced the Tech dept?

Well we wouldn't have contacted sales as they'd just say send it back irrespective of the problem whether there was one or not.

I would contact tech support using their on-line form.



"Our technical team have advised that they believed this to be faulty"

Yes believe....


The bigger issue is if we are talking a one off or the whole batch
here.

Yes I know, but at least one of them has been passed by our electrical tester bloke so it must be safe to use.

Just tested the 3rd heater with the same 'problem' or is it a 'feature'.
I'll contact customer services as indicated on the site after I;'ve tested the 5 x 2KW ones maybe I'll check the smaller 1.6KW too


Support:
Suggest Feedback / Report A Problem,
Return A Product
Telephone:
03447 11 11 55



Cheers, T i m

On Wed, 22 Nov 2017 06:29:44 -0800 (PST), whisky-dave
wrote:

snip

True, I might take one out of action at a time and test it in my office although if I find them all behaving the same the receptionist at CPC might then call it a feature.

I'm not sure what the CPC receptionist has to do with it as the
contact I made was in Sales and they had referenced the Tech dept?

Well we wouldn't have contacted sales as they'd just say send it back irrespective of the problem whether there was one or not.

Are you intent on being difficult on everything, even when people are
trying to help you?

I would contact tech support using their on-line form.

And I emailed them using the link for tech support and not expecting
any thanks (luckily).


"Our technical team have advised that they believed this to be faulty"

Yes believe....

Of course, they haven't had them beck to test them themselves have
they?


The bigger issue is if we are talking a one off or the whole batch
here.

Yes I know, but at least one of them has been passed by our electrical tester bloke so it must be safe to use.

Are you just trolling again? What sort of 'tests' do you think they
would do that could go any way to justifying your poor understanding
of the entire situation?

Just tested the 3rd heater with the same 'problem' or is it a 'feature'.

Ok, so, like I suggested right at the beginning, this could well be a
batch problem.

I'll contact customer services as indicated on the site

As I did you mean?

after I;'ve tested the 5 x 2KW ones maybe I'll check the smaller 1.6KW too

Ok. So you have two different sizes of heater (as you were going on
about the 2kW only being 1.6kW and you didn't know why).


Support:
Suggest Feedback / Report A Problem,
Return A Product
Telephone:
03447 11 11 55

You don't think there is a problem so why are you reporting one?

Why is it that you continually insist that you are right and everyone
else is wrong? Or is it you just have a strange way of asking a
question?


"Technical Support
Online Query Form: Log a Technical Query
Email:
Telephone: 03447 880088"


I was asking *if* there was a problem so felt 'Logging a technical
query' was most suitable.

But hey, if you don't approve of the way I was helping you ...

Cheers, T i m

On Wednesday, 22 November 2017 16:06:31 UTC, T i m wrote:
On Wed, 22 Nov 2017 06:29:44 -0800 (PST), whisky-dave
wrote:

snip

True, I might take one out of action at a time and test it in my office although if I find them all behaving the same the receptionist at CPC might then call it a feature.

I'm not sure what the CPC receptionist has to do with it as the
contact I made was in Sales and they had referenced the Tech dept?

Well we wouldn't have contacted sales as they'd just say send it back irrespective of the problem whether there was one or not.

Are you intent on being difficult on everything, even when people are
trying to help you?

Telling me to send the items back when I don't believe anything is worng isn;t helpful and then we'd have to close the lab resulting in delayed corses and a back log.

I have also delt with CPC before, when I asked them why their extractor fans at about ~£28 were so much cheaper than the farnell versions about ~£42 or so.

They said it was because CPC didn't supply any technical details or any help.




Tested all 5 heaters and they apparently all have exactly the same 'problem' or 'fault' which to me seems unlikely.



I would contact tech support using their on-line form.

And I emailed them using the link for tech support and not expecting
any thanks (luckily).

what link ?

http://cpc.farnell.com/contact-us

there is NO tech suport email link.



"Our technical team have advised that they believed this to be faulty"

Yes believe....

Of course, they haven't had them beck to test them themselves have
they?

Well they have 346 in stock.
No sign of them withdrawing them as yet.




The bigger issue is if we are talking a one off or the whole batch
here.

Yes I know, but at least one of them has been passed by our electrical tester bloke so it must be safe to use.

Are you just trolling again?

No stating the facts.

What sort of 'tests' do you think they
would do that could go any way to justifying your poor understanding
of the entire situation?

The test they legally have to do in order for a product to be considered safe.


Just tested the 3rd heater with the same 'problem' or is it a 'feature'..

Ok, so, like I suggested right at the beginning, this could well be a
batch problem.

and it might well not be, but a design feature.
Do you have any reason to think it is a batch fault ?



I'll contact customer services as indicated on the site

As I did you mean?

No you went through sales.


Support:
Suggest Feedback / Report A Problem,
Return A Product
Telephone:
03447 11 11 55




after I;'ve tested the 5 x 2KW ones maybe I'll check the smaller 1.6KW too

Ok. So you have two different sizes of heater (as you were going on
about the 2kW only being 1.6kW and you didn't know why).

3 or 4 actually.
I did know why they are only 1.6KW.




Support:
Suggest Feedback / Report A Problem,
Return A Product
Telephone:
03447 11 11 55

You don't think there is a problem so why are you reporting one?

you're the one that there's a problem.

I don't think there is.



Why is it that you continually insist that you are right and everyone
else is wrong?

Because I believe that is the case with these heaters.

Or is it you just have a strange way of asking a
question?

I wanted to know what someone here might expect to get from a 2KW heater, would they expect 2KW of heat or 700W of heat ?
What would you expect ?



"Technical Support
Online Query Form: Log a Technical Query
Email:
Telephone: 03447 880088"


I was asking *if* there was a problem so felt 'Logging a technical
query' was most suitable.

Did you log a technical query', you said sales contacted you.
rememeber.

Support:
Suggest Feedback / Report A Problem,
Return A Product
Telephone:
03447 11 11 55



But hey, if you don't approve of the way I was helping you ...

you think that is help ?

yuo contact sales they tell yuo their tech people have said what exaclty.
Where is this tech report ?




Cheers, T i m

On Wed, 22 Nov 2017 09:08:47 -0800 (PST), whisky-dave
wrote:

snip

Well we wouldn't have contacted sales as they'd just say send it back irrespective of the problem whether there was one or not.

Are you intent on being difficult on everything, even when people are
trying to help you?

Telling me to send the items back

I told you to do no such thing. I suggested they may be faulty and you
might raise the issue with the supplier and / or the manufacturer.

when I don't believe anything is worng

What you *believe* is irrelevant (especially considering how many
people have put so much effort into trying to *help* you / understand)
and you were the one raising the whole issue here in the first place
etc, *especially* why it was cutting from 1600 to 700, not zero watts.

isn;t helpful

I think it's all over the top helpful and I suggest 'most people'
would agree.

and then we'd have to close the lab resulting in delayed corses and a back log.

Irrelevant. That's possibly like leaving the same insulation tiles on
other buildings like Grenfell because it will make them colder.

I have also delt with CPC before, when I asked them why their extractor fans at about ~£28 were so much cheaper than the farnell versions about ~£42 or so.

That's a sales, not a safety issue.

They said it was because CPC didn't supply any technical details or any help.

shrug See how much help they 'don't give' if you suggest there could
be a safety issue and therefore a potential threat to life.

I asked them a question without buying the product in question myself
and they replied pretty quickly and willing to take it further?


Tested all 5 heaters and they apparently all have exactly the same 'problem' or 'fault' which to me seems unlikely.

There you go again with you blinkers and denial.

"When you have eliminated the impossible, whatever remains, however
improbable, must be the truth".



I would contact tech support using their on-line form.

And I emailed them using the link for tech support and not expecting
any thanks (luckily).

what link ?

http://cpc.farnell.com/contact-us

there is NO tech suport email link.

What? Are you blind or just trolling?

"Technical Support
Email: "

*That* is the link given for emailing tech support, that's why I used
it! Maybe it confuses you that it isn't
?



"Our technical team have advised that they believed this to be faulty"

Yes believe....

Of course, they haven't had them beck to test them themselves have
they?

Well they have 346 in stock.

OK.

No sign of them withdrawing them as yet.

I'm guessing they haven't had your order number to check that the
person asking for the technical advice has actually bought such from
them etc yet?


The bigger issue is if we are talking a one off or the whole batch
here.

Yes I know, but at least one of them has been passed by our electrical tester bloke so it must be safe to use.

Are you just trolling again?

No stating the facts.

Yes, you are stating 'facts' that are irrelevant to the point. A std
PAT wouldn't test for a (potential) design fault. (I nearly added
'would it' to the end of that but how would you know)?

What sort of 'tests' do you think they
would do that could go any way to justifying your poor understanding
of the entire situation?

The test they legally have to do in order for a product to be considered safe.

Quite. And that test took ~2 hours did it (the time it takes for the
rad to overheat)? They monitored the current for over two hours?


Just tested the 3rd heater with the same 'problem' or is it a 'feature'.

Ok, so, like I suggested right at the beginning, this could well be a
batch problem.

and it might well not be, but a design feature.

Yes, it *might* not be but we don't know yet do we (other than my
opinion and Tech support at CPC etc)?

Do you have any reason to think it is a batch fault ?

Yes (and I'm not going to explain it yet again).



I'll contact customer services as indicated on the site

As I did you mean?

No you went through sales.

Yes, because that *IS* the (email) path for a 'Technical question'.


Support:
Suggest Feedback / Report A Problem,
Return A Product
Telephone:
03447 11 11 55




after I;'ve tested the 5 x 2KW ones maybe I'll check the smaller 1.6KW too

Ok. So you have two different sizes of heater (as you were going on
about the 2kW only being 1.6kW and you didn't know why).

3 or 4 actually.

Ok.

I did know why they are only 1.6KW.

Not at the beginning you didn't or presumably you wouldn't have asked
here?

"So how much power does an oil filled radiator actually use.
But what really surprised me was the power consumption of a radiator
when it was full on, anyone care to guess what it was."




Support:
Suggest Feedback / Report A Problem,
Return A Product
Telephone:
03447 11 11 55

You don't think there is a problem so why are you reporting one?

you're the one that there's a problem.

Eh?

I don't think there is.

But we have already seen just how little you actually understand about
any of this by the amount of time people who do know have spent
correcting and re-explaining it all to you.



Why is it that you continually insist that you are right and everyone
else is wrong?

Because I believe that is the case with these heaters.

But we should be dealing with science here, not what you happen to
believe.

Or is it you just have a strange way of asking a
question?

I wanted to know what someone here might expect to get from a 2KW heater, would they expect 2KW of heat or 700W of heat ?

Sounds like a stupid / trolling question then, especially if you
already knew all the answers?

What would you expect ?

Why do you care, you don't listen to anything else people say?



"Technical Support
Online Query Form: Log a Technical Query
Email:
Telephone: 03447 880088"


I was asking *if* there was a problem so felt 'Logging a technical
query' was most suitable.

Did you log a technical query', you said sales contacted you.
rememeber.

No, I emailed them a technical question. How are sales going to know
to contact me (and I don't need to remember what I said, because it is
all fact / truth).

Support:
Suggest Feedback / Report A Problem,
Return A Product
Telephone:
03447 11 11 55



But hey, if you don't approve of the way I was helping you ...

you think that is help ?

And you think your responses to any of the help you have been given
(by me or others here) are appropriate I'm guessing.

yuo contact sales they tell yuo their tech people have said what exaclty.

I posted their reply previously. I'm not your wet nurse.

Where is this tech report ?

It will be in reply to you following up what I started on your behalf
by giving them the order details and / or even suggesting they get one
out of stock (if they haven't already) and prove an issue one way on
another. But you have already explained why nothing is going to happen
so ...

Cheers, T i m

On 22/11/2017 12:36, whisky-dave wrote:
On Tuesday, 21 November 2017 20:13:50 UTC, John Rumm wrote:
On 21/11/2017 11:29, whisky-dave wrote:
On Monday, 20 November 2017 19:03:54 UTC, John Rumm wrote:

Its much the same situation with a fuse. Both will permit
small overloads for a long duration. In some cases (much
depending on the installation method used for the cable) even
that may result in cable damage, or at the very least premature
ageing.

Yes I know, but what I don't know is whether a 32A MCB would be
used with cable that the MCB is not up to protecting.

For a general purpose ring circuit,


I'm not sure if that is the case, we are an electronics teaching
lab.

If it has a ring, and lots of sockets into which you can plug any device
you fancy, then its classed as a general purpose circuit.

(a non general purpose socket would be circuits for individual bits of
kit, like an immersion heater circuit or a fire alarm one - i.e.
situations where you know all about the specific equipment/load at
design time)

The only thing you might encounter different is in IT labs where the
large quantities of switched mode PSUs would often require that high
integrity earthing be used as well since there is often quite high earth
leakage when in normal operation as a result of all the input filters /
suppressors.

However the circuit protection would still be the same.

the short answer is it should offer both fault and overload
protection. That means the cable in each leg (as installed) has at
least 21A of current carrying capacity, that loads are not
disproportionately bundled right at the end of the ring, and that
the overall loop impedance at the furthest reach of the circuit is
low enough to ensure "instant" disconnection in the event of a
fault.

I would assume this had been checked and done and was even safer than
a home system as we are dealing with students.

If working correctly then it should be on par with that done in domestic
situations.

Commercial installs often tend to make use of larger numbers of radial
circuits with smaller MCBs - usually just to provide better
discrimination in the event of a fault, and the ability to isolate
smaller sections of the infrastructure.

For a general purpose socket circuit, then the MCB will serve two
purposes. Protecting against faults, and protecting against
excessive overload.

and in our case ?

Same.

The one it *must* provide is the fault protection. In some
circumstances the overload protection can be provided elsewhere
and/or by other means (the most obvious example of which is a spur
from a ring - where the maximum current carrying capacity of the
spur cable is less than the nominal trip of the MCB - the overload
protection then comes from the limitation to only a single socket
(unfused spur). It will still adequately offer fault protection
though)

Well here;s a link to our riser.
https://www.dropbox.com/s/u1rnoxretw..._0844.JPG?dl=0

the box opened is the one in the centre of the riser

https://www.dropbox.com/s/jr39axmtv0..._0845.JPG?dl=0

If that tells you anything.

Yup, normal memshield2 commercial style 3 phase CU. Which "room" is yours?

However, some interesting things to note the

Many of those MCBs are not actually MCBs but RCBOs - i.e. they include
RCD and MCB functionality in one unit (this is good for a situation like
yours since any earth leakage faults will only take out the affected
circuit and not others). Although this does mean that when seeing a
trip, you need to decide if its an over-current one or an earth leakage
one.

(I don't know if the memshield2 RCBOs have a different "RCD tripped"
dolly position from the normal overcurrent tripped one - Adam might know?).

The other interesting thing is that most of those socket circuits are
protected by C type devices. That means that the fault current required
to open one is double that which we have been discussing! However the
good news is that only applies to Live to Neutral faults and not Live to
Earth faults since the RCD part of the RCBO will take care of those at
=30mA.


Yes I know I buy then, Quick blow, anti surge, time delay,
semi-delay, 'normal' I'm just glad I don't have to worry about
male and female and LGBTQ versions. Don't seem to nhave those
options with MCBs

You have a similar choice (at least for the larger loads): Common
nominal ratings of 3, 6, 10, 16, 20, 32, 40, 45, 50, 63 (and
possibly others depending on range and brand)

Three different fault / inrush characteristics: Types B, C, & D

And often a range of maximum breaking currents, typically 6kA, and
10KA, but again there are others. (those plug in wylex 3036
rewireable replacements often only do 3kA)

The memshield2 breakers are often 10kA rated BTW - so higher breaking
capacity than most domestic stuff.

I would assume that the installers that charged us £30k last
renovation had the sorted.

You know what they say about assumptions!

(to be fair, the kit they have used is decent stuff)

Is there any likelihood that your combination of loads will
have exceeded 100A?

No, we no longer have a power lab, labs here. well they are all
done on the ELVIS system now or which we have about 30 in use.
http://www.ni.com/en-gb/shop/select/...ab-workstation


Yup, cute, but not really "power electrics" is it? ;-)

Yes it is for teaching purposes we run a course on it and it's the
standard kit for teaching power at degree level anyway.


(I recall an ex Marconi college engineer lamenting the lack of
exposure to things over 5V by most of the current generation of new
engineers - he used to like demoing drawing an arc a couple of
metres long from the output of a high power transmitter!)

5V ! we're trying to keep them to 3.3V. ;-)

Mmmm smell the ozone, feel the hairs on your arms perk up in the
electric field!

If your fault current is not high enough to trip the magnetic
response of the MCB,

What current is that then ? Is 32 amp OK and 33 a fault. ?

For a B32 MCB the minimum fault current to be *sure* of getting an
instant trip would be the 5x In rating, or 160A

Given your pictures above, make that 320A... however:

I've always thought that fault tripping was meant to go at 30ma or
less when there was an inbalance between of curretn detected in the
earth.

With a RCD or RCBO yes. Since you have RCBOs on the socket circuits,
then faults to earth will be cleared even if the circuits don't meet the
maximum allowed earth loop impedance.

That makes the whole situation less worrying, since the only fault that
won't be cleared is the less common L to N fault. (although given the
type C device there is probably no chance of clearing such a fault on
the instant part of the trip if you ever did get one of those)

Its the one in the table on the RHS of the graph:

http://wiki.diyfaq.org.uk/images/d/d...e-MCBTypeB.png

I wonder if 160A going through a student would cause any damage or
would it even be noticable, woul,d oit wake them up ?

To me it seems a bit late to trigger a fault if it takes 160A.

No, that's fine. You don't want it too low, or it would trip with switch
on surges. Remember the magnetic trip facility in the MCB/RCBO is only
there to deal with short circuit style situations, and currents of
hundreds of amps are normally commonplace in these situations.

The thermal part of the trip takes care of the everyday overloads like
someone plugging in a bunch of radiators.

And the RCD trip response will take care of most electrocution risk
events (unless you manage to get a student between L & N while not earthed!)

Yes. The "On Site guide" has a table that gives a maximum length
of cable permitted for each of the standard circuit types to save
needing to do the sums. In some cases the limitation is that of
voltage drop, and in others its maximum earth loop impedance. Where
the limitation is the latter, the level is set so that the cable
should always have proper fault protection.

I would hope a teaching lab would come up to those standards.

Pretty much all wiring should.

Since you have RCBOs on each circuit, that generally makes things much
simpler.

But the fault curretn is a bit obsure because you;re factoring
in time. What is the fault current of a 32amp MCB.

Nominally 160A for a type B device. 320A for a type C, and 640A for
a D type D.

I would assume that each MCB would be used with the appropriate
cables in place.

Usually yes. (In many cases they could be the same cables).

Threre must be some reason why a B C or D would be
installed.

Yup, B is general purpose and what you see in most domestic installs
(although I usually use type C on lighting circuits to minimise nuisance
trips on filament lamp failures).

Type C is often used with high inrush loads (large transformers,
induction motors, large banks of strip lights etc).

Type D is only usually used in industrial settings for things with very
high inrush.

what happened to A ?

It was left out to avoid any confusion with the current rating of the
device, which would often be specified with "A".

Then perhaps we have a faulty instalation.

Possibly.

Just as well we've been promised a new lab then for the past 10+
years.

;-)

Your exercise highlighted to Adam and I that there may be
problem with the circuit. We have simply tried to explain to
you, why this *could* be dangerous in some circumstances.
Should our fears be correct, then a genuine fault may not be
cleared in time to prevent bad things happening.

True but I've no idea what a genuine fault might be.

The most likely case is when a cable gets physically damaged - say
penetrated, crushed, burnt etc.

Unlikely in this lab.

Unlikely in most cases - but unlikely is not the same as never.

Hopefully you are now aware of this and may choose to do with
this information whatever you like.

Nothing I can do with it.

Probably nothing you can do directly...

For those wishing to don the tin knickers, a memo to "the powers
that be" would be about all you could do.

'Powers' interesting phrase. I wonder if these are the powers that
women lack and that is why they don't reprot sexual misconduct.


i.e. When we did this, we observed that. It has been drawn to my
attention this may indicate a problem, but I am no expert and can't
say for certain. You might want to consider having the wiring
checked etc.



If it then blows up later, or someone gets electrocuted, you can
put on the "I told you so" tee shirt and wash your hands of it.

That's OK in theory but not in practice.

Well the presence of all those RCBOs changes the picture somewhat. The
electrocution risk is very much lower (from direct contact anyway). I
would expect there is still a fire risk in the even of a L to N short
though.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

T i m wrote:

On Wed, 22 Nov 2017 09:08:47 -0800 (PST), whisky-dave
wrote:

snip

Well we wouldn't have contacted sales as they'd just say send it back
irrespective of the problem whether there was one or not.

Are you intent on being difficult on everything, even when people are
trying to help you?

Telling me to send the items back

I told you to do no such thing. I suggested they may be faulty and you
might raise the issue with the supplier and / or the manufacturer.

when I don't believe anything is worng

What you *believe* is irrelevant (especially considering how many
people have put so much effort into trying to *help* you / understand)
and you were the one raising the whole issue here in the first place
etc, *especially* why it was cutting from 1600 to 700, not zero watts.

snip

It seems to me more likely that the cut in power to 700W is part of the
radiators' normal, non-fault, temperature control, as this is likely to
be the sort of amount of heat they can dissipate at a normal room
temperature. Beyond this they probably have a thermostat which turns
them completely off when the room reaches the required temperature, and,
very probably, a cut-out which turns them completely off if they reach a
higher, fautl temperature, which should not occur in normal use. I
suggest, as someone else did earlier, covering one up in a couple of
thick blankets and see if it cuts out completely once it gets much
hotter then the temperature where i reduces power to 700W

I am also by no means convinced there is any evidence of a defect
(except for the marketing problem that they probably can't provide more
than about 1kW of space heating at normal room temperature despite being
marked 2kW.)


--

Roger Hayter

On Wed, 22 Nov 2017 20:03:12 +0000, (Roger Hayter)
wrote:

T i m wrote:

On Wed, 22 Nov 2017 09:08:47 -0800 (PST), whisky-dave
wrote:

snip

Well we wouldn't have contacted sales as they'd just say send it back
irrespective of the problem whether there was one or not.

Are you intent on being difficult on everything, even when people are
trying to help you?

Telling me to send the items back

I told you to do no such thing. I suggested they may be faulty and you
might raise the issue with the supplier and / or the manufacturer.

when I don't believe anything is worng

What you *believe* is irrelevant (especially considering how many
people have put so much effort into trying to *help* you / understand)
and you were the one raising the whole issue here in the first place
etc, *especially* why it was cutting from 1600 to 700, not zero watts.

snip

It seems to me more likely that the cut in power to 700W is part of the
radiators' normal, non-fault, temperature control, as this is likely to
be the sort of amount of heat they can dissipate at a normal room
temperature.

True, assuming a curtain hadn't fallen over the radiator etc?

Beyond this they probably have a thermostat which turns
them completely off when the room reaches the required temperature,

Yes, the conventional / adjustable 'main' thermostat, assuming it
isn't shorted etc (and presumably why most such radiators have some
form of 'overtemp' stat).

and,
very probably, a cut-out which turns them completely off if they reach a
higher, fautl temperature, which should not occur in normal use.

A thermal fuse, yes.

suggest, as someone else did earlier, covering one up in a couple of
thick blankets and see if it cuts out completely once it gets much
hotter then the temperature where i reduces power to 700W

That was probably me. ;-)

I am also by no means convinced there is any evidence of a defect
(except for the marketing problem that they probably can't provide more
than about 1kW of space heating at normal room temperature despite being
marked 2kW.)

No, you may well be right but that might at least open up a trading
standards question?

Cheers, T i m

On 22/11/2017 20:45, T i m wrote:
On Wed, 22 Nov 2017 20:03:12 +0000, (Roger Hayter)
wrote:

T i m wrote:

On Wed, 22 Nov 2017 09:08:47 -0800 (PST), whisky-dave
wrote:

snip

Well we wouldn't have contacted sales as they'd just say send it back
irrespective of the problem whether there was one or not.

Are you intent on being difficult on everything, even when people are
trying to help you?

Telling me to send the items back

I told you to do no such thing. I suggested they may be faulty and you
might raise the issue with the supplier and / or the manufacturer.

when I don't believe anything is worng

What you *believe* is irrelevant (especially considering how many
people have put so much effort into trying to *help* you / understand)
and you were the one raising the whole issue here in the first place
etc, *especially* why it was cutting from 1600 to 700, not zero watts.

snip

It seems to me more likely that the cut in power to 700W is part of the
radiators' normal, non-fault, temperature control, as this is likely to
be the sort of amount of heat they can dissipate at a normal room
temperature.

True, assuming a curtain hadn't fallen over the radiator etc?

Beyond this they probably have a thermostat which turns
them completely off when the room reaches the required temperature,

Yes, the conventional / adjustable 'main' thermostat, assuming it
isn't shorted etc (and presumably why most such radiators have some
form of 'overtemp' stat).

and,
very probably, a cut-out which turns them completely off if they reach a
higher, fautl temperature, which should not occur in normal use.

A thermal fuse, yes.

suggest, as someone else did earlier, covering one up in a couple of
thick blankets and see if it cuts out completely once it gets much
hotter then the temperature where i reduces power to 700W

That was probably me. ;-)

I am also by no means convinced there is any evidence of a defect
(except for the marketing problem that they probably can't provide more
than about 1kW of space heating at normal room temperature despite being
marked 2kW.)

No, you may well be right but that might at least open up a trading
standards question?

Cheers, T i m


Stick it in the garden and see how much power it draws.

--
Adam

T i m wrote:

On Wed, 22 Nov 2017 20:03:12 +0000, (Roger Hayter)
wrote:

T i m wrote:

On Wed, 22 Nov 2017 09:08:47 -0800 (PST), whisky-dave
wrote:

snip

Well we wouldn't have contacted sales as they'd just say send it back
irrespective of the problem whether there was one or not.

Are you intent on being difficult on everything, even when people are
trying to help you?

Telling me to send the items back

I told you to do no such thing. I suggested they may be faulty and you
might raise the issue with the supplier and / or the manufacturer.

when I don't believe anything is worng

What you *believe* is irrelevant (especially considering how many
people have put so much effort into trying to *help* you / understand)
and you were the one raising the whole issue here in the first place
etc, *especially* why it was cutting from 1600 to 700, not zero watts.

snip

It seems to me more likely that the cut in power to 700W is part of the
radiators' normal, non-fault, temperature control, as this is likely to
be the sort of amount of heat they can dissipate at a normal room
temperature.

True, assuming a curtain hadn't fallen over the radiator etc?

Beyond this they probably have a thermostat which turns
them completely off when the room reaches the required temperature,

Yes, the conventional / adjustable 'main' thermostat, assuming it
isn't shorted etc (and presumably why most such radiators have some
form of 'overtemp' stat).

and,
very probably, a cut-out which turns them completely off if they reach a
higher, fautl temperature, which should not occur in normal use.

A thermal fuse, yes.

suggest, as someone else did earlier, covering one up in a couple of
thick blankets and see if it cuts out completely once it gets much
hotter then the temperature where i reduces power to 700W

That was probably me. ;-)

I am also by no means convinced there is any evidence of a defect
(except for the marketing problem that they probably can't provide more
than about 1kW of space heating at normal room temperature despite being
marked 2kW.)

No, you may well be right but that might at least open up a trading
standards question?

Cheers, T i m

It would have to be an unequivocally false statement rather than simply
marketing hype for a business customer to have a valid complaint.
After all, they probably do provide 2kW in the open air at -10degC.


--

Roger Hayter

On Wed, 22 Nov 2017 21:38:47 +0000, ARW
wrote:

snip
I am also by no means convinced there is any evidence of a defect
(except for the marketing problem that they probably can't provide more
than about 1kW of space heating at normal room temperature despite being
marked 2kW.)

No, you may well be right but that might at least open up a trading
standards question?


Stick it in the garden and see how much power it draws.

I believe dave has already told us it didn't overtemp trip when a fan
was left blowing on it.

The issue is, how would one *expect* such a heater to behave under
'ordinary' conditions?

Let's say this heater is considered a domestic one and that say the
typical room temperature it was supposed to attain was 20 DegC then I
think most people would 'expect' it to give off 2kW worth of heat
continuously till that temperate was achieved?

I had a similar sort of discussion with a manager of one of the sheds
when I took a 'Desk lamp' back for replacement because the base had
started to melt!

It had actually been used to illuminate a small tropical fish tank, (I
fitted a clear plastic window in the lid) but whilst standing on a
desk and only for the sort of periods one might typically have a desk
lamp on in the evening.

He *tried* to say that it wasn't for that purpose but also couldn't
define where 'that purpose' was excluded anywhere on the box or in the
instructions.

When I asked him for his name as Trading Standards would probably ask
me who I spoke to, he suddenly changed his mind and allowed me to get
a replacement as an offer of 'goodwill'. I suggested that if this one
failed under the exact same circumstances (and didn't burn the house
down) I'd be back, probably *with* the Trading Standards Officer. ;-)

I even asked him to confirm that he was suggesting that this desk lamp
could only be used on a desk but only for a specified period (no
mention of that in the handbook) but he quickly backtracked on that.
;-)

I volunteered to buy another and possibly more expensive one, if he
suggested that the one I bought wasn't able to do as it described and
I required but he (then) wouldn't (of course). ;-)

Cheers, T i m