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.

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