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