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.
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