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.