'Chattering' MCB

Heard a sort of chattering sound from the MCB (30A) supplying one of the
ring mains in our house this afternoon. Turn of the MCB, turn it back on
again, all is ok for a little while (maybe 10 minutes?) then it starts
again.
The circuit in question is one of four ring mains in the house, it
supplies the sitting room, one bedroom , a socket in the hall, in the
conservatory and the dressing room. generally lightly loaded - a few
table/standard lamps, TV, VCR, DVD etc.
Whilst investigating this problem I noticed that when this circuit was
switched on the disc on the meter started fair whizzing round - 30
rev/minute up from the say about 3 rev/minute is was doing normally.
My quick calcs indicate that 30 rev/min implies something like 10kW/h
being consumed - around 40A? , or have I made some big boo-boo?
Anyway, there is nothing on the circuit drawing anything like that (no
we don't have any secret electric showers, or unknown immersion heaters
etc.) These symptoms continue to occur once everything on the circuit
has been disconnected.
I've not had time today to investigate this more thoroughly, the circuit
is isolated now via the MCB pending time tomorrow.
So, my thought so far is that there is some fault in the fixed wiring,
causing a high current draw, and that the MCB is faulty in some way,
hence the noise rather than tripping.
Does the above sound a reasonable diagnosis?
I can check out the MCB by swopping it with others in the CU I guess, if
it is faulty then I'm not sure if replacing it is going to be a problem
or not.
The CU is a pretty old Crabtree one, you can see a couple of piccies
here:
Does anyone know if I will be able to find something to fit?
Thanks
Reply to
chris French
...
They look the same type as used in a couple of my factory distribution boards and they haven't been available for the past ten years.
Colin Bignell
Reply to
nightjar
In article , chris French writes:
I wonder if next door's cannabis plants are now in the dark?
It's very unlikely that a fault in the wiring dissipating 10kW will stay that way for more than a few seconds. It will quickly burn out, either sort or open circuit. Besides which, you'd be quite likely to notice 10kW of heat coming into the house from an unexpected location.
I would not be surprised at a 30A breaker making a noise at 40A, and I wouldn't immediately assume something wrong with the breaker in this case. It's probably right on the virge of tripping.
Seems like you need to find what the high load is, which your meter is also confirming.
Reply to
Andrew Gabriel
In article ,
I can't really think of a fault which would result in a very high current draw on the circuit with no load applied to it. I'd say even with the smaller ECC used on early rings and one end disconnected and shorted to line (the worst possible fault condition) it should still trip the MCB.
But a heavy current draw like this should be easily found with a simple DVM - after disconnecting the ring from the CU of course.
Reply to
Dave Plowman (News)
In message , Andrew Gabriel writes
:-)
Detached house, unless someone has burrowed into the house to connect something up it's pretty safe to say that it's in our property somewhere.
TBh, that was what I was thinking.
Well yes, that was indeed my thought, problem is, I'm at a loss as to what/where it could be, there is nothing in the house that I can think of that could possibly be drawing that sort of current, that isn't already accounted for. OK it's an old installation, and I still haven't got my head round all of it's idiosyncrasies, but this is very odd.
At the moment I'm flummoxed, but I've got the house to my self now till mid afternoon so will try and investigate some more.
Reply to
chris French
In message , nightjar writes
OK, thanks Colin, I was sort of expecting that. I'll worry about that bit if I need too.
Reply to
chris French
On Oct 17, 10:29 am, chris French wrote:
Hi
Disconnect everything from the ring, and disconnect the ring from the CU.
With your multimeter set to the ohms range, do the following measurements:-
L>L (should read low) N>N (should read low) E>E (should read low)
That proves your ring is intact from end to end.
Next, on either of the cables measure between L>E, L>N, and E>N. They should all read open circuit.
If you get any low resistance readings, perform the same test on the other leg of the ring as whichever gives the lowest reading indicates the fault is nearer that end of the ring.
Steve
Reply to
stevelup
Personally I would prefer to start by using a clamp meter on the running circuit - clamp round the live(s) at the exit from the MCB and take a direct reading of the current. This will verify what the real load is.
One test with a DVM would be to disconnect the circuit (live and neutral) at the CU and then measure the resistance between live and neutral.
If the circuit was drawing 40A at 240V, then you may see a resistance as low as 6 ohms. However this does assume that the load you are seeing will be visible at the low voltage of the DVM, and also that it is purely resistive[1].
If you can see the load with the DVM like this, then it ought to be a case of just breaking the circuit at various places until you isolate where it is. So if it is a ring for example, splitting in the middle and seeing which leg of it at the CU shows the low impedance load would immediately eliminate half the circuit.
[1] A large reactive load would be an interesting possibility in that it would not need to be accompanied by a large power dissipation. However your electricity meter ought not record this as power consumption.
Reply to
John Rumm
Have you recently moved to this house? A massive power consumption like this would be reflected in your electricity bills.
A possible explanation is that the wiring is very different to what you think it is - and a heavy load of this sort is normal (e.g. shower, cooker, storage heaters) - or the wiring is so mad that a heavy current device is actually supplied through 2 different MCB's.
I would start by thoroughly mapping each circuit from the consumer unit. All mcb's off - check every single socket, light and appliance is without power (in case something has been wired before the mcb's!). Check zero consumption on the meter. Then one mcb at once, verify every single socket, light & appliance - would be a good time to use a plug-in polarity & earth tester.
If the behaviour persists, and there's a massive current load on only one socket ring with no plugged in loads, and the behaviour is repeatable by switching that mcb with all others isolated - first panic, then consider opening up sockets, disconnecting parts of the ring etc.
Reply to
dom
In message , " snipped-for-privacy@gglz.com" writes
No we've been here 2 1/2 years, certainly we have not had 10Kw/h running continuously since then. Certainly the bills/my meter records don't reflect this. AFAIK this just started happening yesterday.
Thing is we don't have many heavy loads. Cooker (which is isolated a sit is a 'spare') and Immersion Heater - which is, as it usually is, turned off. No electric shower, storage heater etc.
There di use to be storage heaters before our time, and one thing I did think about was that ther was one somewhere that had sparked into life. But if there is then I certainly have never seen it.
It's all very odd (and yep, if I was answering this thread I'd be saying - 'he's got somethign somewhere running he's forgotten about....).
I have checked out what is on which circuit before, but as part of today's checks I'm having another check.
Thanks for everyones comments and suggestions/instructions, Turning off now, I'll be reporting back
Reply to
chris French
In message , stevelup writes
Thanks for that Steve.
A quick lunch break update.
OK, checked over the circuits again, but could not find anything lurking in the woodpile.
So been trying the above tests.
Looks like a Live to Earth fault- Low resistance/continuity between Live and Earth. I think I've now managed to narrow down the section of the ring that the fault is on - there is probably a junction box lurking somewhere as there is a spur off that probably comes off this leg somewhere.
I'm thinking that maybe my RCD isn't doing it's job properly? (We have a TT earth system with just one main incoming RCD - about as old looking as the CU) The test button works ok though.
Reply to
chris French
Are you sure it is a RCD and not an older "voltage operated" ELCB?
There is a picture of a common one here:
formatting link
are also details of how to test the RCD, if it is a RCD.
Reply to
John Rumm
On 17 Oct, 14:27, chris French wrote:
Are you sure it is an RCD and not a VOELCB? Have you or anyone else drilled any holes in walls or driven any nails into one lately? A defective immersion heater might also be a candidate for consideration. It should not be supplied from a ring final circuit but the possibility of it happening does exist.
Reply to
cynic
Hi,
I would be inclined to think that as well. 10KW is a lot of energy - probably enough to meet the entire heating needs of a medium sized house at this time of year and I have to wonder where it is going without you noticing something at least getting hot and probably catching fire.
One place that you might be able to loose that kind of energy for a while would be heating the ground around your earth rod.
Martin.
Reply to
Martin Wiseman
Switch everything off, lift the earth connection in the CU, switch on - does fault persist?
(Immediately reinstate that earth!)
Reply to
dom
In message , John Rumm writes > >> I'm thinking that maybe my RCD isn't doing it's job properly? (We >>have a TT earth system with just one main incoming RCD - about as old >>looking as the CU) The test button works ok though. > >Are you sure it is a RCD and not an older "voltage operated" ELCB? > No I'm not. >There is a picture of a common one here: >
formatting link
fact it looks exactly like that.....
Reply to
chris French
In article , John Rumm writes:
If that's the case, then the leak could be via a water main or similar to earth (you won't get 40A or anything like through a VOELCB). Either this water main (or whatever) is not connected to the house earthing (and has virtually full mains voltage on it), or it is connected to the house earthing but has an overlapping resistance area with the VOELCB earth rod, preventing the VOELCB from working.
Another option is the VOELCB has become disconnected from the house earthing, either due to an internal fault or deliberate action (possibly to prevent tripping in the past).
Probably other options too, but those two spring to mind as matching the symptoms described.
Beware, as you may have live metalwork around the house.
Reply to
Andrew Gabriel
Aha, the plot thickens... ;-)
Like Andy said these can fail to operate for a number of reasons. So it is possible to have a live to earth fault and not have it trip.
First job sounds like finding the live to earth fault. You can worry about replacing the ELCB with something else after!
Reply to
John Rumm
In message , John Rumm writes
It's an old house, the plot is always thick....
At some point I did consider that it could be an ELCB, but seemed to forget about the option at this point.
I think I've found the fault, after much rummaging behind sockets.
A live wire in one has a small bit of damaged insulation - just like a cut or maybe getting trapped and damaged at some point - this must have been in contact with an earth wire (old-ish installation - no sleeving) or maybe the backbox. There was a little tiny bit of what looked like heat damage, but only slight, I was expecting a bit more.
Just need to put everything back now, and see if it's ok.
Reply to
chris French

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