MCB fault or dodgy circuit??

solderer · Posted: Tue Jun 07, 2005 8:58 pm Post Subject: MCB fault or dodgy circuit??

Hi.
I have a 5A MCB on a lighting circuit which quite frequently trips. This is often but not always due the bulbs blowing, but my problem is it is very difficult to encourage it to go on again, even after switching off all of the lights on the circuit.
For instance the most recent time i turned off the lights, but the breaker would only reset after a good 10 mins, otherise there is no mechanical resistance when you flip the switch.
I cant see the point of it waiting so long to reset, do you think it is a fault?
(btw, I have NOT been turning off the main switch while doing this - is this relevant?)

The wiring in our house is of variable quality so it could well be some problem somewhere, but there is never any sign of anything sinister.

My next question is how to safely replace it if so. I have successfully done some household wiring tasks, but i feel much more at home with things which can be isolated by unplugging them!! I dont like the idea the meter tails cannot be isolated. Surely it is not safe to fiddle with the customer unit without getting the meter turned off at source?? Thanks

plugwash · Posted: Tue Jun 07, 2005 9:03 pm Post Subject:

if things have been done properly in the CU it should be possible to take the lid off safely with the tails still live (you should turn off the main switch first though).

HOWEVER take graet care the first time you open it as you don't know if whoever installed it did a bodge job.

kai · Posted: Tue Jun 07, 2005 9:05 pm Post Subject: MCB fault.

It could be a high resistance earth fault causing the MCB to pop out once the fault current rises above the certain level, but not quite high enough to operate the "magnetic" high speed trip mechanism.

How many bulbs do you have on the circuit - is it overloaded somehow ?

Spark123 · Posted: Tue Jun 07, 2005 9:23 pm Post Subject:

The next thing to try, as you suggest is to switch off the main switch before trying to reset the MCB. If, when you turn the main switch back on the MCB trips instantaneously it is time to call in an electrician as there may be a fault on your lighting circuit. If however, the MCB still refuses to reset with the main switch off, it is more than likely the MCB at fault.

solderer · Posted: Tue Jun 07, 2005 9:24 pm Post Subject:

I have quite a lot of lamps on it - but these still only total about 650W in my estimation, and i presume it should handle 1200W on the circuit. We do have problems if most of them are turned on, for sure.

There is one 30ma RCD in the CU and this does not trip when the circuit goes. I'm sure the regular MCBs do not detect earth leakage? Btw, this is not new equipment - looks like 70's or 80's i guess.

Thanks.

andy · Posted: Tue Jun 07, 2005 9:24 pm Post Subject:

if you do decide to replace the MCB, i would suggest you get some cheap, insulated screwdrivers. just incase it manages to hit something live, you wont get a shock

andy · Posted: Tue Jun 07, 2005 9:25 pm Post Subject:

solderer wrote:

I have quite a lot of lamps on it - but these still only total about 650W in my estimation, and i presume it should handle 1200W on the circuit. We do have problems if most of them are turned on, for sure.

There is one 30ma RCD in the CU and this does not trip when the circuit goes. I'm sure the regular MCBs do not detect earth leakage? Btw, this is not new equipment - looks like 70's or 80's i guess.

Thanks.

RCD's detect earth leakage. MCB do not. they work by detecting the current flowing thru and switching off if there is more than what sould be flowing

Damocles · Posted: Tue Jun 07, 2005 9:27 pm Post Subject:

If you turned off the main switch and it was still not possible to switch back the mcb, then I think the mcb is faulty. (but see below)

If you turn off the main switch and then there is no problem switching the mcb back on, then it is fine but overloaded somehow.

Only alternative I see is if the mcb is being loaded very near to trip so it is getting hot internally. Then it might take a while to cool down. This should not easily happen on a lighting circuit, but count how many lights are on and add up how many watts of power they are using. If your wiring is a bit strange, then is it possible that something big has been connected to the lighting circuit? Maybe some wild guy connected a socket to it?

You said it sometimes trips without any bulb failing. Not good, it should never do this. The obvious explanation would be that it was overloaded. count the lights on when this happens and look for anything else which could have been connected. You could try checking if power is being drawn by switching off all other breakers and see if the meter is still moving. One or two lights would not move it much, 6A is only 1.5 units in an hour. But if it was still going with nothing obvious switched on then you have found that electricity is going somewhere and could be causing genuine trips.

Got any big floodlights on the circuit?

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solderer · Posted: Tue Jun 07, 2005 9:34 pm Post Subject:

Spark123 - thanks for the advice. Can I clarify, you mean turn off the main switch, then try to reset the tripped breaker. In this case it would have no excuse to fail to reset, so must be dodgy. Sounds like a very good plan!
Is it usual that they would take time to reset even when isolated from the circuit?

Regarding changing it, i've got a proper DMM so i can confirm whats live. However, I hate the idea of working on something which is so close to a high current live circuit. I'm surprised switches are not installed before the meter...
Presumably, apart from potentially frying yourself, replacing an MCB is otherwise straightforward?

solderer · Posted: Tue Jun 07, 2005 9:45 pm Post Subject:

Damocles - thanks also for your advice, I'll definitely try this next time it goes.

I estimate i've got 650W of lights on the circuit, and i'm not aware that anything other than these are connected. The only other thing i'm thinking is there are some 'old' lights in our kitchen which dont seem to be connected, but perhaps the wires are loose somewhere. I'd expect to hear a big crack if it was shorting though, and never noticed this.

I'll follow the advice of isolating the whole circuit before reset, then go from there.

Thanks for your advice everyone.

Jaymack · Posted: Tue Jun 07, 2005 9:59 pm Post Subject:

solderer wrote:

Quote:

Regarding changing it, i've got a proper DMM so i can confirm whats live.

These meters should not be used for testing in this situation. Refer to GS38 and EAW 89. You wouldn't have a leg to stand on if there was a problem.

Spark123 · Posted: Tue Jun 07, 2005 10:20 pm Post Subject:

solderer wrote:

Can I clarify, you mean turn off the main switch, then try to reset the tripped breaker. In this case it would have no excuse to fail to reset, so must be dodgy.

Yes,

solderer wrote:

Is it usual that they would take time to reset even when isolated from the circuit?

No,
if a lamp blowing is the only cause of the circuit breaker tripping, it would lead me to suspect the circuit breaker is operating on the magnetic trip, hence it should reset immediately.
Another idea, if you have two identical 5A circuit breakers in the fuse box, try swopping them over.

solderer · Posted: Tue Jun 07, 2005 10:20 pm Post Subject:

Jaymack, i know its a bit off the original topic now, but why do you say it isnt safe? I dont have copies of the hse docs. The meter I have is CATIII rated, double insulated, 600v. It is marketed at electrical testing.

I've used it for testing the lighting circuits (other occasions) and found it very reassuring to know for sure the circuits are off. Far safer than common DIY alternative which is using a volt stick or neon screwdriver(!).

Cheers.

Spark123 · Posted: Tue Jun 07, 2005 10:49 pm Post Subject:

solderer wrote:

Jaymack, i know its a bit off the original topic now, but why do you say it isnt safe? I dont have copies of the hse docs. The meter I have is CATIII rated, double insulated, 600v. It is marketed at electrical testing.

I've used it for testing the lighting circuits (other occasions) and found it very reassuring to know for sure the circuits are off. Far safer than common DIY alternative which is using a volt stick or neon screwdriver(!).

Cheers.

Not to say it isn't safer to use a DMM than a volt stick (yuk) or a neon screwdriver (even worse, should be banned), a DMM is still not a recognized tool for testing dead. Recognized testers such as RS 410-0097 conform with the british standard and are permitted for testing dead.

HDRW · Posted: Wed Jun 08, 2005 7:46 am Post Subject:

solderer wrote:

Regarding changing it, i've got a proper DMM so i can confirm whats live. However, I hate the idea of working on something which is so close to a high current live circuit. I'm surprised switches are not installed before the meter...
Presumably, apart from potentially frying yourself, replacing an MCB is otherwise straightforward?

That's a pretty big "otherwise"! ("Apart from that, Mrs.Lincoln, what did you think of the play?")

Since you said the MCB is 5A I assume it's a plug-in replacement for a rewireable fuse. If it's a Wylex CU, beware that the incoming live terminal (not disconnected by the main switch) is well exposed when you open the lid - I replaced one in my house a while back and was a tad surprised to see that (no, I didn't find out the hard way!).

You are right to be cautious because touching that could spoil your whole day, and that would be a shame for your last one!

Cheers,

Howard

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