Strange MCB trip

[Roger Mills]

We woke up to a cold house this morning because the MCB on the ring main to which the CH is connected had tripped in the middle of the night.

I reset it as soon as I realised, and it didn't trip again - so I'm wondering what is likely to have caused it.

We're quite used to having 6A lighting circuit MCBs trip if a bulb fails at switch-on, but this is different.

The one which tripped is a 32A MCB on a ring circuit which has no RCD protection. According to a mains-driven clock, it appears to have tripped at about 3AM. Nothing significant would have been happening at that time. There are quite a few electronic devices on that ring - mainly using wallwarts - but nothing using much current at that time of night. [The freezers are on a different ring]. I can't find any device which has obviously failed - but surely if a device had failed, its own fuse would have blown rather than tripping the MCB?

Do MCBs sometimes trip for no good reason, or is there bound to be a genuine fault somewhere? If the latter, what should I be looking for?

[John Rumm]

Not always guaranteed - there have even been rare cases where an appliance fault has left its plug fuse and the circuit MCB untouched, but blown the main incomer fuse!

Not usually unless faulty. Perhaps in cases of parasitic heating from adjacent "hot" heavily loaded MCBs.

Might be worth sticking a clamp meter round the circuit live or neutral in the CU and checking the current draw is inline with your expectations...

Not powering an outside socket etc by any chance?

[Bill Wright]

Got gypsies next door?

Bill

[Brian Gaff]

One thing that happened to me some years back is that I had a surge protector on my wander lead for the hi fi and computer. It was as far as I could see, a medium sized resistor looking thing in the socket bar. Apparently if the voltage exceeds whatever its set for it puts a momentary short on the mains. This is definitely enough to trip things and more worryingly reveal how many nasty over voltage pulses were cominginto the house. Strangely, the device did not look stressed at all, and as it stopped quite a few spikes, one assumes it is still ok. Of late either its died or the spikes have gone away.

Brian

[ARW]

I would wait and see what happens. It could be a MCB on it's death bed. Unless you have an old/odd CU then it should only be a few quid to swap the MCB - that would be first my first suggestion - and if that fails then you start to look for fixed wiring or appliance faults

BTW Did you get in touch with Geoff?

[meow2222]

no, MCBs are generally faster,so either can go.

NT

[Roger Mills]

Not easy to isolate a single conductor from that MCB, but when I put my clamp meter on one of the tails feeding the CU in question, it measures

2.5 amps - and there are more things running than there would have been at 3am - including some things on the other MCBs.

Funny you should ask that! Yes, there is an outside circuit on that ring, which powers the pond electrics (circulator pump and air pump) via an RCD-protected spur.

The RCD had not tripped and the pond pumps are still working ok.

[Roger Mills]

It's a Wylex CU which I fitted about 30 years ago. It originally had re-wireable fuses, but I replaced them with MCBs later.

I do, in effect, have a spare MCB. When we moved the kitchen to the other side of the house a few years ago, we had a new dedicated CU for it. The MCB which used to power the cooker in the old kitchen is now largely redundant - the cooker control box is still there but nothing is connected to it. Its built-in 13A socket isn't used very often. So, if it happens again, I'll swap the MCB with the old cooker one, and see whether that makes any difference.

Not yet, but I will. Would you care to send me a PM with the best email address for him, so that my email won't get zapped by his spam trap?

[I was hoping for a few more responses to my OP - but yours is the only one so far].

[Andy Burns]

Any neighbours with an electric car?

[Roger Mills]

Not as far as I know, and all the outdoor electrics are in the back garden - a long way from where you could park a car. And the circuit whose MCB tripped only has the pond electrics - with nothing accessible to plug into.

[ARW]

That is the way forward,

I'll sort something out when I get to London later tonight. Is your email valid?

[Roger Mills]

Ta! Yes the email is valid. [As I was typing this, a PM arrived from John Rumm with an address for Geoff, so I'll try that].

[Andrew Gabriel]

I've had table lamp bulbs blowing trip the 32A MCB.

I've even had the 15W oven lamp blow and take out the ring's 32A MCB, and the plug fuse. It did cause the bulb to explode, and the force of it blew the glass cover cross-threaded, so I had to take the whole oven out so I could extract the fitting from the rear, and un-jam the glass cover.