Downstairs lighting circuit MCB randomly tripping since electrician's visit last week.

JohnW2 · 13 Nov 2017

EFLImpudence said:
There's no way that such a fault could be over 2kW. So, was the meter flashing rate a red herring - or some kind of warning?

If it were a near dead short, and IF it were TT, then the great majority of the energy would be dissipated in the vicinity of the earth rod, where it probably would not be noticed.

Admittedly, to get 2kW would require an earth rod of about 25Ω, which is rather lower than normal for domestic rods, but it could be 'helped' by an extraneous-c-p - and the current (~8.7A for 2kW, in reality perhaps a bit more than 2kW) would probably heat up and trip a B6 fairly quickly.

Kind Regards, John

JohnW2 · 13 Nov 2017

garyo said:
Any sign of melting/blackening in the switch back box?

As I've just written, if it were something approaching a 'dead short', but resulted in only a modest fault current (e.g. if it were a TT installation), then nearly all the heat would be generated somewhere else.

Kind Regards, John

echoes · 13 Nov 2017

EFLImpudence said:
There's no way that such a fault could be over 2kW.

*if* it's TT with Ze about 30 ohms, that's 2kW dissipated in the ground, and a lot of live, bonded metalwork !?

echoes · 13 Nov 2017

JohnW2 said:
..Admittedly, to get 2kW would require an earth rod of about 25Ω ...

Kind of impedance I have measured when the only 'earth' is a water main...

EFLImpudence · 13 Nov 2017

Post #4, Nov 5th.

sparkwright said:
Check, with the power off, at the switch again.
Sometimes a wire can get 'pinched' against a faceplate screw, or against one of the fixing lugs.

JohnW2 · 13 Nov 2017

echoes said:
*if* it's TT with Ze about 30 ohms, that's 2kW dissipated in the ground ...

Indeed - as I've said, that would more-or-less explain everything the OP experienced - but, as we've both said, we are only speculating that it's TT (and no-one had even suggested it {and I confess that I hadn't considered the possibility} until I did recently).

echoes said:
... and a lot of live, bonded metalwork !?

If everything that should be bonded were bonded, then that didn't ought to represent much of a hazard - except for someone who touched some live metalwork with one foot on the ground outside the front door (or who touched a bonded outside tap whilst standing on the ground). That is, after all, what equipotential zones are all about.

Kind Regards, John

JohnW2 · 13 Nov 2017

echoes said:
Kind of impedance I have measured when the only 'earth' is a water main...

As often mentioned here, my incoming water pipe is about 0.4Ω - but I think that's 'thanks' to a neighbour's TN-C-S (my installation is TT) !

However, as I said before, a typical 50-100Ω earth rod plus a bit of help from a not-very-low-impedance supply pipe could easily explain 25Ω.

Kind Regards, John

ceecjc1 · 13 Nov 2017

You're all very smart, the installation is indeed TT. It's hammered into the ground outside the front of the house, very close to the switch in question.

sparkwright said:
Check, with the power off, at the switch again.

Sometimes a wire can get 'pinched' against a faceplate screw, or against one of the fixing lugs.

I did actually check both switches (there is one either end of the hall) but didn't notice this because I obviously didn't look very well expecting to see something more obvious, and in my defence it wasn't obviously pinched, just the electrician noticed the resulting hole in the cable.

JohnW2 · 13 Nov 2017

ceecjc1 said:
You're all very smart, the installation is indeed TT. It's hammered into the ground outside the front of the house, very close to the switch in question.

Thanks for confirming. As I recently wrote, none of us had previously considered that possibility and it only occurred to me very recently (I should have known better, since my house has TT!).

As I've said, I think this really explains everything (given that your RCD is not working - if it were, that would have tripped). As discussed at some length, it seems that, as a result of the short, about 2kW was being dissipated somewhere, and had it been in your house, there very probably would have been smells, smoke and possibly fire. However, with TT (and that's what limited it to ~2kW) nearly all that heat will be dissipated in the ground around the earth rod. The current caused by the short is again limited by the TT, and was presumably not high enough to trip the MCB 'instantly' (which is what would have happened had it not been TT) but nevertheless high enough to heat up the MCB and cause it to trip fairly quickly (minutes).

So all now seems explained. However, as you have now seen, you really do need to get that RCD replaced asap - since, as you have seen, with TT the MCB does not trip immediately in the event of a short to earth (but a functioining RCD would), but leaves a lot of the metalwork within your house potentially 'live' until the MCB eventually trips (if it ever does, which is not a foregone conclusion).

Kind Regards, John

ceecjc1 · 13 Nov 2017

JohnW2 said:
you really do need to get that RCD replaced asap

Absolutely, I agree. I'm in the process of getting some quotes.

ban-all-sheds · 13 Nov 2017

Good Idea, but..

ceecjc1 said:
Indeed! Need to take a look on check-a-trade for some more quotes for my RCD.

That's one of the last places you should look.

sparkwright · 13 Nov 2017

That's true. They pressure vulnerable people into giving them good reports, then get unreliable.

ceecjc1 · 13 Nov 2017

ban-all-sheds said:
Good Idea, but..That's one of the last places you should look.

I used to use Rated People back in the day but didn't have much luck with it, seems like they can remove bad reviews pretty easily. Dunno if the same thing happens with check a trade reviews but I've usually if admittedly not always been happy with the trades I've got through that.

What would you recommend then?

JohnW2 · 13 Nov 2017

ceecjc1 said:
What would you recommend then?

It's traditional to say that 'personal recommendations' are the best, but the problem there is that, whilst family/friends/contacts can judge whether a tradesman was pleasant, polite, punctual and tidy etc., they will rarely be in a position to make any real judgement about the quality, even safety, of the actual electrical work. However, it may still be your best bet.

What about the electrician who diagnosed and rectified your problem? Did he give a quote for replacing the RCD?

Kind Regards, John

ceecjc1 · 14 Nov 2017

JohnW2 said:
What about the electrician who diagnosed and rectified your problem? Did he give a quote for replacing the RCD?

He was a little vague about pricing, maybe it's too small a job for him to come back and do (he is not all that local).

The problem with personal recommendations is it only really makes sense if someone has had a lot of electrical work done otherwise it's a single review. It makes more sense to take your chances on 20 check-a-trade reviews for one guy than on 20 friends' recommendations for 20 different guys.

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Downstairs lighting circuit MCB randomly tripping since electrician's visit last week.

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