Fault finding. Mind boggling

normanton · Jan 11, 2016

I'm stumped on this one. My mate has a two way garage board in his garage. Circuit 1 is lighting, circuit 2 is 3 sockets on a radial. Lighting works fine. Sockets trip main RCD in DB when switched on. 3 sockets are now disconnected. Tried to reset 16A MCB (for radial circuit) and still trips RCD. So... Thought, simple, fault is on cable between DB and first socket. Disconnects live from out of top of MCB and it stays on. But.... When I push cable into top of MCB again, (even though the MCB is switched off), the lights go out (but don't trip) and arc like crazy from the bottom of the MCB). Bearing in mind this cable goes from the DB to the first socket and is disconnected at both ends!! Only things I haven't done is disconnect the neutral as well whilst trying to connect it back in. I realise this is an absolute headache to read. But if you manage to understand, could you give your opinions. Much appreciated, Lee

DPG · Jan 11, 2016

I suspect people will want to know what test results you have got - assuming you haven't just done the 'keep switching it on' method

Daz

normanton · Jan 11, 2016

DPG said:
I suspect people will want to know what test results you have got - assuming you haven't just done the 'keep switching it on' method Daz

Haha, all IR results are >299.

NDG Elecs · Jan 11, 2016

The lights stop working you say, is that without the B6 or RCD tripping? Where is it arcing from? Could possibly be the busbar is not in the cage clamps properly or tightened. I have had that tripping out seemingly random circuits before. But if the B16 is off when you try to connect the live in to it, that suggests a neutral fault maybe. Thinking out aloud here so may be off track!

Have you tried a different breaker? Or checked for continuity on the existing one and that you get open cct when it's off.

DPG · Jan 11, 2016

If there's arcing from the bottom of the MCB then there is a bad connection there - can't be an arc if there is a good solid connection made. I would take both MCBs out and all cables (including neutrals) and connect everything up fresh. IR test of all cables including neutrals as well. Daz

normanton · Jan 11, 2016

TheGayOwl said:
The lights stop working you say, is that without the B6 or RCD tripping? Where is it arcing from? Could possibly be the busbar is not in the cage clamps properly or tightened. I have had that tripping out seemingly random circuits before. But if the B16 is off when you try to connect the live in to it, that suggests a neutral fault maybe. Thinking out aloud here so may be off track!

Have you tried a different breaker? Or checked for continuity on the existing one and that you get open cct when it's off.

yes that's correct. The 6A MCB doesn't trip which would suggest to me the problem is on the incoming side of the breaker. ie, loose connection on busbar. Will try securing connections now and report back. Lee

Lucien Nunes · Jan 11, 2016

If making a connection to the 16A MCB when off causes the RCD to trip, either it's not really off (unlikely) or the trip resulted from something else being disturbed. You can confirm that the MCB is really opening with an insulation tester, then move on to find what is being disturbed.

A golden rule of fault finding is never to let your mind boggle! There's usually a very simple explanation for everything, which you will invariably find if you break a system down into small parts that can be conclusively proven to be, or not to be, part of the problem. E.g., something that arcs (other than a welder!) is either a poor connection or contact. You've got that symptom so find the cause, never mind RCD tripping or not tripping or whatever, find the cause of the arcing (probably busbar not properly located). Once you've done that, move on to the next bit. etc.

NDG Elecs · Jan 11, 2016

As I said earlier and others have also said check the busbar. I would remove the din rail wholesale with RCD and MCBs attached still and check that the busbar is correctly located in all three cage clamps. As I said loose connections can cause RCD to trip. Are there any signs of heat damage to the MCBs or RCD.

Midwest · Jan 11, 2016

Give your mate the Spanish Inquisition as well. I've been lead on a merry dance, by customers not telling the full 'story'.

NDG Elecs · Jan 11, 2016

Midwest said:
Give your mate the Spanish Inquisition as well. I've been lead on a merry dance, by customers not telling the full 'story'.

Yeah, annoys me that one. I often ask whether they have done something they don't don't want to mention for fear of looking stupid! I say I may find the problem much much quicker (and cheaper) if they tell me the truth, the whole truth and nothing but the truth! The prospect of it being cheaper normally entices them to swallow their pride!

davesparks · Jan 11, 2016

So you're working on a live board poking cables into terminals which is causing arcing to occur?

HandySparks · Jan 11, 2016

TheGayOwl said:
Are there any signs of heat damage to the MCBs or RCD.

This was a recent weekend callout. House is about 15 years old, with no significant modifications, so I recon the busbar was installed like this from new.

Note that this is not high current stuff. All the breakers on the affected busbar are 6A. Two alarms and three lighting circuits. Quite surprised at how the busbar has been eroded away. Somewhat amazed that the occupants had only recently noticed anything wrong.

[ElectriciansForums.net] Fault finding. Mind boggling

DPG · Jan 11, 2016

Great photos handysparks. Just shows what can happen even as you say at low currents. Daz

ruston · Jan 11, 2016

And a very clear picture of the reason for board fires.

R G · Jan 11, 2016

One thing you have to watch out for is the Wylex NSB breakers and also the equivalent crabtree and the Steeple breakers too , these were all subject to the Electrium recall and breakers should have been checked and sticker fixed on the board , I did quite a few of the recall visits and replaced loads of breakers , but I notice on many call outs with this problem that theres loads out there that didn't get replaced , where sparks have accounts at the wholesalers the use of these boards could be chased , but theres loads that had been bought for cash etc that hadn't ........