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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> MICC cable to bathroom fitting. Tripping https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1487251159 Message started by beekeeper21 on Feb 16^th, 2017, 1:19pm

Title: MICC cable to bathroom fitting. Tripping
Post by beekeeper21 on Feb 16^th, 2017, 1:19pm

HI.

I have an ex local authority flat (built late 60's). Recently the mcb on the upstairs lighting circuit kept tripping. I traced it to the just beyond the bathroom light fitting. I had to scrape the plaster away to expose the problem. It is MICC cable coming into the two (brass?) pots. They are contained in a metal junction box and fixed in place with a metal saddle.

The issue seemed to be that one of the connections was touching the metal plate of the junction box. When moved it slightly away from the metal backplate, it would not trip (or spark), but pushing it back to the plate created a spark and tripped the circuit. I don’t see what could have ‘suddenly’ corroded. There has been no movement for at least 20 years. As I say, the fitting was buried in plaster (in the ceiling- all under artex). For now I've put some pvc sleeving around it just at the point between the brass pots and the orange sleeve where the bare copper is. Trying to trace the fitting back to the next junction would be really difficult. I'd like to solve the problem where it is. I'm tempted to insulate it with some WRMX sealant and then cover it back up. I'm aware that trying to get a MICC capable sparky with imperial fittings will be difficult. Would that be a reasonable solution? I’m surprised that it was ‘buried’ in plaster. I thought these cable connections would suck up the moisture of new plaster. Any advice gratefully received. Photo's attached.

[album]beekeeper21&1487254763[/album]

Title: Re: MICC cable to bathroom fitting. Tripping
Post by Lectrician on Feb 17^th, 2017, 6:42am

What you are suggesting is DANGEROUS, not just for that part of the lighting circuit, but the entire lighting circuit.

By moving it away from the metal box, you are breaking earth continuity (the safety conductor) and the fault can therefor not be cleared by the MCB. The fault is still there, but as your earth is then not connected, it cannot trip the MCB. The cable or metal box would also become live without the earth continuous!!!!!

You need to locate the fault with an insulation resistance tester, although it is likely a failed pot which requires a decent electrician to repair. I would say rewiring this section, after locating the faulty section, is the smartest move.

Title: Re: MICC cable to bathroom fitting. Tripping
Post by beekeeper21 on Feb 17^th, 2017, 2:29pm

Thanks so much for this Lectician. I'm glad I asked the question. I'm trying to find an electrician who will deal with pyro. As I say I'd like to deal with the problem where it is (and probably as you say it is a failed pot). In the meantime is there something I can do to isolate that particular light fitting and make that particular connection safe but still allow the lighting circuit to operate? I imagine it may take a while to get somebody out and I'd like to use the lights in the meantime.

Title: Re: MICC cable to bathroom fitting. Tripping
Post by Lectrician on Feb 18^th, 2017, 7:34am

The fault may not be were you have opened the ceiling. Thats were you have split the earth, disconnected the fault which is likely further down the cable somewhere.

You need someone who can test properly with an insulation resistance test meter to locate the fault.

Why do you suspect it is at this location? Does it only trip when you switch that light on, or at all times? With all other lights off, does it trip?

Title: Re: MICC cable to bathroom fitting. Tripping
Post by beekeeper21 on Feb 18^th, 2017, 11:38am

Thanks again. I've got a better sense of it now. I've never encountered MICC cable before so thought it was some strange dark art that had it's own set of rules. Having done some research and thanks to your advice I'm (a bit) more clued up now. If it hadn't been pyro cable I would have maybe made different judgements.
The bottom line is that I need to just isolate the whole circuit and live without upstairs lights until I get somebody round who can do the insulations test, find the fault and if necessary work with MICC.
Hopefully I've found somebody who will come on Monday.

I'm really grateful Lectrician, you may have saved me from a frazzle!