Foil backed plasterboard causing fault.

'tripping the lighting circuit' ?

Do you have light switches with metal backboxes in the foil-backed p/b ?

Mr. HandyAndy - Really

 

I think of the Emma Shaw court case which started 2007 and was only finished this year OK today we have RCD protection so could not happen that way but the report on how the fault had blown clear and in doing so lost earth completely and then she was killed a year latter after a water leak makes me say the fault has to be found.
With consideration of the above even I would be testing every switch earth connection to ensure no earths have been lost during the event.
As to what to do if you can't find it at the switches that becomes hard. But I am sure you don't want to have to stand where Mr Hoult stood and be blamed for a death.

 

Hi Ian

Very difficult one. Guessing with it being light the probability is on a drop so that will probably be a switch. Can you test back from the switches to earth/foil? Drop the switch plates off and test to see if you can detect anything? Th only other option I can think of would be to disconnect at the roses and see if you can isolate to a particular cable giving a lower reading.

Hope that helps

Kind regards

BS

 

When you're IR testing, what are you testing between?

Can you connect to the foil back?

 

Hi Ian.

I'm not a sparky, so the test readings you gave in your first post mean little to me - ie: I know what the 'terms' are, but not if the actual readings are good or bad.

So, standing back and looking at this as a layman.

You give two possible scenarios in your first post. (2) Induced voltage large enough to trip the MCB and go seriously 'bang'? No, surely next to impossible. The foil is in large, flat sheets as opposed to loops or coils of any sort; any voltages induced would be tiny. Large flat sheets do bring to mind 'capacitance', but even then the 'other' plate is a cable,which cannot be that close, so capacitance effect also would be teeny, tiny.

In any event, if induction or capacitance were possible scenarios, there would be exploding walls all over the country.

So to (1). I'm guessing that, if the foil backing were 'live', then the 'bang' was caused by the earthed drill bit? Ie: the drill bit/screw didn't cut through an actual cable and short two wires therein - it touched only a single conductor (the foil) so that foil must have been 'live'?

No (1) strikes me as being the only likely scenario, in which case thank your lucky stars that this plumber happened to accidentally locate the problem - or else the potential for the customer to locate it themselves in the future using a screw and screwdriver would have been very high; imagine living with a 'live' wall?

I can only think of one likely way this could have happened - and it's what you say yourself; a plasterboard screw nipped the live wire and turned the whole sheet live too. And it's probably also very likely that the foil was also 'blown' around that screw as you say, breaking that bond afterwards. But, it'll be close.

(Or, the reverse happened - the plumber's drill was live and he earthed it through the foil... Nah - not likely!)

How else could that foil have become live? Via a metal back-box? Via some weird cross-wiring in a socket/switch plate? Hmmm, surely it's almost certainly a p'board screw?

If the foil in the area of 'bang' is really isolated from the rest of the house's foil, it might just have to be a case of rip the darned thing off in that porch area?

 

Suggest contacting Kingspan Tec Support for advice, as they may well have come across before.
And you'll be giving them valuable feedback if not
Tel: +44 (0) 1544 387 382
Fax: +44 (0) 1544 387 482
Email: technical@kingspaninsulation.co.uk

Your approval body eg NIC etc may have some useful advice too of course.

 

retest your IR between Line and Foil with the switches in the Closed Position. That should give you and indication if the fault in on the Loops. Retest then with each switch drop closed one at a time. This will give you a clue to the room you need to be investigating. Did you carry out the IR with the Lamps removed?

 

Have you removed the area behind the black screw? That would be my first check. H e must surely have hit a cable with the screw. Keep your focus on that bit first before you go around removing all your boxes.

 

I think Ian has said that they exposed the area where the screw went 'bang'. "When he took of radiator one of the screws was black so he thought that he had hit a cable behind the wall. He took a patch of plasterboard off but there was nothing behind but the screw had arced of the foil tripping the lighting circuit.No sign of cables, but blackened"

From what he says, the screw arced across to the foil itself. He also says that the foil-backed board in that area (and where the fuse box is located) is very likely 'isolated' from the rest of the house's wallboards, so it would seem the worst they'd have to do is rip off a couple of sheets of board for a looksee.

I hope he comes back to tell us what was found

 

This is an interesting case and it does show how Ali-tube may in the future become the norm rather than twin and earth. From the IET report on the Emma Shaw court case that also caused the fault to clear it's self after the plasters screw hit the cable. It was only when the plaster got wet when the fault was found again. However they also stated that the earth was lost so doing a test and inspection should have high lighted the fault. With that in mind I would be testing the ELI at all accessible points in the hope of finding which cable is involved.

 

Me too DA!

 

I don't believe so - Providing RCD protection was their attempt to address this type of fault.