Hello all,

I was called to a job where the client reported that the 3 phase mcb which supplied the D/B in their garage had tripped and would not reset. The supply to the garage is by way of a 4core SWA - and it turns out that one of the cores is down to earth. It is virtually impossible and not cost effective to run in a new cable.

There is very little load on the garage board and it is in perfect condition, with just 3 circuits and none are in constant use. My question is: can I turn this 3phase board into single phase, is there anything is the Regs that prohibits this etc?

I was intending to use a 3 tooth piece of insulated comb busbar to link out the main switch - I have never had to do this before and do not know if a conversion kit is available?

Thanks

Well,
the first thing I`d be asking is WHY is one of the cores down to earth?
Is whatever caused it likely to affect the other cores in time?
Will it definitely not do?

If, and only if, you are happy with your answers you might just use the remaining cores for 2 or 3 phase operation .

I take it that you are going to disconnect the bad core both ends and one good core both ends then use the three tooth comb on the incoming side of the DP switch? If so the very least you ought to do is make it so apparent as to warn the unwary future persons as to just what you have done in a belts and braces sort of way. Changing to two pole DP would be best if possible

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik

Thanks for the reply.

Yes I have asked the question of why a single core is down to earth -and I have not ruled out that there may be a joint somewhere along this run of cable. It's a difficult situation as I could spend a lot of time trying to find this fault and still may not solve the problem - and this would be hard to justify when charging the client.

Yes - I would definitely label the board up so it was apparent to anyone working on this installation what had been done.

Too be totally honest I dont like this solution - but other solutions are labour intensive and costly.

I think re using the faulty cable is a bad idea. I suspect the rest of the cable will fail in a few years.

Did someone put a garden fork through the cable? Can what has happened this time happen again? How can you be sure that one of the currently good cores hasn't been damaged e.g. Made into a bit of 1mm? Do you have any IR readings?

If you really must then an RCD in the feed to the swa might be a fraction safer.

If the cable has been "spiked" down one side with a fork or similar there is a fair chance the other cores may survive without a problem. I'd IR test it all ways round, L1,L2,L3,N,E to each other to confirm that there is just isolated damage to the one core.

Can't really see the logic in DP11's suggestion of an RCD in the feed to the SWA being a great benefit. If the cable goes pop in style it should clear the existing protection if the circuit is designed correctly.


Stu

Single-phase conversion kits are standard accessories for most brands of boards, e.g. see: http://www.tlc-direct.co.uk/Products/HGJK250SP.html
- Andy.

Cheers Andy - that is just what I need.

Thank you all for the replies.