DIY Doctor

Flak jacket = ON

Postby Devad » Mon Jun 02, 2008 8:15 pm

Ok, first my mitigating circumstances...

1) I have fully rewired a previous house I owned (before part p and all that)
2) I have a good understanding of electrics

1) I am not a qualified leccy
2) I have no real gear apart from a decent multimeter

now to confess...

I have recently replaced an old consumer unit with a split load (RCD 30mA), the unit being fed via a 100mA RCD (ie all circuits protected to 100mA, some now protected to 30mA). I do intend to get my electrics fully tested as I'm getting some new circuits added for various reasons (this should happen within the next 3 months).

So I have donned my jacket and am ready to take your flak however a few serious questions first -

1) I know that a comprehensive amount of testing should be carried out (and will get that done) however is it not fair to say that I am no worse off with the new CU and in fact from a safety viewpoint should be better off as at least now I have all circuits on mcbs, and some on 30mA RCD?

2) Under what circumstances would I now be worse off (from a safety viewpoint) than before I replaced the old CU (given that it has not been fully tested)

3) There is an earth wire running along the incoming mains cable (I'm assuming that this suggests the supply is TN-S) however the wire is 10mm can this be upgraded (ie by a leccy, not me)


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Simply Build It

Postby ericmark » Tue Jun 03, 2008 10:35 am

Forgetting Part P and problems in selling your house.
Even us qualified electricians make mistakes, so many times I here "I can't believe I did that." even to getting neutral and Line swapped. It is the testing that shows up the silly mistakes. You could have by accident for example turned a ring main into two radials and then an large overload could result. I would agree that earth leakage trips make things safer but only if they work the button on the trip it's self only checks the mechanics of the trip you need expensive meters to test trip. I will admit years ago we did very little testing if it worked it was OK but this means there could be existing faults in your house and in the main it is not one fault that causes a problem but a number of faults all added up.
As to 10mm earth that is ample and as to TN-S well in which case you would have not required the 100ma RCD here we have an insurance problem with things like a power failure and loss of food in fridge etc. If it is required then it would be covered but if not required it becomes an undeclared risk.
With the new regulations requiring 30ma RCD's for nearly all circuits I can't see that being a problem any more but it may be.
There is also a small chance that the earth loop impedance or prospective short circuit current are too low or high to use MCB's and in your area you need fuses very unlikely but without a test you don't know.
So although it is likely you now have a safer system it is not automatically the case and it should be tested.
One problem faced by many DIY'ers is that post event only a periodic inspection and test can be done only the person doing the installation can sign an installation certificate and you must be able to show you have the knowledge required to sign it. And more and more the certificates are being requested when selling houses.

All best Eric

Postby jardov » Fri Jun 06, 2008 10:24 am

Thanks Eric for a considered (as ever) reply - sorry for the delay but been away for a few days.
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