RCD or not to RCD ... <sigh>

Feb 28, 2004 2 Replies
T

So,



Re wiring Dads garage.



Non RCD CU in the house, new (not by me) fused DP switch, Armoured into garage into another fused DP switch (after that it's all old and coming out).



So another Q here got me pointed to a neat little mini garage CU with RCD and twin MCD's (Screwfix, +AKM-24.99).



However, went to B&Q today (Screwfix's retail outlet) to get the plastic trunking (not so easy to post ) and the guy there suggested that you shouldnt 'string' RCD's *but* the external cable *should* be protected by a RCD (which it currently isn't)?



So what *do* I do here ..?



I unserstand that I could fit a 100mA RCD in the house CU (that I assume would be good enough to protect the external cable) but are they good enough to protect against a fatal shock / current?



The (older) guy did seem pretty clued up so is this all down to 'interpretation' ?



All the best ..



T i m


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U

I don't believe he's right. As long as you are using the house earth on the cable to the garage then the earth loop impedance will be low enough to trip an MCB (which is has) if there is a fault in the cable. The only need for an RCD to protect it would be if you want to protect people (not for a buried cable) or to protect the cable if you have high earth impedance (e.g. a local earth rod).

What you are proposing is fine, an MCB to pretect the whole garage installation from overload/fault and and RCD in the garage to protect you from electrocution.

You'd need a 100mA time delayed RCD to protect the cable, then a 30mA RCD in the garage to protect you. This would be the right way to do it if you want to use a local earth for the whole lot from house outwards.

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L

It all depends on your exact situation, I think he is covering himself by being over precautious.

I think by stringing the RCD's the B&Q "expert" was referring to discrimination. Basically if there are two 30mA RCD's in line only one will trip and it may not be the one that you want, eg a fault in the garage could trip the house RCD first. (usually that's the way it works anyway) ..

SJW A.C.S. Ltd.

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