RCD or not to RCD ... <sigh>


T

T i m

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 <g>) 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

usenet

T i m said:
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 <g>) 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)?
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.
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?
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

Lurch

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 <g>) 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)?
It all depends on your exact situation, I think he is covering himself
by being over precautious.
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' ?
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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