RCD tripping

12 Jul 2013
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United Kingdom
I have a 30mA RCD at my electric meter as well as a consumer unit.

I wired a spur from a mains ring for sockets yesterday with it disabled at the consumer unit and tested it was off with a multimeter at the socket I was wiring from - I checked every wire's voltage in relation to the others and all read 0. I tested the multimeter on a live circuit before this. The socket had 2 sets of live and neutral but only 1 earth which I thought was odd. I haven't traced all the wiring on the ring.

When I'd wired up the socket side of the T+E I went to cut the fused spur side and the electrical meter RCD tripped. Then later when I was inserting the stripped wires in to the metal box, it tripped again.

What would cause this?

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That happens when you short the neutral and earth together eg when cutting cables or catching the neutral on the backbox. Remember that the MCB only disconnects the line conductor and not the neutral.

Maybe you should post a photo of the socket in question. I'm curious about this missing CPC wire.
Okay great :).

Here is a picture from just before I added the spur, there are two neutral wires though it's hard to make out in this picture

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The RCBO RCD used in main consumer units monitors line and neutral but only switches the line wire. So switching off at the RCBO will only remove power it will not isolate the two live feeds (neutral is classed as live).

As a result switching off at the RCBO will not stop neutral - earth connections from tripping a main RCD which should be 3 times larger than the RCD built into the RBCO and to comply with fire requirement main RCD where supplying RBCO's would normally be between 100ma and 300ma not 30ma. The only place a 30ma RCD feeds another 30ma RCD is caravan sites and marinas.

Where the supply is TT then having two RCD's is common the main 100ma either in separate box or instead of main isolator in consumer unit and then smaller 30ma after that. Only where the consumer unit is metal do you normally have the main one is a separate box. However pre the RCD we had ELCB-v (A RCD is a ELCB-c) the ELCB-v was outlawed as two easy to be disabled but earth bonding these were normally on their own between consumer unit and meter not built into the consumer unit. Where latter a RCD is fitted it was common not to bother removing the ELCB-v but only use as an isolator. It should not trip with neutral - earth faults but I question why you have two 30ma RCD's fitted and wonder if there is something missed? A single 30ma RCD on incoming supply would not normally comply with 314.1 regulation.

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