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Electrical

Why would RCD trip when I cut a wire on a circuit where MCB is switched off?

Anonymous user 09/03/2024 - 2.45 PM

I'm changing a single socket for a double socket in the garage, and I needed to shorten the cable a little. The RCBO for the garage sockets was OFF in the garage consumer unit. However when I cut the cable, the RCD in the main house tripped (but NOT the garage RCBO). How can this happen? If the RCBO is off, the only wire still connected to the house is Earth. How can cutting the earth (or shorting it to a switched off circuit) trip the RCD? Could it just be a tiny amount of current induced in the circuit from perhaps where the feed for the socket, runs adjacent to the feed for the lighting circuit which was still switched on?

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2 Answers

Electrical Safety Services

Rating: 5 out of 5
Dereham
When you cut the wire you create a neutral to earth fault so the RCD trips. Both neutral and earth are still connected when fuse is removed or MCB is in the off position.
Answered8 January 2017
21

DEACON ELECTRICAL SERVICES

Rating: 5 out of 5
Grimsby
Because you cut through the neutral also! An RCBO or RCD sense an imbalance the RCBO is not double pole. An RCD is double pole, had you switched the main switch off in the garage consumer unit which hopefully you have got! is DP, you could have isolated the garage completely you then could have cut it.( safely) never take anything for granted with electrical work, did you have a voltage tester? By the way, you should not have 2 or more 30ma RCBO's or RCD's on the same circuit as they conflict. You should have the garage circuit Earth protection at the consumer unit,RCD or ideally an RCBO with a consumer unit with just mcbs in the garage or a MCB in the consumer unit and an RCD in a consumer unit in the garage. An electrician would be able to tell you all this on a visit, which is what you need as your current set up is wrong if how you explain it is correct.
Answered18 December 2016
8