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Problems with RCB wiring - solved!

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Simon036219/02/2019 22:19:53
219 forum posts
77 photos

This is intended as an aid to others who may have similar problems and are looking for inspiration...I have an electronics background and a reasonable experience in house electrics, but no expert by a long way. To anyone who thought the solution was obvious, I admire your intellectual abilities, to those others, I hope it may help finding an obscure fault.


When we moved to our current house in France some 3 years ago, I had the garage packed to the brim with workshop stuff, piled in with no chance to sort on delivery. Once the moving dust had settled, I started attempting to sort things out in the garage and fairly early on, needed power and light from the sockets placed on both sides of the building. The pair that were situated directly under the garage distribution box worked perfectly. Not so those on the other side of the garage floor that instantly pulled the house RCB’s as soon as something was plugged in and turned on. Due to the requirements of time spent elsewhere it was only last weekend that I had a serious delve into the problems, having managed with a series of extension cables up until this point.


The source power for the garage is single phase and comes via a ‘power’ RCB and a ‘lighting’ RCB both placed in the main consumer box in the house. All of our circuits are either lighting or power and are entirely separate.

The garage lighting and power supplies come through a separate consumer box and have separate trips. There is a further lighting complication since the 3 way switching uses a relay in the box to turn the lights on and off, adding to the wiring.

The cabling is single core, Red, Black, Blue (N) and Green (E) inserted into plastic conduit which is concealed behind tongue and groove covering the roof insulation, so completely invisible…

All Earths and all Neutrals are ganged together in the garage consumer box except those on the 2 functioning sockets.


Plug any device into the socket and power up results in RCB trips going off. This is the case for both earthed and two pin equipment.


  • Removed socket earth – no change
  • Socket L-N = 230V
  • Socket L-E = 228V
  • E-N =2V at socket and all the way back to the main consumer unit
  • Disconnect power trip, socket L-N = 140V

No obvious other shorting faults.

Simon036219/02/2019 22:20:32
219 forum posts
77 photos

(Part 2, posting too long...)


Part 1 – removed neutrals on by one from the bus bar in the consumer box. Equipment still causes a RCB fail but now ONLY the power RCB associated with one of the neutral wires (assumed to be that from the faulty sockets).

Part 2 – linked that neutral wire to the functioning power socket neutral – Bingo, all works perfectly.


The previous owner had installed the sockets or had rewired them for some reason and had failed to notice that he had used a neutral associated with the lighting circuit rather than that of the power circuit. The RCB balances current out on the Live with that returning on the Neutral so in the power RCB, since there was never anything returned on the neutral, it tripped the switch. Similarly on the lighting RCB, this suddenly saw a surge of current on the neutral side unbalanced by any outbound Live side and also tripped.


RCBs that have unknown wiring or that you have just installed and don’t work – ensure that the Live and Neutral are on the same circuit.

And there goes a Saturday afternoon…

Emgee19/02/2019 22:56:12
2426 forum posts
290 photos

Simon, I think the moral is to keep with 1 supply to the garage with the required consumer unit to distribute power and lighting.


Mike Poole20/02/2019 07:01:54
3339 forum posts
73 photos

Remember to keep the neutrals on the correct neutral bar on a split load consumer unit or it will trip the RCD.


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