RCD tripping

22 Oct 2006
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United Kingdom
I have just finished my wiring in the new garage, so I put the 40A mcb in the house consumer unit on, then go down the garage and switch the rcd in the garage cu on, everything fine. Then I switch the 32A mcb on for the sockets and it trips the rcd in the house. I leave the 32A mcb and the rcd on in the garage, and I go back up and reset the rcd in the house, everything fine. As soon as I plug a radio into a socket in the garage and switch on, the rcd in the house trips again. Can anyone shed any light into what is going on?

By the way, the cu in the house is set up as a split load unit. The main switch is 100A double pole, and it feeds the lighting circuits and also the 30mA 80A rcd. This in turn feeds all the power circuits in the house and also now the garage.

Should the garage circuit be fed from an mcb straight off the main switch, and not from the rcd?

I have done the usual tests on the circuits before I switched on. There are no shorts to earth, insulation resistance is good, polarity is good etc.

Is it possible that the rcd in the garage cu could be bad? It was an old one that I had lying around, could it possibly cause this problem?
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In the house CU, have you connected the live for the garage side to an RCD protected MCB, and the neutral to the non RCD neutral bar?

Double check this.
I think you have possibly connected the neutral to the garage at the house CU to the non-rcd protected neutral bar or you have a fault somewhere in the garage that is causing a lack of discrimination between the 2 rcds ( are they both 30mA?) and the house one is operating first. I suggest you re-check ALL your connections and possibly put the sub-main to garage on the non-rcd side of the house CU (I predict another do we rcd protect buried cable discussion here!! ;) )
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Just checked, and you were spot on RF (and you ricycle :) ), neutral was on the non-rcd protected neutral bar. All sorted now, everything working as it should.
Schoolboy error, that will teach me for working this late. I am getting tired, I think I will pack it in for the night before I do any real damage.

Thanks a lot guys.
Ps. If you put the outbuilding feed on the non-rcd side & fit an RCD incomer in the outbuilding, it will save the inconvenience if it trips again, not to say that if it did, and you were away, you would stand to lose a lot of food...
Good thinking, but like ricicle said, do you not need it supplied from the rcd side of the house CU to protect the armoured cable that is buried underground?

There is already an rcd in the garage CU, it is a 40A 30mA unit.
As long as you've got RCD in garage CU you can connect the SWA supply cable to non RCD of main CU.
RCD is for protection against direct contact which will not occur on a properly earthed sheath on SWA.

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