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Topic Title: RCD trips when bits of the lighting circuit are used
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Created On: 24 November 2012 07:18 PM
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 24 November 2012 07:18 PM
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Posts: 790
Joined: 16 November 2006

Further to the post below about a lighting circuit taking out an RCD, i have a similar fault.

Lighing circuit fed from a CU with single up front 30mA RCD (so no errors with wrong neautral bars!).

Three points on this circuit 1-kitchen, 2-dining room, 3- outside light.

customer calls to say that the RCD has tripped and thw lights are out. with all switches off, the RCD resets OK.

With kitchen light on RCD is fine. Turn on dining room or outside light and RCD trips. Remove lamps from dining room light fitting and outside light and RCD stays put even if switches are on.

IR test shows a N-E fault (0.03M Ohms) somewhere after the kitchen light.

There's a mystery JB somewhere that i need to locate as the lives and neutrals are not looped to the fittings or switches.

My query is; why will the kitchen light stay on? Presumable removing the lamps downstream of the kitchen light stops the current flowing in the N and leaking to earth.?

Is my thinking correct here?


 25 November 2012 08:43 AM
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Posts: 4616
Joined: 21 November 2008

Yes that right, the neutral to earth fault does no cause an imbalance until a load is applied if the neutral and erth are at the same potential, i.e. a TNCS system. The low IR reading will be between the kitchen and the second light

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