RCBO

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If both of the 'borrowed' circuits are dead there will not be an imbalance. As soon as current flows then one or the other (maybe both) will trip.

Unless there is an Neutral-Earth fault taking some of the neutral current from active circuits. The circuit for this is from the neutral bar, ""backwards"" through the RCBO sensor onto the neutral of the dead circuit, through the neutral-earth fault on the dead circuit and then via the CPC to the neutral at the cut out where CPC and neutral are connected.
 
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DAJ

Checked with meter today and it seems there IS a neutral/earth fault on the lighting circuit. Must've been like it for years 'n' years!

Seems my "borrowed neutral" theory has just been blown out of the water! Rectification might be just as lengthy as tracing the fault!
 
D

DAJ

THANKS GUYS - Took many hours of testing but now all done! The internal wiring in an old brass wall light fitting had overheated causing a naked neutral to earth. All internal wiring removed and replaced with heat resistant wire, lower (15w) wattage bulbs fitted; recommended 25w max too high in my opinion.
 

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