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I'm replacing a light in my hallway. Before starting, I turned the light on, turned off the lighting-ring breaker, confirmed the light went out, and then turned off the light-switch for good measure.

Once I had removed the conductors from the terminal-block, I started pulling the bare wires back through the housing of the old light. At this point, presumably two of the three wires touched, or shorted through the light housing, and the RCD for the whole property went off.

I don't understand why this happened. Could the Neutral conductor still have been energised, and that is what flipped the RCD? Or, could it be that the breaker for my lighting ring and the light-switch for this outlet are on the Neutral rather than the Live? Is this expected behaviour on a UK installation, or is it time for me to get my friendly local electrician in to investigate further?

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Shorting neutral to earth can be sufficient to trip a RCD, so there's a good chance that's what happened.

It's unlikely that the breaker or switch is on the neutral - though it is good practice to check that there is no live supply to the fitting before working on it, even with the breaker and switch turned off just in case it is fed from a different circuit.

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  • Thanks John. I had the idea of temporarily wiring a 13A socket to the lighting wires and plugging in my 13A Socket Tester. That proved that there were no cross-overs, and no missing legs (including no missing Neutral when the light-switch or breaker were off), so I'm happy it's not a wiring fault. This test together with your info all ties up.
    – dfluff
    Jan 1, 2016 at 9:35

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