Consumer unit RCD oddity

Jeff Layman · 2015-12-29T11:05:41+00:00

We have a 3-years old split-load consumer unit, with a separate RCD for each half. I had to replace 3 old wall-light fittings with some new ones yesterday. I switched off the CU lighting circuit MCB, replaced the first fitting, and switched on the circuit to check the new one was ok. No problems, so changed the second one. No problems again. The third one needed a bit of fiddling, as the hole the cable exited from wasn't central, and the cable had to be moved a bit (the light fitting fixture diameter was about 10mm less than the one it replaced). So I removed about 5mm of plaster from the right side of the hole, moved the cable over, and used some filler on the left side . Of course, some plaster bits went where they weren't supposed to, and I went to get the vacuum cleaner before The Management returned from shopping. It was then I noticed that all the power was off. On checking, I found that /both/ the CU RCDs had tripped. I reset them without problem, and finished the third wall-light. It worked without problem too. Anyone got any idea why /both/ RCDs tripped? I was working on the lighting circuit only, and not even using any power tools. I checked with neighbours, and none of them had any problem with CU RCDs tripping.