In article . com,
wrote:
Just to satisfy my remaining curiosity, I take it an RCD only trips
when power from it's own circuit goes to earth (as opposed to my test
which was live from the lighting circuit to socket earth).

An RCD looks for an unbalanced current between neutral and line - which of
course may mean a 'short' to earth. But it will also protect on a two wire
circuit with no earth.

However, if you have a split load CU and the lighting circuit is on the
non RCD side running a lighting load to an RCD protected circuit earth
won't cause it to trip.

--
*Did you ever notice when you blow in a dog's face he gets mad at you? *

Dave Plowman London SW
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