My guess would be if three are 3 other sub rcd in the house with a 30ma
sensitivity and a 11 ma leakage each then the main rcd would always trip.
only a guess but plausible.


Andrew

John Wakefield wrote:
I've read that current practice is to use several RCDs in a domestic
installation. A time delayed 100mA RCD to protect the whole installation and
30mA RCDs for sockets, outside wiring etc. My question is why can't the main
RCD be set at a safer 30mA sensitivity since it's time delay should prevent
it from tripping before any other ones?
Also if you install a garage unit with a 30mA RCD on an installation which
is protected by another 30mA RCD at the CU, which one trips if an earth
fault occurs?