On Sun, 19 Oct 2003 09:23:30 +0100, John Wakefield wrote:

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?

Because 30mA isn't very much leakage current, modern appliances with
SMPSUs and other things with suppression components "leak" to earth
quite a bit. ISTR that you shouldn't have more than 10 such
appliancies on a ordinary 30mA RCD because of the leakage level
causing nusiance trips.

Only certain types of installation need a 100mA time delayed whole
house RCD. An 100mA delayed RCD is required when the earth loop
impedance is (or maybe) to high to allow large enough fault currents
to flow enabling the protective devices (fuses or MCBs) to disconnect
in the required times.

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?

Already been said "indeterminate". But the garage should be fed from
it's own MCB and RCD at the CU not just be part of one of the houses
RCD protected ring mains.

--
Cheers
Dave. pam is missing e-mail