"John Wakefield" wrote in message
...
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?
Time delay 30ma is a rare beast & it's possible these days for there to be
enough cummulative (but normal) leakage in a system to make it a nuisance.
Whole installation RCDs are to protect against fire not human contact.

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?
Indeterminate.