Hello,

I'd be very grateful of some advice / guidance with respect to time delay RCDs. I've Googled, but not found
any definitive answers.


At present, the major wiring is as follows:

Meter Tails feed 100A Master Switch. This connects to a Henley Block, which feeds a 60A Switch Fuse, and the
house CU. The 60A SF is then connected via 16mm^2 SWA out to a garage CU, which is earthed separately. The
garage CU is a Split Load, with a 50mA time delay RCD for lights and freezer, and a 30mA RCD for the sockets.

Shortly, an additional SF will be added, feeding a summerhouse CU via 4mm^2 SWA, which will also be earthed
separately, and will have all its circuits protected by a 30mA RCD.

The new SWA will be very much more exposed thn the existing run, being laid along the edges of flower beds
etc, and it seemed a good idea to provide some form RCD protection to it. It also appealed to add some form
of RCD protection to the entire installation, so a good solution seemed to be to replace the 100A master
switch with a 100A 100mA time delay RCD. I am however concerned about discrimination between this, and the
50mA time delay RCD in the garage. Clearly, it will always discriminate based upon current, but will a 100mA
RCD also have a larger time delay built in? Are there standard delays associated with each value of leakage
current detection, obeyed by all manufacturers? Do I need to look for a 'extra time delayed' RCD?

I'm also concerned about nuisance tripping of this master RCD. Is a 100mA time delay RCD reasonably immune
from tripping under circumstances where there really isn't a problem?

Many thanks for any advice,

Chris Key