"Christian McArdle" wrote
| 1. Whole system 30mA RCD
| 2. Split load 100mA time delay+30mA RCD
| 3. 100mA time delay + RCBOs
| The problem with system 1 is that it kills the lights in the event
| of an earth fault, which could be very dangerous, either due to
| rotating machinery or fire escape. Some would say that system
| type 1 is no longer allowed by the regulations, although it isn't
| very explicit about it, if I recall.


| Obviously, system 3 can be very expensive if many circuits must be
| 30mA protected. You would probably err on the side of protecting
| more circuits on a TT system, as the consequences of a nuisance
| trip on the non-RCD protected circuits are greater, as they would
| trip the time delay RCD and kill the lights.

An alternative could be to use a split load CU, with *all* the circuits off
the non-RCD main switch being RCBOs (say socket rings and lights), with the
RCD side being used for items where discrimination is less essential, eg
cooker, shower, immersion heater, radials to wall heaters, etc. (There would
be no whole-house RCD at all.)

That would provide the advantages of system 3 on power and lighting
circuits, which are more likely to suffer high quiescent residual current
and also form the greatest danger/inconvenience in the event of a trip,
whilst reducing the number of RCBOs required. If the washing machine and
dishwasher are wired to radials on the RCD side, a separate kitchen ring
might not be required, so most houses would be sufficed by 2 socket rings
and 2 lighting radials only requiring 4 RCBOs.

The downside is that most split-loads only have about 60A available through
the RCD.

Owain