On Tuesday, 3 July 2018 12:19:41 UTC+1, John Rumm wrote:
On 03/07/2018 00:23, tabbypurr wrote:

OK, here's where things are so far...

TT & metal CU means a 100mA S RCD - can that go in the CU or must it
be external to protect against a tail to case short? If external can
it be in plastic or must be metal? And if metal does it need another
before it to...

You may be able to get an extra insulation kit for TT installs to allow
them to be done in a metal enclosure.

Rented rooms can all go on one DP S RCD, with one SP RCBO then
feeding each meter, which feeds each socket circuit which feeds each
lighting circuit via a 5A fused FCU. The purpose of the SP RCBOs is
to give discrimination against L-E leakage trips.

A SP RCBO will discriminate on L E faults, but not N & E faults as
previously discussed. Since you are feeding both lights and sockets
from the same circuit, then a normal DP RCD and MCBs would be adequate
and also protect against tripping the head end RCD on a NE fault.

Yes, but that isn't wanted. The expense of multiple DP MCBs is unwelcome, and the lack of discrimination of a single DP RCD is also not wanted. Hence the plan to split the protection: L-E discriminating, N-E not.


Communal areas I take it must be fed from 2 RCDs not 1 because losing
lighting could be an issue due to cooking being potentially
hazardous.

They also want to be independent of the rooms, so that you can "escape"
to a light communal area, if a trip takes out all power to a room.

Fire alarm circuit requires its own MCB or SP RCBO: does it need its
own separate DP RCD?

You don't want any other fault being able to disable it.

That takes the count up to a minimum of 4 main RCDs, effectively a 4 way split CU. Not going to be cheap.


NT