On 20/03/2019 08:30, PeterC wrote:
On Tue, 19 Mar 2019 23:12:14 +0000, John Rumm wrote:

On 19/03/2019 13:39, PeterC wrote:

The CU is a Wylex from the early '90s ('91 - 2 IIRC). All MCB, 30mA RCD on
rings, cooker, shower etc. and 100mA on lighting circuits.

Is this a TT earthed install? And does the 100mA RCD have a time delay
feature? (often denoted with a s suffix)

As I'd need 30mA on the lights for the fitting in the bathroom I thought of
replacing the two MCBs with miniature Wylex RCBOs, so it's 100mA - 30mA.
I rather like the mini-RCBOs as they are 2-pole.
Would this be a valid arrangement (assuming that the RCBOs will actually
fit!)?

Well it will work, however if the 100mA RCD is not a time delay types,
then in the event of an earth leakage trip, you may trip either one or
both of the RCDs. You may not get any selectivity or "discrimination"
between them.

Thank John and Adam. I haven't looked closely at the RCD but, as it's no
worse and it'll comply I'm happy with it.


BTW what now requires YOU to use a 30mA RCD for the bathroom lights? The
regs are not retrospective so swapping a bathroom light should not
require you to install RCD protection.

--
Adam