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.


--
Cheers,

John.

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