100mA RCD + RCBO

If the 100mA RCD is time delayed then this is correct.

However even if the 100mA RCD is not time delayed then you not made the situation worse and complied with the 17th edition on bathroom lighting circuits.

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

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.

Thanks Adam - I didn't know about that. Around 25 quid saved!

The regs are retrospective to any 'material alteration'

Whether fitting a new bathroom light is 'material alteration' is a moot point

Swapping a light fitting would be a like for like change and not an alteration to the fixed wiring. Its not notifiable under part P either - even in a bathroom.

Moving a light or fitting an additional one in a bathroom would require

I'll have to check but I presume that now means in all rooms under the

You could argue that it would be good practice to fit the RCD protection if the supplementary bonding is not present.

Probably... how much movement counts as moved I wonder?

Yup, and main bonding is there and up to current standards.

Interesting points, thanks. Sopplementary bonding: er, well, there's no way to do that as all of the water side is plastic and the incoming main is MDPE. CH is copper, but it seems to me that creating an earth path where there's no earth is pointless.

There's an old spur for the immersion heater. It was redundant so I used it for a couple of sockets in the loft. As it's on the 100mA RCD I've been thinking of using a double socket with integral RCD but, for about the same money I could use an RCBO. I must measure a spare MCB to see if there's space.

As a note it's worth mentioning that I spent most of a day bonding the metal sinks at a school the other day. The hot and cold pipes were already cross bonded and there was no way that a pd could be introduced to the sink.

Schools must have money to burn.

Bonding the metal sinks to what, the hot and cold pipes?

I guess they had an inspection and this was an arse covering exercise. H&S spending trumps teaching.