RCBO: CU or in-line?

[Inchiquin]

Does an RCBO have to be mounted in the CU or can it go in-line somewhere, for example in a control box nearer to the circuits it's protecting?

Don't worry - I'm not planning to mess around with the CU myself but I'm at the planning stage for my bathroom and was intending to put a control box somewhere for LED dimmers, smart-home connections etc., and wondered if I could kill two birds with one stone by leaving enough space for an RCBO to protect all the bathroom circuits.

 

[EFLImpudence]

RCBOs can go in separate enclosures.

Depending on what you are going to install in the bathroom, you might not be able to have an RCBO for all the circuits.

 

[JohnW2]

Does an RCBO have to be mounted in the CU or can it go in-line somewhere, for example in a control box nearer to the circuits it's protecting?

An RCBO will protect the circuit downstream of it equally well regardless of where it is in the circuit.

HOWEVER, an RCBO remote from the CU would not (at least in some respects) protect the cable feeding it (i.e. between the CU and RCBO), so it is therefore probable that additional protection (of that cable) would be needed at the CU. In particular, if any of the cable between CU and RCBO were 'buried in a wall', it would need to be protected by an RCD (or RCBO) at the CU end, in which case the remote RCBO would become redundant.

Maybe I've missed something, but I'm not sure I can see much point in doing as you suggest.

Kind Regards, John

 

[Inchiquin]

an RCBO remote from the CU would not (at least in some respects) protect the cable feeding it (i.e. between the CU and RCBO), so it is therefore probable that additional protection (of that cable) would be needed at the CU. In particular, if any of the cable between CU and RCBO were 'buried in a wall', it would need to be protected by an RCD (or RCBO) at the CU end, in which case the remote RCBO would become redundant.

Those are fair points, thanks. I was thinking only about protecting what's inside the bathroom rather than the whole circuit back to the CU. The main reason for thinking about doing it this way is that I assumed it would be a lot cheaper to pick up an off-the-shelf RCBO for £12 (doesn't need to be high-power as there will only be an extractor fan and a few LED lights - probably no more than 100w in total), than to find one to match the Memera 2000 CU.

Alan

 

[JohnW2]

Those are fair points, thanks. I was thinking only about protecting what's inside the bathroom rather than the whole circuit back to the CU.

Yes, I realise that but, as aI pointed outr, the cable between CU and RCBO may well need protection (certainly if any of it is 'buried') and, as I implied, it would be a bit silly to also install a remote RCBO if you had had to install an RCD/RCBO in CU in order to protect the cable feeding it!

Would any of the cable between CU and RCBO be buried in walls?

The main reason for thinking about doing it this way is that I assumed it would be a lot cheaper to pick up an off-the-shelf RCBO for £12 (doesn't need to be high-power as there will only be an extractor fan and a few LED lights - probably no more than 100w in total), than to find one to match the Memera 2000 CU.

OK - but, quite apart from the above, if you are 're-doing your bathroom', I would have thought that the marginal extra cost of an RCBO for your CU would be pretty trivial in relation to the 'big picture'.

Also, given the very modest needs you speak of, why cannot you use an existing (presumably already 'protected') circuit?

Kind Regards, John

 

[Inchiquin]

Yes, I realise that but, as aI pointed outr, the cable between CU and RCBO may well need protection (certainly if any of it is 'buried') and, as I implied, it would be a bit silly to also install a remote RCBO if you had had to install an RCD/RCBO in CU in order to protect the cable feeding it!

Would any of the cable between CU and RCBO be buried in walls?
OK - but, quite apart from the above, if you are 're-doing your bathroom', I would have thought that the marginal extra cost of an RCBO for your CU would be pretty trivial in relation to the 'big picture'.

Also, given the very modest needs you speak of, why cannot you use an existing (presumably already 'protected') circuit?

All the circuits in my house are currently protected by MCBs in the CU. I thought an RCBO specifically for the bathroom would give extra protection in a watery environment. As for the cost, my bathroom renovation has already gone 150% over budget and I haven't started on the rest of the house yet!

It's not a big issue. I'm quite willing to pay a little bit extra, I was just exploring possibilities before finalising the plans. Currently the bathroom is just part of the upstairs lighting circuit but I have the option to utilise the redundant circuit that was used by the immersion heater, with the added advantage that the wiring already runs to the bathroom.

Thanks for your replies,
Alan

 

[flameport]

MCBs in the CU

Just MCBs? No RCD / RCBOs at all?

 

[JohnW2]

All the circuits in my house are currently protected by MCBs in the CU.

No RCDs at all?

Kind Regards, John

 

[plugwash]

If installing in a seperate enclosure i'd use a plain RCD rather than a RCBO, you have a MCB at the consumer unit already so overcurrent protection would be redundant and while RCBOs designed for "cables in, cables out" do exist they are a bit esoteric in the UK.

 

[winston1]

Why not just change the upstairs lighting MCB for a RCBO?

 

[ericmark]

Back around 1992 when RCD's were not required and finding a consumer unit to take RCD's never mind RCBO's was near impossible I fitted two RCD's outside the fuse box. However today there would be little point, the unit

is discontinued and even second hand cost around £60 each, so yes with the Memera 2000 either swap whole board or a RCD before or after the board.

Problem is RCD's can fail to work simply due to strain from the cables, so after fitting they need testing, the testers are expensive, so in real terms not a DIY job, so to fit one you need an electrician willing to do the work, to me this is a problem, as electricians need to follow BS 7671 even if it is not law.

 

[JohnW2]

... Problem is RCD's can fail to work simply due to strain from the cables ...

That's a new one, at least for me - do I take it that you have experienced that?

I must say that I would have expected it to take an awful lot of 'strain on the cables' to have an effect on the internal workings of an RCD (which usually have pretty rugged enclosures.

Kind Regards, John

 

[JohnD]

one to match the Memera 2000 CU.

I have a shelf full. How many do you need, and what rating?

 

[ericmark]

That's a new one, at least for me - do I take it that you have experienced that?

Yes, early days before electronics used I will admit, fitted RCD, tested it, failed, re-bent the supply cables so no strain on casing re-tested and passed. This was when no independent isolator so it was the SWA cores direct into the RCD, today normally a far more flexible cable from isolator to RCD so less likely to cause strain.

 

[Inchiquin]

Lots of useful information here!

Just to clarify everything I've said: there are no RCDs at present, just MCBs. I think the house was rewired in the 90s but it was a professional job so I saw no reason to change anything. I'm putting a main LED light and an 8w extractor fan in the bathroom plus a couple of RGB LED strips. My original idea was to build a control box into a stud wall, accessible from outside the bathroom, which would simply re-route the existing bathroom light circuit to the fan and the LED drivers, and house RGB dimmers as well as some smart-home cables in case I decide to install sensors later. This is why I wondered about including an RCBO in the box which would protect everything in the bathroom. I've no intention of messing with the CU so I would call in an electrician if that's necessary.

Alan