Opinions on all RCBOs vs Dual RCD

[rebuke]

I'm currently planning the replacement of my consumer unit. Before anybody asks, I've been in touch with the LABC, and I am going to be submitting notice to them next week before doing the work, and testing it all thoroughly myself (I have a full set of test kit) regardless of what the LABC do etc etc...

The circuits I have/will have are (ratings are based on load and I have verified that existing cable sizes are appropriate based on installation method and length etc using the tables in the wiring regs, I'm replacing the one cable that isn't sufficient at the moment):

Circuits that will be direct from the main switch regardless, either as they don't want to be RCD protected, or have a high likelihood of tripping:
Supply to consumer unit in brick outbuilding (40A RCBO)
Shower (40A RCBO)
Supply to computer servers (16A MCB)
Smoke alarms (6A MCB)
Burglar alarm (6A MCB)
Switched maintained emergency light by consumer unit (6A MCB)

Circuits that will either be RCBOs, or MCBs fed from an RCD:
Electric oven/hob (40A)
Downstairs ring main (32A)
Upstairs ring main (32A)
Immersion heater (16A)
Downstairs lighting (6A)
Upstairs lighting (6A)

What I'm trying to decide is whether to use a dual RCD board and split the last group up, or just get RCBOs. The cost difference isn't huge (about £40 more for the RCBO approach), so that isn't really a factor.

The obvious positive of the all RCBO approach is that one circuit tripping will not take out any other, but I'm worried as to how tricky it will make getting the cabling neat inside the consumer unit with all the extra neutrals (I'm currently looking at getting a contactum metal clad unit if anybody has any experience with those?).

Does anybody have any thoughts on this, in particular can anybody think of any other pros/cons to either approach?

 

[ccrowe]

Whichever way you choose to go, you should make sure that the upstairs and downstairs lighting circuits are protected independantly of each other. You don't want the whole house going dark. Ideally put upstairs ring final with downstairs lights and vice versa. Obviously if you go the all-RCBO route this won't be an issue.

While you're doing this it may be a good idea, if it's possible, to create a seperate circuit for your fridge/freezer. Not essential by any means but while you've got the chance...

 

[securespark]

Your supply to the servers...does it terminate in socket outlets <20A?

Is the wiring surface-mounted or protected by earthed metal?

Same Q about the smokes, BA panel & EM lt ftg?

Do you have shared neutrals on your lighting circuits? If so, the best approach would be to rectify this.

 

[rebuke]

Supply to servers will be going to a FCU, supplying a UPS via an IEC connector, so I believe 411.3.3 doesn't apply as it's not intended for general use. I don't want to put this on an RCD as I've had issues with UPSs on RCDs before. The servers are in the cupboard with the consumer unit, so the wiring will be going in earthed metal conduit directly from CU to the FCU.

Smoke alarms are ceiling mounted, the cable runs are above plasterboard ceilings to the actual alarms, buried in a wall at >50mm depth getting from the CU, and run inside surface trunking in a cupboard to get in to the loft. The manufacturers instructions say not to RCD these where possible, so I'm not (unless I've missed something that means they should be under the regs of course!)

BA panel will be another FCU in same location as servers. EM lighting fitting will be mounted next to the CU, with cabling run in earthed metal conduit again.

No shared/borrowed neutrals - I've traced both lighting circuits and they're wired up correctly.

I've thought about fridge/freezer socket, but only way I could do it easily would be to surface mount it which I don't like the idea of, as the kitchen is relatively new and nicely tiled, so don't want to trash them and have to re-tile it. It's a nice new efficient f/f so should cope for a reasonable period if the power goes out...

 

[GaryMo]

Your supply to the servers...does it terminate in socket outlets <20A?

If it does and is only to supply the server and nothing else (not for general use) and is suitably labelled as such then no RCD protection is needed (411.3.3(b)).

Obviously, as you say, RCD may still be needed due to the type and route of the cable supplying the possible socket-outlet which may not exceed 20A.

[EDIT] Position now clarified by Rebuke as I was typing this post.

 

[hairyben]

If it's at all possible, I'd try to get the immersion and cooker on non-RCD protected supplies, obviously if the wirings in and buried you have to go with RCD's but for a little bit of trunking in a cupboard it might be worth the bother.

Technically speaking, Dual RCD boards are non-compliant (reg 314.1) but they seem to be regarded as acceptable.

Personally I've yet to fit a dual RCD board, I've talked everyone into RCBO units, although I do offer the dual RCD with a price differential. On a small board, say <6 RCBO's, there's little cost difference (but a 25% smaller unit), on a larger installation I think there should be limits to how many circuits should be fed via a single RCD.

my way of thinking is, should there be an earth fault, I'd much rather have a customer ringing me about the loss of a specific circuit- something which may be a little inconvenience and likely fixed via simple instructions over the phone- than a customer ringing me hollering about half their house being out, with a) the complexity of explaining how a customer might remedy it and b) the urgency with which they'll want it fixed.

 

[rebuke]

The cooker has cables buried less than 50mm in a wall, so that has to be RCD'd (I can't put in conduit or whatever without destroying my kitchen tiling which I don't want to do).

I don't tend to use the immersion heater (it's just there in case the boiler dies), so I'd assumed an RCD would be a good idea since it's electricity and water etc. The wiring's run under the floor, or clipped direct in the cupboard itself, so it could be just an MCB...

I do prefer the all RCBO idea, but as stated I'm just concerned as to what it will mean in terms of making the cables neat in the CU, as obviously they'll take up a fair amount of space!

 

[hairyben]

I do prefer the all RCBO idea, but as stated I'm just concerned as to what it will mean in terms of making the cables neat in the CU, as obviously they'll take up a fair amount of space!

You're right RCBOs can be a cow for doing a neat job but they're the way forward. You'll gain space in not having two rcd's, in fact if you want to be fastidious you could use double width RCBO's.

The immersion I'd go MCB, I understand your thinking re water/electric but it's more of an inconvenience than a safety feature.

 

[securespark]

Your supply to the servers...does it terminate in socket outlets <20A?

If it does and is only to supply the server and nothing else (not for general use) and is suitably labelled as such then no RCD protection is needed (411.3.3(b)).

Fair comment that I omitted to mention.