Split Board General Question

firebladerrw · 2 Jun 2010

Whilst I understand the concept of split boards ie, RCD protected or not, could anyone enlighten me for a general house application, on what circuits normally would be protected by the RCD and what wouldnt? I know that 'mission critical circuits' shouldnt go via the RCD (fire alarms etc), but just wonder what is the genreal recommendation?

ie: downstairs & upstairs lighting circuit (Downstairs with outside lights)
downstairs & upstairs rings
Imersion heater
Cooker
Heating

etc etc

Lastly, are there any specific design issues when selecting a CU/RCD for a house with PME installation?

Thanks in advance

LoveRocket · 2 Jun 2010

Under the latest edition of the regs (17th), pretty much everything must be RCD protected. This is mainly due to the requirement that cables buried in plaster less than 50mm from the surface require RCD protection (unless they are in earthed steel conduit, or SWA, Pyro etc).

You split board is not unusable though: Put sockets, cookers, showers etc on MCBs on the RCD side, then put lights, alarms, boilers, on RCBOs on the other side. (RCBOs can be a bit pricey though!)

ericmark · 2 Jun 2010

My house everything is RCD protected.
Items not RCD protected would be:-

- non battery backed fire alarms and these would be wired in Ali-tube cable. Unlikely to use non battery backed in a house. Not even sure if permitted? To me to rely on wiring to remain intact when a house is on fire seems to have a flaw?

- Cooker where because the elements are mineral insulated they can absorb moisture, however because Ali-tube cable is not easy available, so it would likely need SWA feed cables, I would say not worth the problems involved in supplying without a RCD, and very few cookers have problems today. Only exception seems to be multi fuel types not sure why will guess they are not sold as much and held in stock longer!

- Fridge/Freezer again would need Ali-tube cable supply, although I have never lost contains due to RCD tripping. Insurance companies have in the past quibbled at lost due to RCD tripping, now it is in regulations can't see they can still object? Protecting with an RCD is something which has been talked about a lot. If on it's own RCD (RBCO) less likely to trip, but you may miss it if it does. Selecting to be with rest of house or independent is something electrician can't agree on. However if really worried then UPS is answer as even a MCB can trip.

- Extra low voltage equipment. This seems to be an error in BS7671:2008 as specially with separated extra low voltage (12v) can't see how a RCD can possibly protect it? So I would still run door bell, LAN, Telephone etc without RCD protection.

STI · 2 Jun 2010

Bathroom circuits must now be RCD protected, which means, if sticking to the letter of the law, the lights must be. As most people i guess,dont have a separate light circuit for the bathroom it will mean all lights on the same circuit as the bathroom will be RCD protected. Personally if possible and you have two light circuits minimum i would not put the other on the RCD side. At least then you can have some light to find your way around if the RCD trips.

ericmark · 2 Jun 2010

STI said:
Bathroom circuits must now be RCD protected, which means, if sticking to the letter of the law, the lights must be. As most people i guess,dont have a separate light circuit for the bathroom it will mean all lights on the same circuit as the bathroom will be RCD protected. Personally if possible and you have two light circuits minimum i would not put the other on the RCD side. At least then you can have some light to find your way around if the RCD trips.

I would agree lights in bathrooms are now RCD protected but since not to RCD protect the rest would mean running with Ali-tube cables to lights if buried to not protect any lighting circuit with a RCD in practice would be hard.

To me this leaves a problem that unless street lights etc mean one will get enough light not to be in danger should the lighting circuits trip you have two options.

1) Supply lights with RCBO's so likelihood of tripping is at a min.
2) Install some battery backed lighting.

I have the latter with box holding batteries and florescent inverter in loft and a fluorescent tube in a small fitting (minus all lamp gear) above stairs lighting the stairs and landing and through glass at top of all doors into all upstairs rooms. Open stair case give enough light in living room to reach front door and garage has it's own emergency lamp to see consumer unit.

Before I fitted emergency lamp in garage I had a touch in charger which would auto switch on in case of a power cut but if only lights tripped and not RCD then it of course did not work.

Since my stairs are central in the house to rely on outside light at night I don't consider an option. However with stairs well lit from outside this may not be required. It is all down to a risk assessment.

Son has best 400AH of 12v batteries and all 12v lighting. Well even all 230vac power comes from an inverter and that is powered by the batteries so power cuts are rare!

securespark · 2 Jun 2010

If you just want to RCD bathroom lighting and no other, then use the lighting loop into the bathroom to feed an RCD spur, with the bathroom lighting fed from the outgoing terminals. But the spur needs to be accessible.

LoveRocket · 2 Jun 2010

When the 17th was released I was vehemently opposed to the new RCD requirements, as it meant - in effect - lighting circuits needed RCD protection. However, I have now worked out that it's not a problem that lights are RCD protected, it's a problem that lights are supplied from the same RCD as other circuits. In my opinion, designing an installation such that a fault on a socket or shower or whatever circuit causes the lights to go out doesn't meet the regs (can't think of the reg number, but "a fault on one circuit shall not affect another" is the jist). I don't see how a "17th edition CU" can be installed in good faith, as it surely doesn't meet this requirement. I wish the IET had gone further and explicitly stated that a fault on ANY other circuit must NOT affect a lighting circuit.

So, so summarise my ramblings:
RCD protection for lighting circuits = good. It could save a life if someone drills through a lighting cable
but
Lights should be on their own RCBO - not sharing an RCD with half a dozen other circuits.

Goodnight, and god bless.

AndyPRK · 2 Jun 2010

Another option is to put downstairs skts, cooker, immersion(backup only?) and upstairs lights on one RCD.

And upstairs skts and downstairs lights, boiler on the other RCD.

This way you will always have some sort of power in every room and always have hot water available.

LoveRocket · 2 Jun 2010

AndyPRK said:
Another option is to put downstairs skts, cooker, immersion(backup only?) and upstairs lights on one RCD.

And upstairs skts and downstairs lights, boiler on the other RCD.

This way you will always have some sort of power in every room and always have hot water available.

That's what a lot of sparks do, but designing an installation so that a fault on something in a bedroom plunges downstairs in to darkness? Yeeee-haw!

AndyPRK · 2 Jun 2010

The downstairs TV will still be on

and any table lamps.

LoveRocket · 2 Jun 2010

AndyPRK said:
The downstairs TV will still be on and any table lamps.

Good thinking! Design it so that the TV amplifier goes off with the lights, ensuring plenty of static on the telly to illuminate the living room.

plugwash · 2 Jun 2010

Split load boards in general whether the old 16th boards or the new 17th ones have IMO always been skirting the very edge of acceptability.

LoveRocket · 2 Jun 2010

plugwash said:
Split load boards in general whether the old 16th boards or the new 17th ones have IMO always been skirting the very edge of acceptability.

A "16th Edition" split-load CU skirted towards the edge, but stayed on the correct side owing to the small amount of circuits needing RCD protection.
"17th Edition" dual-RCD installs (with nowt else) go head-first over the edge of acceptability.

IMHO.

firebladerrw · 2 Jun 2010

Gosh worm can well & truly opened?

TBH, I anticipated a mixed reply on this, and it looks as though there is a balance between common sense, and taking regulations to the nth degree.

I think that every post on this has some good points, and as ever with regulation, its how its interpreted.

In engineering, and in particular safety case work, there is a good methodology used to determine risk of failure, or injury - ALARP analysis. (As Low As Reasonably Possible)

For a risk to be ALARP it must be possible to demonstrate that the cost involved in reducing the risk further would be grossly disproportionate to the benefit gained. The ALARP principle arises from the fact that infinite time, effort and money could be spent on the attempt of reducing a risk to zero.

In other words, if you had limitless funds to rip apart an existing installation, and install every safety device & measure known to man, then you could make it extremely safe. However, on the other hand, common sense and funding dictates that this isn't really practical, and spending more on an installation would only marginally decrease the risk factor. Of course installing lots of super sensitive safety devices would probably render the installation unreliable & costly as well.

So I guess, there is a balance to be had here between strict adherence with regulation and practicality !

1john · 3 Jun 2010

Domestically there are three or four 'off the shelf' options

1 - dual rcd board, two banks of circuits on seperate RCD's
2 - dual RCD board with additional non protected ways for use with MCB's or RCBO's
3 - Split load, two 'banks, one RCD protected, the other non, for use with MCB's and RCBO's
4 - Non protected board installed with RCBO's and MCB's

1 being the basic bare minimum where as a fault to earth would leave you without half of an installation. 4 would leave only the circuit at fault off.

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Split Board General Question

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