RCD Protection for outside circuit

[Ian]

I have a question on how I should protect a circuit to some outhouses.

In case it makes any difference, the outhouses are physically attached to the main house in an 'offshot' arrangement - they are the old outside loo and coal shed).

I would like to get some power to the outhouses. This would be to:

- provide lights inside the outhouses (I'm going to use the coal shed as a storage cupboard)

- supply the power for a couple of outside lights

- power an outside socket to be placed on one of the walls of the outhouse.

Fortunately, there is already a cable routed from indside my house at a point close to the consumer unit to the outhouse, but unconnected (at both ends!). I understand from the previous owner that he put this in prior to having some work done in the house that would have made routing it more difficult, but he never got round to connecting anything up.

Anyway, the cable is modern 2.5mm T&E so that gives me the basis of a

20A radial circuit. My plan is to use a junction box to spur off to the socket and to a 5A FCU for a lighting circuit that will comprise the lights inside the outhouses and the exterior lights (note: these *aren't* going to be the high-wattage security types).

I'd like some advice on the best way to protect the circuit. The consumer unit is a relatively modern Wylex with MCBs and the whole thing is RCD protected (I gather this is not considered a great arrangement these days). I understand it was installed when the bathroom was refurbished and a shower was installed.

I believe I need a 30 mA RCD to protect the outside socket but I am not sure what would be considered the best way to provide this:

- use an RCD in the consumer unit. (But I don't see a 20A one in the Wylex range at TLC, for example.)

- use an RCD connection unit to protect the socket spur in the outhouse.

- use an RCD connection unit to protect the whole of the outside circuit. Presumably I could install this near to the CU inside my house rather than in the outhouse?

I have also come across 'RCBO's. I understand that these are a combination of RCD and MCB but I don't really understand what that means - e.g. if I installed a 32A RCD in my consumer unit wouldn't that break the circuit when the current exceeds 32A? So what does the MCB element of a RCBO give in addition? This may be moot anyway as there don't seem to be any RCBO's available for the Wylex CU.

Thanks for any help.

-- Ian

[Owain]

"Ian" wrote | I have a question on how I should protect a circuit to [attached shed]. | I would like to get some power to the outhouses. This would be to: | - provide lights inside the outhouses (I'm going to use the coal shed | as a storage cupboard) | - supply the power for a couple of outside lights | - power an outside socket to be placed on one of the walls of the | outhouse.

Only the outside socket *has* to be RCD protected.

| Anyway, the cable is modern 2.5mm T&E so that gives me the basis of a | 20A radial circuit. My plan is to use a junction box to spur off to | the socket and to a 5A FCU for a lighting circuit that will comprise | the lights inside the outhouses and the exterior lights (note: these | *aren't* going to be the high-wattage security types).

All fine.

| I'd like some advice on the best way to protect the circuit. The | consumer unit is a relatively modern Wylex with MCBs and the whole | thing is RCD protected (I gather this is not considered a great | arrangement these days).

This will probably be a 30mA whole-house RCD and this should provide RCD protection to the socket. However as you say it is not a great arrangement, and many consider it to be unacceptable under the IEE regs. It makes your whole house vulnerable to an outside fault taking out the whole electrical installation (including smoke detectors and intruder alarm).

There is no point in adding any further RCD protection to this circuit whilst it is still supplied through the whole-house RCD though, as there will be no discrimination (in the event of a fault, either device may trip).

| - use an RCD connection unit to protect the socket spur in the | outhouse. | - use an RCD connection unit to protect the whole of the outside | circuit. Presumably I could install this near to the CU inside | my house rather than in the outhouse?

You could, but no point. See above re discrimination.

| I have also come across 'RCBO's. I understand that these are a | combination of RCD and MCB but I don't really understand what that | means - e.g. if I installed a 32A RCD in my consumer unit wouldn't | that break the circuit when the current exceeds 32A?

No, an RCD provides earth fault protection. It does not provide overcurrent protection. 32A is simply the maximum current the RCD is rated to carry.

| So what does the MCB element of a RCBO give in addition?

Overcurrent protection.

To improve your RCD protection by providing localisation of protection to circuits, you would need to do one of:

- replace the CU with a split-load one, put the shed circuit on the non-RCD side, and use RCD socket.

- replace the CU with a split-load one, put the shed circuit on the non-RCD side, and use RCBO instead of MCB for that circuit

- replace the CU with one that can take RCBOs and use them for the shed and other circuits that require RCD protection.

- Install a second mini-CU for the shed circuit (this could just be a switchfuse with a 20A cartridge fuse) teed into the meter tails using 'henley blocks' and use RCD socket in the shed.

Note that if your house has TT (earth electrode) earthing then all the above should have a 100mA time-delay RCD as the main switch, with split-load or RCBOs providing 30mA protection for socket circuits. Suggestion 4 above, of mini-CU, would need to be 100mA/30mA protected also, but you wouldn't be doing a mini-CU for the shed if you were changing your existing CU.

Owain

[Ian]

Thanks for your detailed reply Owain.

You are correct - it is a 30 mA RCD. I am going to consider having a split load one installed as per your advice. The annoying thing is that I don't think the current one was installed all that long ago - within the last 5 years I think. Oh well, at least our burglar alarm/smoke alarm system has a battery back-up.

OK - I guess what what confused me was that I assumed that some kind of overcurrent protection is a basic requirement on a mains circuit and yet that doesn't seem to be the case given that standalone RCDs are available to go in consumer units.

[Toby]

That would be an RCD immediately upstream of one (or more) MCB(s).

[Lurch]

On 30 May 2004 11:21:09 -0700, ian [e-mail] (Ian) strung together this:

Yes but each circuit from the consumer unit is protected by an RCD and MCB so an RCBO is just these two units combined. An RCD is meant to be installed where there is already overcurrent protection available. In the case of a consumer unit the overcurrent protection for the RCD is provided by the service fuse and the MCBs just provide overcurrent protection for the individual circuits.