RCD Protection for outside circuit


I

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.
 
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O

Owain

| 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
 
I

Ian

Thanks for your detailed reply Owain.
| 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).
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.
| 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.
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.
 
T

Toby

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

Lurch

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.
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.
 

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