Running electricity outdoors

[Pandora]

A twin-and-earth cable is run from from a 32A MCB (not RCD) at the house consumer unit to an internal 25A 30mA RCD close to the back door of the house. From the LOAD side of this RCD, SWA single phase+neutral+earth is laid underground to an outdoor socket about 8m from the house (the earth at the socket is connected to earth at the consumer unit). From the SUPPLY side of the RCD, SWA is run underground to the garage to a small consumer unit protected by a 32A/30mA RCD. The SWA earth is not connected to the garage consumer unit, which is instead earthed to an electrode just outside the garage. Four garage sockets and a small radial garage lighting circuit are wired from the garage consumer unit (via 20A and 6A MCBs respectively)

Is the above method legitimate (Wiring Regs)?

Could I install a switch for the garage circuit next to the back door RCD (so that I can isolate the garage power at the back door)?

What size SWA cable should be run to the garage?

Is the location of the outdoor socket's RCD alright? Or should it be right next to the socket?

Any other suggestions?

[Andrew Gabriel]

You didn't give enough information to be sure it conforms, but nothing you said is contrary to the wiring regs. It sounds like it was done by someone who knew what they were doing -- the isolated earth in the garage is a good thing to do, but usually not done. It would be slightly better if the socket also had its own earth rod like the garage does.

Yes. One issue you might stumble on is that some RCD's include logic to check for broken neutral supply conductor and trip in this case. If your double pole switch happens to make the phase contact fractionally before the neutral contact, it would cause such an RCD to trip. However, I suspect that an RCD intregral with a CU is unlikely to include such logic; it tends to be a feature of more expensive devices that are normally found in CU's.

Don't have tables on me -- someone else will have to check. Might also depend on the length and size of T&E back to the CU.

It's fine.

The earth rod one I made above.

[Lurch]

On Fri, 16 Apr 2004 11:23:58 +0100, in uk.d-i-y "Pandora" strung together this:

Sounds like a good bit of wiring to me. Just one thing, see below.

Yep, as long as it's rated at 32A or more. There should be a DP isolator for that circuit anyway as per the regs.

Depends how far, 4mm will probably be ok, 6mm will be ample.

Nope, it's fine where it is.

No, I think you've got all bases covered there.

[John Aston]

I understand that a socket on the outside wall of the house does not need a separate earth electrode. The OP's socket is 8m from the wall. How far away from the house can you put an outside socket without using a separate electrode?

[Andrew Gabriel]

It's not this clear cut, and it isn't a requirement either way. Generally, I think it's better for all socket outlets outside the house to be locally grounded but this is just a recommendation and not a requirement. (RCD protection is a requirement though.) I would worry less about a socket near the house if the house has a TT earthing system (i.e. it uses an earth rod itself as its primary earth connection).

[John Rumm]

Not sure if there is a distance limit specified in the regs, but the need for a separate earth would depend a couple of factors.

The desire is that should a fault occur resulting in the live coming into contact with the earth, the fault current that will flow to earth is high enough to cause the protective device in the supply to operate quickly enough. This fault current will be limited by the earth fault loop impedance measured at the socket (and hence include the earth impedance at the CU + the total impedance of all the cable from there to the socket).

Even in cases where you are within permissable disconection times, you might also want to factor in the likely fault scenario, and make a subjective judgment as to if that is good enough. Say that external socket was likely to be powering garden tools with a high likelyhood of coming into contact with bare live wires (i.e. mowers, hedge trimmers etc), you may find that the fault loop impedance is low enough to cause disconnection within 0.35 secs (i.e. just inside the 0.4 sec recommended for appliances you are likely to be holding on to). Subjectively however, you may decide that a local earth rod will reduce this to under

0.1 seconds which would hurt less!

[Andrew Gabriel]

The protective device trip current is only 30mA in this case, as the socket has to be RCD protected at no more than 30mA. There is no requirement for the earth fault loop impedance to be low enough to ensure the fault current protective device will trip within 0.4 seconds (or ever even) on a short to earth in this case. Generally, thats pretty difficult to meet when using earth rods, which is why RCDs are mandatory on TT system socket outlets.

That's not the reason for the local earth rod. The local earth rod is to ensure the earth potential is than of the local ground, and not the ground at a substation half a mile away plus possible voltage drops in the earth conductors inbetween.

[Andrew Mawson]

What size is the twin and earth that runs from the 32A MCB to the 25A one at the back door. If it's 2.5mm then I understand a single run is rated at

24amps so the 32A one needs replacing with a lower rated one.

Andrew Mawson

[John Rumm]

Yes you are right - I mentally skipped over that, and was only thinking of the MCB disconnect requirements ;-)

"in this case" being when there is an RCD present I take it?

This is assuming the OP does not have a TT system. The inclusion of the RCD makes it a bit of a moot point however.

[Andrew Gabriel]

Yes.