Re: Socket Outlet Back Boxes

[Paul Lakra]

Yes. Absolutely. Should not be a problem because you must run twin and earth cable anyway.

[Chris Oates]

We fail NIC inspection if it's not done.

plus it's a good way of finding nicked cables

(you did grommet the holes)

[Dave Plowman]

Right.

That takes some doing, nicking cables through the entry hole.

Probably those who can't be bothered to add the tails will save the cost of the grommets too.

[Andy Wade]

The only situation in which it's mandated by BS 7671 is where the circuit earthing (CPC) is through metal conduit or trunking attached to the back-box. Then you must fit a separate protective conductor between the earth terminal in the back-box and the earth terminal of the wiring accessory [Reg. 543-02-07].

[Dave Plowman]

Interesting. MK backing boxes used to have only one movable terminal, but the no brand ones I usually get these days have both.

In domestic terms, this would usually be plastic. Does it still apply?

[Andrew Gabriel]

It's not required by the regs, but as you say, it's good practice. If the back box is exposed, then it obviously must be earthed.

[Dave Plowman]

Any more for any more? ;-)

[LOZ34]

sorry should have said metal surface box.

loz

snip

a surface mounted box must be earthed.

[ARWadsworth]

So 99% of the time there is no need for the back box earth in a domestic setup. I have never fitted them and no NICEIC inspector has failed my work for this.

Adam

[Chris Oates]

Failed me - on plastic socket which was heavily used by staff for a brew-up, screw heads possibly live. and also for decorative metal sockets.

[ARWadsworth]

True. But on most sockets the eyelet for the screw is usually part of the sockets earth bar. To achieve your scenario would need the earth to fall out of the socket terminals and "float" in the back box whilst at the same time the live falls out of the socket terminals and touch the back box. In this case the earth bonding lead is useless. Refer to LOZ34's earlier reply.

It does not matter if the socket is in contact with the wall or back box, the socket is made from plastic!!!. A bit of corrosion, as you put it, contradicts your first statement and would make the socket safer.

I agree. But it is unnecessary.

Adam

[Dave Plowman]

Yes. But the connection between this and the screw relies on the screw being clean and tight.

No - if the screws were making poor contact and the cable had frayed so the line was touching the box, the *screws* could become live.

I'm referring to the screws themselves being live.

Why then do they provide a terminal in every box these days? Once, you could buy them without, but I've not seen those recently.

[ARWadsworth]

In reality it does not. Even if the screws are corroded or loose they will still make an electrical contact with the eyelet and blow the fuse if the back box is live.

Only if the screws were insulated against the eyelet, and I do not believe this happens through corrosion. So yes it is possible but the NICEIC seem to think it is irrelevant

Same again. If the screws are tight no problem, if they are loose then the movement of the screws against the eyelet will cause an electrical connection and again blow the fuse.

They are there so that you can wire a circuit with a high integrity earth, should you ever need to do so. What I never see are new back boxes with 2 adjustable lugs (on the same plane).

Adam

[Dave Plowman]

I'm sorry, but there we disagree. I've repaired curcuits that ceased working after some time - years possibly - due to the screws for the live or neutral not being correctly tightened in a socket or sockets when installed. The conductor corrodes and ceases to make contact.

[ARWadsworth]

So how many live screws have you seen?

Adam

[Andrew Gabriel]

There has to be -- aluminium oxide is a damn good insulator.

I was breadboarding a circuit which uses power MOSFETs for directly switching mains. I temporarily used a bulldog clip to clamp the MOSFET to a heatsink. Later, I clamped the second MOSFET to the same heatsink.

It was only when I was disassembling the circuit to build it properly that it dawned on me that I hadn't used any insulating washers between the live MOSFET tabs and the heatsink (OK so far, I was expecting the heatsink to be live whilst breadboarded), and that the two MOSFETs had full mains voltage between their heatsinks (oops!). The thing that saved them was that the aluminium heatsink, like all aluminium, had coated itself in a thin invisible hard layer of oxide, which was enough to insulate it from mains even against the force of a strong bulldog clip. The final build uses insulating washers obviously, and although the heatsink is bolted to the base, it also has a separate earth bonding connection using a sharp earth tag to cut through the oxide layer.

This is also why aluminium mains wiring (common in US homes) is rather a bad idea.