earthing back boxes

Hello,

Does the OSG (17th ed.) cover something as basic as wiring a socket? Or do they think that people who read the OSG already know something as simple as that? I wanted to look-up the requirements for earthing a metal back box. I had a quick look through the OSG but could not find it mentioned.

I seem to remember someone else asking a similar question on this group. If I remember correctly, the reply was that if you were wiring a socket, which included an earthed screw terminal, you did not have to run a separate earth wire from the box's earth terminal; provided that the earthed screw hole was screwed onto a fixed tab in the box, not the moveable one. If you are wiring something without an earth, such as a light switch, then you have to connect to the box. Have I remembered that correctly?

I have bought some round pin sockets to use with a table lamp and some back boxes from Toolstation. The sockets only earth one screw hole and guess what, it's the one above the moveable tab! The problem is that I put the boxes in, plastered, and papered around them so I do not want to try removing and rotating the boxes. Am I right to think that in this case I need to run the extra earth wire from the box to the socket?

I want to have two boxes with a switch in the top one, running to a socket in the one below. I was hoping to fit a 2-gang light switch to use a system like they have in hotels: switch one for the main ceiling light and the adjacent switch for a table lamp. That means quite a few wires:

L, switched L, and E from the ceiling rose

3-core and E for two way switching of ceiling light twin and earth to table lamp switch twin and earth from table lamp switch to socket

Can I comfortably fit all four earth wires into the box's earth terminal? Or would it be advisable to crimp or otherwise connect some of them together separately?

TIA

Reply to
Fred
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Personally, I would always run a flying lead from the socket to the back box, so that if the socket or switch plate was unscrewed, the back box will still be earthed - so in the event the plate if off, and the live finds it's way to the back box (loose or damaged cable), it is still protected, when it is fully exposed and most vulnerable (in my opinion anyway)

Toby...

Reply to
Toby

There's no absolute right answer here. The requirement is that exposed-conductive-parts must be reliably earthed. Some authorities (e.g. the NICEIC/ESC, IIRC) are happy with the above interpretation, others say good practice is always to include the link wire.

There's a further debate: is a flush box (sunk in plaster or drywall construction and fitted with a plastic-fronted accessory) an exposed-conductive-part at all? Arguably not since it's not accessible to be touched. If you're using wiring accessories with metal front plates though, there's definitely E-C-Ps to be earthed.

So, FWIW, my view is that it's only OK to rely on earthing via a fixing screw to a fixed tab if the box ends up inaccessible and a plastic accessory is fitted. With metal-faced accessories it's best to earth to a terminal on the accessory, if there is one, and link to the back box if necessary.

Clearly you do.

It sounds as if the first-fix wiring is all done now, but you could have saved a cable there, assuming the ceiling and table lamps are on the same final circuit. A 3C+E cable from the ceiling light could have been used to bring down a neutral (grey core, blue sleeve) as well as the L & SL wires in the 'switch drop. Then a separate feed cable for the table lamp would have been unnecessary.

With 1.0 or 1.5 mm^2 T&E (1.0 mm^2 CPCs) That's unlikely to be a problem.

Reply to
Andy Wade

With a metal backbox the screws are, but usually at least 1 screw enters a fixed lug so is earthed via the wiring accessory.

Then again, you can argue a rusty screw doesn't conduct well enough to disconnect a fault should one arise.

-- Jason.

Reply to
js.b1

Just how much extra work is it to run an earth from the socket to the box terminal? There's usually enough ECC left after stripping back so the only real cost is a piece of sleeving...

Reply to
Dave Plowman (News)

More work than an electrician can be bothered to do:-)

I have however, seen one DIY garage rewire using trunking and plastic surface mount boxes where the DIYer took an earth lead from the socket to the earth lug in the plastic box.

Adam

Reply to
ARWadsworth

Well, imagine the scenario where a metal box sits below the plaster - or has tiles over it. Would you really rely on the thread of a screw to make a decent contact throughout its likely life?

To be sure to be sure. ;-)

Reply to
Dave Plowman (News)

I actually would not worry. The backbox is not touchable in day to day use and the screws sit on the sockets "earth bar".

However, as you say, it is not that much extra work for someone just fitting a couple of sockets. I carry a roll of 1.5mm green/yellow cable for jobs that specify the earth link.

Adam

Reply to
ARWadsworth

I put it in the same class as using grommets in knockouts.

Reply to
Dave Plowman (News)

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