That earth connector in the corner of the wall box.

Yes, for what it's worth. Light switches, for example, typically don't have this bonding "built-in"

Thanks for that.

The main advantage of having the backbox well earthed is that it may help clear some fault conditions more quickly (or at all). If a live wire were to work loose due to thermal cycling for example, or a cable were to abrade on the entry hole[1] there is more chance of causing an earth fault and operating the circuits protective device.

It is. Although you are not supposed to rely on the moveable earth tags (most boxes have at least one fixed one though). Having the hard wired connection gives some added assurance that there will always be a low impedance earth connection to the box - even if someone loosens the screws to paper behind the socket and forgets to tighten them again.

Even then the cable will be passing through the entry hole. A damaged cable could render it live - especially when refitting. That could give the decorator a nasty surprise!

as above. Also on lighting circuits the switch drop cable will have an earth that may not be required by the switch in question. The terminal on the box gives you somewhere to park it.

Having a good earth connection to the box ensures the circuit will trip on the RCD. This would also probably apply to *both* lives (i.e. phase and neutral) making contact to earth. Without an earthed back box the RCD may trip - but it is far less certain (and will depend on materials and moisture content of the wall).

On the RCD protected circuit you potentially help detect a failure mode in a ring circuit (disconnected neutral) that otherwise may go unnoticed. On circuits without RCD protection it will at least ensure the MCB/Fuse operates quickly on a phase to earth fault.

Are you confusing the faceplate mounting lugs (one fixed, one moveable) with the earth tag (always fixed)? Or am I missing the point?

Having the hard wired

Forgot to add:

Sorry, obviously not being clear.

Some back boxes have two (or four) moveable lugs. In this case you must provide a dedicated earth wire to the back box's earth terminal - since you can't rely on movable lugs for a reliable connection.

For back boxes that have a fixed lug (i.e. most modern ones) you could rely on the screw connection alone for earth continuity, however it is still preferable and considered good practice to have the dedicated wire for the reasons mentioned in the previous post.

If might be - but if the box is recessed slightly into the plaster the connection could be poor. If the fitting was screwed tight to the box rather than to plaster etc it should make a good connection but this isn't always the case.

The obvious one is plastic fittings like light switches which need no earth and have no terminal on them. The cable earth is then made off neatly to the box one - all ready for future use when the switch gets changed to a metal one.

No surprise there. Perhaps a Google on this group would have been better as it's come up before. But probably as thread drift which can make things more difficult to find.

OK, I was under the impression that an earth bond between the faceplate and the backbox was *required* (not just good practice) irrespective of whether the mounting lugs are fixed or moveable (or any other factor for that matter). Can you cite any references? Thanks.

Dave,

I was assuming that the screw under tension would connect the metal strip on the socket (which is exposed where the screw goes in) to the lug on the back box.

I don't think there is anywhere the metal earth strip is expected to contact the back box itself.

Cheers

Dave R

Hmm tricky one that... I can find one for cases where conduit or armour etc is used for earthing:

But the general case of "good practice" I don't have a reference to at the mo.

A screw thread doesn't make a decent contact - it's it holding two contact areas together which does.

Certainly with some the earth 'strip' will be clamped to the box by the screws if the box isn't recessed.

But, in this case the protective conductor is *not* "formed by conduit, trunking, ducting or the metal sheath and/or armour of a cable", so this reg doesn't apply...

I was thinking maybe the "requirement" for bonding backboxes came from Selection and Erection or the On-Site Guide, neither of which I have to hand. Ho hum.

Actually, what I really was thinking was that (notwithstanding Mr Rumm's full answer to the OP's original question) that the reason we bond a flush backbox to an accessory plate is because some committee somewhere decided it would be "a good idea".

There are many comparable situations where such a bond is not required (unless things have changed recently - correct me if I'm wrong). For example, there is no requirement to bond the front cover of a metal consumer unit; there is no requirement to bond a self-assembly 19" equipment rack (bonding kits are available, but optional); there is no requirement to bond the door of an industrial enclosure (unless there are controls attached to the door which have mains voltage present), etc.

Hi If the screws were loose and making poor contact to the faceplate earth and the backbox rubbed through the live insulation then the screws could be live without blowing any fuses rcds etc (esp since metal backboxes will be on older systems). A seperate cable would be useful for the above unlikely senario? Al

Which leads me to a related question. Why do plastic surface mount boxes have earth terminals but plastic plasterboard mounted ones do not? At least, the ones I have do/don't respectively.

Andrew

I would guess because the dry lining ones are made from rather soft thin plastic that would not take a screw terminal without it breaking off at the first turn of a screwdriver....

It is required for the case when metal conduit is used as the circuit protective conductor (earth). It is acceptable according to the Regs to have only phase / neutral conductors pulled through when using conduit and rely on the conduit as an earth, although common proactice these days is to pull through an earth wire too.

The connection made by the faceplate screws to the backbox was considered to be inadequate and so the requirement for a dedicated earth terminal was introduced.

It is, as I understand, optional (but good practice) to use this terminal when there is an adequate earth connection to the accessory which does not involve the backbox.

On Mon, 8 Oct 2007 13:07:19 +0100 someone who may be Alistair Riddell wrote this:-

This is because the "connection" may consist of 0, 1 or 2 screws. In the latter two cases the screws may or may not be done up tightly enough to provide a reasonably low impedance protective conductor.

Indeed.