Earthing metal back boxes

[blup]

As I've often said, and for what it's worth, I personally always do earth metal back boxes - but that's 'just me', and doesn't necessarily mean that it is 'required'.

Likewise

Both in terms of common sense and the spirit of BS7671, your argument above is essentially academic/semantic. The concept ('spirit') obviously is that anything which could become live under fault conditions and which is "exposed" (i.e. touchable) should be earthed. Whether or not something else which is not exposed (i.e. not touchable) is in electrical continuity with the exposed/touchable part is not really relevant.

It's not something else when the metal screw becomes mechanically fixed to the back box. Two separate components can become one exposed conductive part. The semantics, if there are any, relate to separating back box socket and screws for the purpose of analysis.

As I said, if one is really concerned, one should probably use nylon screws, in which case I can'ts see how anyone could suggest that the back box was an 'exposed-c-p'..

I wouldn't trust nylon screws

Kind Regards, John

 

[SUNRAY]

I wouldn't trust nylon screws

Have you used them? They are a lot more stable and secure than I expected.

 

[JohnW2]

It's not something else when the metal screw becomes mechanically fixed to the back box. Two separate components can become one exposed conductive part. The semantics, if there are any, relate to separating back box socket and screws for the purpose of analysis.

I'm nor sure what you are arguing about. We aree agreed that theee screws are ('touchable') xposed-c-ps and theerefore, per BS7671, theoretically should be earthed. If, as a consequence, one wishes o earth thee screws, I can think of no way of achieving that other than by eartjhing thee back box, can you? It's simply because there is no practical ways of earthing thee screws directly that one has to earth them by earthing 'something else' with which they aree in contact.

I wouldn't trust nylon screws

Wouldn't trust them to do what?

However, there is a problem (I suspect an insuperable one) which one rarely sees discussed. What if it is a plastic switch with a plastic ('plasterboard') back box? With many of the designs of such box, the screw protrudes out of the back of the bush into which it screws (see pic below) - and, since that screw (the head of which is touchable) could come in contact with a live conductor which had come astray, it surely still qualifies as an exposed-c-p, hence required (by BS7671) to be earthed....

... but how on earth (sorry ) could one earth that exposed-c-p? ... and if one wouldn't/couldn't earth it when the back box was plastic, why does it become 'more necessary' if the back box is metal?

Kind Regards, John

 

[blup]

I'm nor sure what you are arguing about. We aree agreed that theee screws are ('touchable') xposed-c-ps and theerefore, per BS7671, theoretically should be earthed. If, as a consequence, one wishes o earth thee screws, I can think of no way of achieving that other than by eartjhing thee back box, can you? It's simply because there is no practical ways of earthing thee screws directly that one has to earth them by earthing 'something else' with which they aree in contact.

Wouldn't trust them to do what?

However, there is a problem (I suspect an insuperable one) which one rarely sees discussed. What if it is a plastic switch with a plastic ('plasterboard') back box? With many of the designs of such box, the screw protrudes out of the back of the bush into which it screws (see pic below) - and, since that screw (the head of which is touchable) could come in contact with a live conductor which had come astray, it surely still qualifies as an exposed-c-p, hence required (by BS7671) to be earthed....

... but how on earth (sorry ) could one earth that exposed-c-p? ... and if one wouldn't/couldn't earth it when the back box was plastic, why does it become 'more necessary' if the back box is metal?

View attachment 331896

Kind Regards, John

How on earth indeed, but we are not talking about plastic on plastic because although they are exposed in the sense argued, they are not conductive

 

[JohnW2]

How on earth indeed, but we are not talking about plastic on plastic because although they are exposed in the sense argued, they are not conductive

No, but unless one uses nylon ones, the screws are conductive and are exposed-c-ps - and so, per BS7671 ought to be earthed - even if I can't think of any easy way in which one could achieve that!

This reinforces what I've been saying - that what (if anything) needs to be earthed it is the ('exposed' and 'touchable') screws, not the back box (whatever material it is made of).

Kind Regards, John

 

[blup]

No, but unless one uses nylon ones, the screws are conductive and are exposed-c-ps - and so, per BS7671 ought to be earthed - even if I can't think of any easy way in which one could achieve that!

This reinforces what I've been saying - that what (if anything) needs to be earthed it is the ('exposed' and 'touchable') screws, not the back box (whatever material it is made of).

Kind Regards, John

That is simply remedied by clipping on the plastic screw caps that are supplied with sockets.

 

[JohnW2]

That is simply remedied by clipping on the plastic screw caps that are supplied with sockets.

As I wrote ....

.... Plastic caps/covers over the screws might be considered adequate to address this theoretical issue ....

... and if you are one of those who believes that they are adequate (to prevent the screws heads being exposed-c-ps which need to be earthed) in the case of a plastic back box, then, if you are consistent, you will presumably also believe that they adequate (as an alternative to leaving the screw heads exposed and earthing the back box) when the back box was metal, won't you?

Kind Regards, John

 

[ebee]

... but how on earth (sorry ) could one earth that exposed-c-p? ... and if one wouldn't/couldn't earth it when the back box was plastic, why does it become 'more necessary' if the back box is metal?

Hows about the chances of the exposed conductive part (the screws) becoming energised is extremely tiny but not non-existent , one could argue that this risk alone means we should earth it or adequately insulate it in order to negate it being classed as exposed. If it is housed in a plastic insulated box then the chance of it becoming energised still very tiny. However if it is housed in a conductive box then the chances of it becoming energises greatly increases ?

 

[blup]

As I wrote ....

... and if you are one of those who believes that they are adequate (to prevent the screws heads being exposed-c-ps which need to be earthed) in the case of a plastic back box, then, if you are consistent, you will presumably also believe that they adequate (as an alternative to leaving the screw heads exposed and earthing the back box) when the back box was metal, won't you?

Kind Regards, John

Yes, in principle, but that does not give the additional practical protection to a sparkie, or a diyer for that matter, when removing the socket that a fly lead does.

 

[Adam_151]

... and if one wouldn't/couldn't earth it when the back box was plastic, why does it become 'more necessary' if the back box is metal?

Because the screws arn't strcitly an ECP in their own right and unlikely to come into contact with electrical parts. What they do result in though, is effectly an extension of the metal backbox, (which could come into contact with electrical parts quite easily) which is not considered an ECP due to being recessed into the wall, into an ECP because the screws bring out whatever potential is on it to the front.

Thats why in the days of lighting cirucits without a CPC/ECC that the 16mm backboxes had nylon lugs to take the screws, and retrofit applications, these days, one can buy the aforementioned nylon screws.

I also have a problem with folk who ram large headed self drilling screws through the back of a plastic box into a piece of (unearthed) racking, the secrewheads entering the enclosure (to which one of the live conductors faulting to would be reasonably foreseable) means the racking effectivly becomes an ECP, now generally, earthing the racking is probably undesirable (although if someone has attached a metal clad accessory instead, we would probably not concern ourselves too much that it was). However the racking being both unearthed and an ECP is not a good situation, in my view the best way is to minimise the size of the screwheads and put a piece of plastic between the fixings and the conductors

 

[JohnW2]

Hows about the chances of the exposed conductive part (the screws) becoming energised is extremely tiny but not non-existent , one could argue that this risk alone means we should earth it or adequately insulate it in order to negate it being classed as exposed.

Indeed - but it's the usual discussion about 'where one should draw lines' in relation to risks which are "extremely tiny bit not non-existent".

If it is housed in a plastic insulated box then the chance of it becoming energised still very tiny. However if it is housed in a conductive box then the chances of it becoming energises greatly increases ?

...and there you are making a personal decision as to where that line should be drawn.

It's obviously the case that the risk is less when the backbox is plastic (hence greater when it's metal), since a 'stray live conductor' then has to actually touch the screw (rather than just touch the back box). However, I would suggest that it's still an 'extremely tiny risk' (even if somewhat less 'extremely tiny') - so, as I've said, you're making a personal decision as to where to draw that line.

I suppose I should repeat what I recently said - namely that I personally do always earth metal back boxes, the discussion (mainly relevant to 'others') being about what it actually 'required'.

Kind Regards, John

 

[JohnW2]

Yes, in principle, but that does not give the additional practical protection to a sparkie, or a diyer for that matter, when removing the socket that a fly lead does.

Very true but, as I've said, anyone who undoes the faceplate screws (let alone 'pulls the faceplate forwards') whilst the circuit is still energised is simply asking for trouble, and there is a limit to how far one can/should go in attempting to mitigate the results of future stupidity.

... and, of course, there's always the 'two-edged sword' issue as regards 'earthing things' .... If an idiot pulls the faceplate of an energised plastic accessory forwards, resulting in a live conductor becoming attached from the accessory and their then touching that live conductor, then the risk of their receiving a significant, or serious, electric shock is much greater if there is an earthed metal back box for them to touch simultaneously.

... so it's certainly not all 'black and white'

Kind Regards, John

 

[ebee]

I suppose I should repeat what I recently said - namely that I personally do always earth metal back boxes, the discussion (mainly relevant to 'others') being about what it actually 'required'.

Yes I think that we pretty much agree there John, If you had seen as many touches and near misses as I have in metal back boxes I think it might increase your favouring of earth links to metal back boxes too. I draw the line at plastic no problem (well very very tiny in practice so OK to ignore usually) but metal best done. As a sort of "lets cover all bases for the sake of safety"

 

[JohnW2]

Because the screws arn't strcitly an ECP in their own right and unlikely to come into contact with electrical parts.

If (as you were) one is talking about "strictly", then I think that I have to disagree with that, since the screws 'strictly' satisfy the BS7671 definition of an exposed-c-p (even in the absence of a metal back box), namely ...

Conductive part of equipment which can be touched and which is not normally live, but which can become live under fault conditions

Admittedly, the probability of a fault resulting in the screws becoming live is appreciably less if the back box is plastic rather than (unearthed) metal, but, per what ebee has just written, not zero ('non-existent risk;').

What they do result in though, is effectly an extension of the metal backbox, (which could come into contact with electrical parts quite easily) which is not considered an ECP due to being recessed into the wall, into an ECP because the screws bring out whatever potential is on it to the front.

All true - but, as above, the screws still strictly speaking being exposed-c-ps (which could become live under certain fault conditions) even when the back box was plastic

Thats why in the days of lighting cirucits without a CPC/ECC that the 16mm backboxes had nylon lugs to take the screws, and retrofit applications, these days, one can buy the aforementioned nylon screws.

Maybe some, but I've seen, and have, plenty of 16mm boxes without 'nylon lugs'.

Kind Regards, John

 

[JohnW2]

Yes I think that we pretty much agree there John, If you had seen as many touches and near misses as I have in metal back boxes I think it might increase your favouring of earth links to metal back boxes too.

As I just wrote, it's not as straightforward as you seem to suggest, since there are certainly some situations (albeit mainly ones involving 'stupidity') in which earthing a back box can considerably increase the risk of electric shock.

Kind Regards, John