Light switch earthing

[JohnW2]

Whether they have or not doesn't come into it does it?

Of course it does. If, an average of 50+ million people have been living with 'faceplate screws' for 50+ years without anyone every having come to significant harm,then that would, in most people's minds, count as a theoretical hazard 'not worth worrying about', wouldn't it?

The simple fact is; injury could be caused and things can be done to take that risk away

Literally true, but if one really wanted to 'take every conceivable risk away', one would have to 'ban' electricity supplies of appreciable voltage (and even 50V would probably be far to high to be 'allowed').

Having said all that, I'm not advocating leaving metal backboxes without earths

However, if one is getting concerned about incredibly improbable 'risks', one really needs to consider the fact that earthing faceplate screws of plastic accessories actually increases another (also incredibly improbable) risk - namely that if someone comes in contact with a live conductor (e.g. damaged vacuum cleaner cable) and simultaneously touches the faceplate screw of a plastic accessory,then one might get a serious shock if the screw is earthed, but not if it isn't earthed!

Kind Regards, John

 

[JohnW2]

John, the problem might not be as inconveivable as you may be suggesting. Let's assume the back box isn't connected via a cpc loop and relies upon the somewhat 'loose' IET guidance of using the faceplate screws. In normal circumstances this may well be ok.

You seem to be talking about metal faceplates, whereas we've been discussing metal faceplate screws in plastic faceplates.

Now assume that someone decides to (for whatever reason) remove the faceplate. ... Both screws are now out and the faceplace is being drawn forward. ... At that moment there is a chance (due to poor installation workmanship) that a live wire comes away from the fitting and the bare end of it springs back and comes to rest against any part of the back-box. If the 'dabbler' should now touch the (live) back-box .........

All true, but if anyone removes the screws and pulls the faceplate forward without de-energising the circuit, then any consequences are down to them - and, indeed, they would be exposing themselves to significant risks by doing that even if the backbox were earthed.

In fact, if they were being that silly, having an earthed backbox might well (in some circumstances) increase the risk to them - since if they came in contact with the 'live wire' that had popped out of its terminal, they might well simultaneously be in contact with the backbox - which (in some circumstances) would mean that they could get a serious shock if the backbox was earthed, but not if it wasn't earthed.

Kind Regards, John

 

[opps]

John, the problem might not be as inconveivable as you may be suggesting.
Let's assume the back box isn't connected via a cpc loop and relies upon the somewhat 'loose' IET guidance of using the faceplate screws.
In normal circumstances this may well be ok.
Now assume that someone decides to (for whatever reason) remove the faceplate.
Both screws are now out and the faceplace is being drawn forward.
At that moment there is a chance (due to poor installation workmanship) that a live wire comes away from the fitting and the bare end of it springs back and comes to rest against any part of the back-box.
If the 'dabbler' should now touch the (live) back-box .........

As a decorator, I often loosen the screws on plastic faceplates so that I can tuck the paint brush behind it, and I have had live (and neutral) wires pop out.

 

[ebee]

Yes it is not that uncommon, not strictly H & S but still happens quite a bit in practce, sometimes even forgeting to retighten - say just a few turns to push paper on with the brush (or even a trowel/scraper/screwdriver) now it is slack, not good contact and heyho your earth loop non existant even with a fixed lug.
Surely 20 seconds extra time and a farthing worth of materials is not a bad option.

 

[SUNRAY]

As a decorator, I often loosen the screws on plastic faceplates so that I can tuck the paint brush behind it, and I have had live (and neutral) wires pop out.

Well I'm very pleased to eventually find a decorator who has the sense to do a proper job, so many these days caulk round accessories and slap paint on to cover it.

A few years back Mrs Sunray wanted the hall and stairs turned into a doctors waiting room with magnolia emulsion decorated in time for Christmas, I was very busy at work trying to catch up on an overrunning job. When I got home from work I found dust sheets everywhere, the wallpaper stripped and prepared for painting. I spent the rest of the evening removing the accessories and cleaning the caulk off the walls so the employed decorator could apply the chosen coats of paint.

 

[ebee]

Well I'm very pleased to eventually find a decorator who has the sense to do a proper job,

Indeed, well done that man!

 

[JohnW2]

As a decorator, I often loosen the screws on plastic faceplates so that I can tuck the paint brush behind it, and I have had live (and neutral) wires pop out.

That doesn't surprise me.

However,as I'm implied, it's not a straightforward situation - if a live wire pops out such that it could be touched, then, depending on the circumstances, the presence of an earthed backbox (and faceplate screws) could either increase or decrease the risk of a serious electric shock.

Of course, you (or anyone else) should really isolate the electrical supply before loosening the screws and pulling the plate forwards!

Kind Regards, John

 

[opps]

That doesn't surprise me.

However,as I'm implied, it's not a straightforward situation - if a live wire pops out such that it could be touched, then, depending on the circumstances, the presence of an earthed backbox (and faceplate screws) could either increase or decrease the risk of a serious electric shock.

Of course, you (or anyone else) should really isolate the electrical supply before loosening the screws and pulling the plate forwards!

Kind Regards, John

But if the socket is on an RCD it would trip the RCD...

 

[JohnW2]

But if the socket is on an RCD it would trip the RCD...

In the situation I was describing, it would if enough current flowed through a human being to result in a serious electric shock, but not otherwise.

Don't forget that I wasn't talking about an L conductor 'popping out of a terminal' and coming in contact with an earth back box (which would result in an RCD trip) but, rather, about the L conductor 'popping out of the terminal' and being touched (only) by someone who was also touching the backbox (or a screw). It that case the person would get a potentially serious shock (which hopefully would trip an RCD) if the backbox were earthed, but not if the backbox were not earthed.

As I said, it's 'swings and roundabouts', since in other situations the situation would be reversed - i.e. a shock if the backbox were not earthed, but no shock if it were earthed.

Kind Regards, John

 

[ebee]

Well I certainly agree with the swings v roundabouts scenario on that one it`s just that we tend to favour it must be earthed usually.
If we do actually favour the must be earthed stance then that is where I get the loop it do not rely on the fidxing screw being tight in a fixed lug idea.

 

[JohnW2]

Well I certainly agree with the swings v roundabouts scenario on that one it`s just that we tend to favour it must be earthed usually.

We do tend to, but that is (or should be) all down to our judgement as to whether the swings or the roundabouts are the 'more likely' scenario. Opinions about that judgement will often differ.

The same discussion (and need for judgement) arises in all cases of 'possibly unnecessary earthing'. ... like bernard';s view about (otherwise electrically 'floating) metal baths - he feels that the more likely scenario is that the bath will somehow 'become live', and therefore should be earthed to avoid electric shocks. Others believe it more likely that a person would touch something else 'live' (e.g. a damaged vacuum cleaner cable) and simultaneously touch the bath - in which case the situation would be 'safer' if the bath were not earthed.

Kind Regards, John

 

[ebee]

Agreed - case in point, the metal strip hook thingy thar killed Mary Wherry, might that have been safer if she did not also touch an earthed aplliance? possibly, we can`t be sure I`m afraid.

 

[EFLImpudence]

This all comes down to which part is an exposed-conductive-part.

The back box is not, therefore does not require earthing.

If anyone considers the screws to be exposed-conductive-parts then they must be earthed.
Presumably the only way to do this is by earthing the back box.

 

[JohnW2]

Agreed - case in point, the metal strip hook thingy thar killed Mary Wherry, might that have been safer if she did not also touch an earthed aplliance? possibly, we can`t be sure I`m afraid.

Exactly.

Other than gambling/guessing, the best we can do is make an intelligent/informed judgement as to which of two possibilities is the 'more likely' - but, of course, even if we get that judgement right, it certainly doesn't mean that the 'less likely' will never occur.

Such is the (probabilistic) world of 'risks'.

Kind Regards, John

 

[JohnW2]

This all comes down to which part is an exposed-conductive-part. .... The back box is not, therefore does not require earthing. .... If anyone considers the screws to be exposed-conductive-parts then they must be earthed. .... presumably the only way to do this is by earthing the back box.

You're now really talking about regulatory definitions, and hence that an "exposed-c-p" is something which can be touched 'in normal service' - and I suppose that, strictly speaking, that does include metal faceplate screws.

However, there's no reason why one should not exceed the minimum requirements imposed by the regs, and it seems that some people's thinking is going beyond 'touchable in normal service' to the situation in which screws are removed/loosened and the faceplate pulled forwards, exposing the backbox.

Kind Regards, John