Earth funnies and phantom voltages on lighting circuit

[Harry Bloomfield]

All true. I don't think anyone has suggested that Class I items should (sensibly) not be earthed.

However, that doesn't alter what I said, that to create additional touchable ('unnecessarily') earthed metal is always going to (very slightly) increase the risks of electric shocks - so 'why do it?'

To ensure, they cannot ever become live! If all conductive items, which are touchable, which have even the slightest chance of becoming live, are bonded to earth, then risks of electric shock are minimised. The only exception, is if that individual makes contact with something which is live, and another item which is earthed.

 

[EFLImpudence]

But then the sheath is stripped and the cores are no longer double insulated. Unless the inside of the metal enclosure is fully insulated then you've lost your DI compliance.

Yes, the sheath will be clamped and these will be the connections of the double-insulated appliance - no CPC present, of course.

IIRC, the last fitting I worked with came with a chock block - the instructions said to wrap it with tape.

Obviously that is no good.

 

[Harry Bloomfield]

All true. I don't think anyone has suggested that Class I items should (sensibly) not be earthed.

Yartin was discussing a Class 2 item!

 

[JohnW2]

To ensure, they cannot ever become live!

If what is beneath an outer touchable metal case would be deemed to be adequately 'safe' (safe to touch) without the outer metal (because it was Class II), then that outer metal would be equally unlikely to ever become live.

If all conductive items, which are touchable, which have even the slightest chance of becoming live, are bonded to earth, then risks of electric shock are minimised.

See above. If you are suggesting that one has to regard touchable metal as being at significant risk of 'becoming live', even if there is nothing in the equation which could/should be able to make it live, then you arre in the territory of suggesting that metal window frames, door knobs etc. etc. should be earthed

The only exception, is if that individual makes contact with something which is live, and another item which is earthed.

What you call "the one exception" is, of course, the very situation I was talking about when I said that unnecessarily creating touchable earthed metal increases (very slightly) the risk of electric shock.

 

[JohnW2]

Yartin was discussing a Class 2 item!

He was, but (as per my quote) I was responding to your post, not his/hers. In your post, you had written (as I quoted) ...

If everything metal, taps, sinks, cookers, fridges, freezers, switch plates, pipes, radiators, and so on, could be replaced with none electrically conductive versions. Unfortunately, that cannot be achieved, so they are earthed/bonded for safety, to ensure they can never at risk of becoming live.

... which, paraphrased, says that Class I items (which inevitably exist obviously must be earthed.

 

[Harry Bloomfield]

then you arre in the territory of suggesting that metal window frames, door knobs etc. etc. should be earthed

Not really, how likely do you think a window frame, or a door knob might become live? By contrast, unearthed radiators, pipes, metal baths, can without need for much imagination, quite easily become live. My electric shower is carefully earthed, and bonded to my metal bath, along with the bath taps, and the radiator.

 

[securespark]

But then the sheath is stripped and the cores are no longer double insulated. Unless the inside of the metal enclosure is fully insulated then you've lost your DI compliance.
IIRC, the last fitting I worked with came with a chock block - the instructions said to wrap it with tape.

Technically, the sheath of a cable isn't insulation. The conductors are insulated then sheathed, not double insulated.

 

[SimonH2]

Technically, the sheath of a cable isn't insulation. The conductors are insulated then sheathed, not double insulated.

Pedanticly correct - but the effect is the same, it's an extra insulating layer over the basic core insulation. Once it's removed to allow termination, the cores only have a single layer of insulation.

 

[SUNRAY]

See above. If you are suggesting that one has to regard touchable metal as being at significant risk of 'becoming live', even if there is nothing in the equation which could/should be able to make it live, then you arre in the territory of suggesting that metal window frames, door knobs etc. etc. should be earthed

Like it or not, back in the 70's there was a lot of that attitude. I did a fair amounnt of work for a small developer and the electric board would go round with a wander lead checking for bonding of all exposed metalwork including hinges and catches on wooden windows etc.

In that era kitchen cupboards had metal trims as handles and metal hinges, it got to the stage cupboard and drawer fronts were not being fitted until after the Boards inspection and energisation. there were a row of coat hooks on a wooden plate, metal handles/hinges for internal doors not being fitted until later. It was all very crazy, stupid, but those were the actions required back then to get power in a new house when 'bond all exposed metalwork' was taken to the nth degree.

 

[JohnW2]

Not really, how likely do you think a window frame, or a door knob might become live?

We both know the answer to that!

By contrast, unearthed radiators, pipes, metal baths, can without need for much imagination, quite easily become live.

They could if they were, as you suggest, not earthed - but, in practice, that won't happen (for more than milliseconds, before the fault is cleared), since earthing of such things is essentially 'unavoidable'; whilst it would be easy enough to isolate pipework, and what is connected to it, from 'the outside world' (potentially 'earth'), things like immersions, CH pumps and MVs etc. will usually inevitably result in al, the pipework being earthed (via CPCs).

However, even if it were possible for things such as you mention to become live,that merely underlines my suggestion that it is preferable not NOT introduce any 'unnecessarily earthed' touchable metal (e.g. of a Class II item) into the building, since that would increase the risk of shock if someone touched that together with one of your live pipes/radiators etc.

My electric shower is carefully earthed, and bonded to my metal bath, along with the bath taps, and the radiator.

What touchable metal part of your electric shower is 'earthed'?

 

[JohnW2]

Like it or not, back in the 70's there was a lot of that attitude.

I know - hence my comment about metal window frames and knobs - and I suppose I could (as others sometimes do!) also mention cutlery

 

[Harry Bloomfield]

They could if they were, as you suggest, not earthed - but, in practice, that won't happen (for more than milliseconds, before the fault is cleared), since earthing of such things is essentially 'unavoidable'; whilst it would be easy enough to isolate pipework, and what is connected to it, from 'the outside world' (potentially 'earth'), things like immersions, CH pumps and MVs etc. will usually inevitably result in al, the pipework being earthed (via CPCs).

There are no bits of plastic, anywhere in my plumbing installation, I would not allow plastic, but you cannot gamble on some wannabe plumber coming along and replacing sections of the copper with plastic. So your earth bonding, is then entirely relying upon the conductivity of the water, in the pipes.

What touchable metal part of your electric shower is 'earthed'?

The copper pipe, leading to the shower, and I would assume (not checked) any internal metal in contact with the water, plus maybe the flexible metal shower hose. The main risk with an electric shower, is that of electrical leakage from the shower elements, to the water. That would be effectively given a path to ground, via the bonded copper supply pipe.

 

[EFLImpudence]

There are no bits of plastic, anywhere in my plumbing installation, I would not allow plastic, but you cannot gamble on some wannabe plumber coming along and replacing sections of the copper with plastic.

The plastic bits might make it safer - depending on where it is

 

[JohnW2]

There are no bits of plastic, anywhere in my plumbing installation, I would not allow plastic, but you cannot gamble on some wannabe plumber coming along and replacing sections of the copper with plastic. So your earth bonding, is then entirely relying upon the conductivity of the water, in the pipes.

It's all down to balancing pros and cons. To get an electric shock (other than by touching L & N simultaneously, which is rare), one usually has to simultaneously touch something 'live' and something 'earthed'. I would suggest that pipework becoming live is less likely than other things (e.g. frayed cables or faulty portable appliances) becoming live - so the more likely of the two scenarios is probably when the pipework is the 'earthed something' and the 'something else' (which one is simultaneously touching) is the live one.

If that is the case, the ideal would be to minimise (ideally 'eliminate') the amount of touchable earthed pipework (hence also radiators, taps etc.). As I wrote before, it would probably be fairly easy to electrically isolating incoming service pipes etc. (although that could not be done with plastic pipe within the house for a gas pipe), the more difficult thing being to isolate most of the pipework from the CPC-connected things like CH pumps/valves, immersions, showers etc. However, if that could be done, the resulting situation (with the great majority of touchable pipes etc. not earthed) would probably, on balance, be the 'safer' option'

The copper pipe, leading to the shower, and I would assume (not checked) any internal metal in contact with the water, plus maybe the flexible metal shower hose. The main risk with an electric shower, is that of electrical leakage from the shower elements, to the water. That would be effectively given a path to ground, via the bonded copper supply pipe.

We've been through this many times before. Earthing of some parts within an electrical shower is unavoidable but, as you say, the only theoretical 'risk' involves electricity being conducted through flowing (pretty 'clean') water, which is probably very unlikely to result in a serious shock - and, as above, the risk of any such shock would be increased by the presence of any touchable earthed metal in the vicinity.

 

[Harry Bloomfield]

I would suggest that pipework becoming live is less likely than other things (e.g. frayed cables or faulty portable appliances) becoming live - so the more likely of the two scenarios is probably when the pipework is the 'earthed something' and the 'something else' (which one is simultaneously touching) is the live one.

I would suggest otherwise... I would suggest from my experience, that metalwork becoming live, is just as common as making direct contact with the likes of a frayed cable, at least for the average member of the public. For those involved in the electrical industrial, obviously, direct contact with live cables is much more likely.