Gas supply earth bonding

[bernardgreen]

Like an earth electrode you mean?

An Earth electrode should have an impedance of 200 Ohms or less. Ensures an RCD will trip.

30 mA would flow if 240 Volts when to ground via an 8,000 Ohm path.

That does not detract from anything I have written.

No it doesn't, but it extends the thought process into the ( apparently ) uncharted areas of sneak paths to Earth with a range of un-predictable impedances.

 

[Harry Bloomfield]

There is also the situation where a metal item that is not earthed or bonded but does have a conductive path to the Earth.

Consider when a person is for some reason touching a Live wire and this metal item.

The impedance of the conductive path to the Earth is now critical.

I agree with all of that Bernard, but I am not sure what point you are making?

 

[bernardgreen]

but I am not sure what point you are making?

My point is that nothing in electrical installations is simple black and white ( or brown blue and green/yellow ) yet the people who write the regulations seem to think only in black ink on white paper.

 

[Harry Bloomfield]

If you earth all your pieces of cutlery, they then may need bonding together; instead of all being quite safe to handle before.

The point about cutlery is obviously ridiculous. I used the word 'large' and of course assumed static or fixed. The work was done mid 80's, not by me, by a specialist large contractor who assured me it was to the new regs - not being a 'house basher', I left it at that. I could see and understand the point of ensuring absolutely everything metallic and fixed, was bonded - it avoids the situation of one unbonded item becoming live through a fault and someone touching that and something which is bonded.

The opposite, would be to ensure that no metalwork was bonded and everything is insulated from earth, which is impossible. Basically - an all or none way of thinking.

Neither way is perfect, each carries its own risks.

 

[Harry Bloomfield]

My point is that nothing in electrical installations is simple black and white ( or brown blue and green/yellow ) yet the people who write the regulations seem to think only in black ink on white paper.

Got you, thanks. Yes, which is why opinions always do differ and over time the regulations change.

 

[EFLImpudence]

The point about cutlery is obviously ridiculous.

No, it isn't. It illustrates the situation perfectly.
Spoon, door handle, bucket, metal window frame etc.

I used the word 'large' and of course assumed static or fixed.

Ok, so what is the difference.

The work was done mid 80's, not by me, by a specialist large contractor who assured me it was to the new regs - not being a 'house basher', I left it at that. I could see and understand the point of ensuring absolutely everything metallic and fixed, was bonded

Yes, but you are using the word 'bonded' - to equalise potential - when you mean 'earthed' - to operate the OPD in the event of a fault.

it avoids the situation of one unbonded item becoming live through a fault and someone touching that and something which is bonded.

No, if your "unbonded item" is likely to become live, then it should be earthed, but that does not apply to a big lump of metal - a table if you don't like spoon - in the middle of a room.

The opposite, would be to ensure that no metalwork was bonded and everything is insulated from earth, which is impossible. Basically - an all or none way of thinking.

Yes, not earthed. It would be better but it is impossible with metal appliances.

Neither way is perfect, each carries its own risks.

Yes, we can only mitigate against the most likely of different situations.


If you want your bath NOT to be a path of negligible impedance when you touch it and a faulty vacuum cleaner flex then don't earth it.
If you want your bath to disconnect the supply when the faulty vacuum cleaner flex touches it then earth it.

 

[EFLImpudence]

The opposite, would be to ensure that no metalwork was earthed and everything is insulated from earth, which is impossible. Basically - an all or none way of thinking.

Surely that contradicts what you are saying.

Why is it not safer to have as much as possible not earthed?

 

[SUNRAY]

The point about cutlery is obviously ridiculous. I used the word 'large' and of course assumed static or fixed. The work was done mid 80's, not by me, by a specialist large contractor who assured me it was to the new regs - not being a 'house basher', I left it at that. I could see and understand the point of ensuring absolutely everything metallic and fixed, was bonded - it avoids the situation of one unbonded item becoming live through a fault and someone touching that and something which is bonded.

The opposite, would be to ensure that no metalwork was bonded and everything is insulated from earth, which is impossible. Basically - an all or none way of thinking.

Neither way is perfect, each carries its own risks.

it's not as rediculous as you say. In the 70's the new builds would be inspected by Seeboard (In my area) before they made the final connexion. They would go round and ensure every bit of metal was bonded and the developers would get the power on before wooden windows, the strip of wood with 6 coathooks, kitchen cupboard doors etc were installed as earthing/bonding hinges, handles etc was a PITA.

EDIT now see this repeats some of EFL

 

[Harry Bloomfield]

Surely that contradicts what you are saying.

Why is it not safer to have as much as possible not earthed?

Because if it is not earthed, then it could be live due to some accidental contact with something which was live - fixing screw through a cable or etc..

 

[EFLImpudence]

But accidents are not one of the considerations on which parts are earthed.

 

[bernardgreen]

But accidents are not one of the considerations on which parts are earthed.

which is precisely why some accidents can have serious repercussions.

 

[EFLImpudence]

No, it isn't. They are less likely than the converse.

 

[JohnW2]

No it is NOT Earthed, it has a significant impedance to Earth, an example is a damp wall where a conductive path can exist from a metal shelf bracket to the Earthed metal back box of a socket in the wall.

It looks as if there is some semantic/terminological quibbling going on here!

I think that EFLI is using 'earthed' to refer to the situation in which something has a "significantly conductive path to the Earth", and I suspect that his criteria for declaring the presence of 'a significantly conductive path' would be those which, with application of the nominal supply voltage, would result in 5MA or 10mA to flow to earth (and if that criterion is satisfied, he would probably conclude that it was 'earthed enough' to need bonding). You seem to be using 'earthed' in a sense which requires a lower impedance path to 'the Earth' than that, but I don't know what criterion you are assuming/suggesting.

Kind Regards, John

 

[JohnW2]

My point is that nothing in electrical installations is simple black and white ( or brown blue and green/yellow ) yet the people who write the regulations seem to think only in black ink on white paper.

I think that the guidelines as to how low an impedance to earth has to be (corresponding to a current of 5mA or 10mA) to require bonding does exist in black ink on white paper somewhere, although, to the best of my recall, not in BS7671.

As I have mentioned before (since I have examples in/around my house) real 'grey areas' exist in relation to conductive parts entering a building which would have an extremely high impedance to earth if measured during a very dry spell but a pretty low impedance to earth if measured under very wet weather conditions. As I've said to EFLI in the past, I would personally not be comfortable to decide that such a conductive part did not require bonding because I had measured a very high (higher than 'my threshold') impedance to earth during a very dry period.

Kind Regards, John

 

[bernardgreen]

would result in 5MA or 10mA to flow to earth (and if that criterion is satisfied, he would probably conclude that it was 'earthed enough' to need bonding)

That current would not be a problem, provided it was not flowing through a person.