Question about earthing

[ebee]

PS

I see nothing wrong with putting rods etc in on TN systems as it helps to keep you installation "earth" near to local "earth" thereby reducing step/touch voltages but you might have to watch out for the gradient close to the rod itself though

 

[bernardgreen]

If your gas supply is metal and in contact with the local earth then it will be at local earth potential, if it is isolated in some way and you rod it to local earth then you might make the danger worse not better .

Rodding it to local ground, on which the meter reader is standing / kneeling will ( should ) ensure meter and meter reader are at the same potential.

The hazard for the meter reader is when the MET potential is different to the true ground potential and this potential is exported through the wall and onto the metal gas meter.

An electric lawn mower with its metal frame and handles connected to the earth CPC presents the same hazard, namely the risk of an electric shock from a potential difference between the MET and true ground. And it is a shock that the RCD will not detect. Is this the reason that is it is strongly recommended that lawn mowers and other out door tools are double insulated and often have the manufacturer's instruction. "This appliance must NOT be earthed". The "earthed" meaning connected via the CPC to the incoming neutral in all but TT supplies.

Why is the gas meter not treated in the same way as the lawn mower, it is a metal case object outside the equipotential zone.

I believe answer may be that the meter reader is a "skilled" person who will have been made aware of the potential shock hazard from gas meters and will take sensible precautions thus allowing gas meters to be bonded even when with a plastic gas pipe supplying the meter there can be no extranous voltage imported.

 

[ebee]

Bernard, I wasn`t thinking so much about the meter reader kneeling outside on the ground to read the meter (nevertheless a good point though!) but more like its connected pipework if metal being inside the location at one "earth" ref and the rest of your system being at another unless you deliberately joined them electrically as one

 

[EFLImpudence]

To surmise -

If a service supply (gas) is through a plastic pipe and tests determine that the internal metal pipework is, because of this, not an extraneous part then it , correctly, does not require main bonding.

However, because of this the (only just) external gas meter may be at a different potential to the ground.
This results in the suggestion that, to protect the meter reader, the meter and internal pipework should be main bonded to the ground, with a rod, thus making them an extraneous conductive part to the internal supply which, because of this, now require main bonding to the MET.

The situation for the meter reader is the same whether or not the internal pipework is or is not main bonded.

Thus, with plastic supply pipes, the only solution to this problem would seem to be that all internal pipework should be main bonded, whether required or not, plus the meter and internal pipework shall be connected to an earth rod.

Perhaps an easier solution would be for the meter readers to carry their own earthing lead and spike.

 

[JohnW2]

If a service supply (gas) is through a plastic pipe and tests determine that the internal metal pipework is, because of this, not an extraneous part then it , correctly, does not require main bonding.

Those 'tests' might be less than straightforward since it might well involve dismantling pipework- there will often be paths to earth from internal metal gas pipework due to mechanical+electrical continuity with water pipework.

This results in the suggestion that, to protect the meter reader, the meter and internal pipework should be main bonded to the ground, with a rod....

Seemingly - but I'm not sure I would call that 'main bonding'...

.... thus making them an extraneous conductive part to the internal supply which, because of this, now require main bonding to the MET.

Indeed, the conductor joining the rod to the internal pipework would then be an extraneous-conductive-part.

Thus, with plastic supply pipes, the only solution to this problem would seem to be that all internal pipework should be main bonded, whether required or not, plus the meter and internal pipework shall be connected to an earth rod.

I'm not sure that even that would be much of a solution for the (outdoor) meter reader. You seem to be assuming that an earth rod would be capable of pulling down a high 'earth' potential (relative to the true earth on which the meter reader was standing) on a TN sytem (particularly TN-C-S) - and I serioulsy doubt that is necessarily the case, particularly in the face of a supply-side fault.

Perhaps an easier solution would be for the meter readers to carry their own earthing lead and spike.

Same problem as above - unless (s)he actually stood on the rod and managed to avoid contact with any 'true true' earth. Maybe carrying an insulating mat (and perhaps also gloves and/or other PPE) would be better!

As Bernard often points out, a similar, probably greater and more common, problem results from outside water taps plumbed with copper to main-bonded pipework.

Kind Regards, John.

 

[ebee]

An outside tap is safer with an insulating link of plastic pipe particularly on TNC-S (PME) supplies yes.
An earth rod to the MET on PME supplies will help to hold the touch voltages down and be of particular help on PME supplies in the rare event of a broken PEN.
Thankfully a broken PEN is relatively rare (but unfortunately not as rare as once envisaged) and bonded parts can still rise quite appreciably towards L thru any connected loads. Although a rod will help keep it down a bit it could still be very high though

 

[JohnW2]

An outside tap is safer with an insulating link of plastic pipe particularly on TNC-S (PME) supplies yes.

Quite. One technical issue is obviously that if metal pipework from the tap enters the house before an ('internal') insulating link, that pipe would theoretically need main bonding.

An An earth rod to the MET on PME supplies will help to hold the touch voltages down and be of particular help on PME supplies in the rare event of a broken PEN.

Yes, it will obviously 'help' - but (as you go on to agree) I was suggesting that it will probably not 'help enough' in the face of a major supply-side fault (like a broken PEN in a bad place) to be of much use.

Kind Regards, John.

 

[EFLImpudence]

If a service supply (gas) is through a plastic pipe and tests determine that the internal metal pipework is, because of this, not an extraneous part then it , correctly, does not require main bonding.

Those 'tests' might be less than straightforward since it might well involve dismantling pipework-

It would.

there will often be paths to earth from internal metal gas pipework due to mechanical+electrical continuity with water pipework.

I agree but, if not extraneous, need not be main bonded.
It could require supplementary bonding in certain locations.

This results in the suggestion that, to protect the meter reader, the meter and internal pipework should be main bonded to the ground, with a rod....

Seemingly - but I'm not sure I would call that 'main bonding'...

What else would it be?
As, from the outside, the internal pipes are extraneous and should be connected to the outside main earth - the ground.

Thus, with plastic supply pipes, the only solution to this problem would seem to be that all internal pipework should be main bonded, whether required or not, plus the meter and internal pipework shall be connected to an earth rod.

I'm not sure that even that would be much of a solution for the (outdoor) meter reader. You seem to be assuming that an earth rod would be capable of pulling down a high 'earth' potential (relative to the true earth on which the meter reader was standing) on a TN sytem (particularly TN-C-S) - and I serioulsy doubt that is necessarily the case, particularly in the face of a supply-side fault.

So, there is no solution.

Perhaps an easier solution would be for the meter readers to carry their own earthing lead and spike.

Same problem as above - unless (s)he actually stood on the rod and managed to avoid contact with any 'true true' earth. Maybe carrying an insulating mat (and perhaps also gloves and/or other PPE) would be better!

That sounds like an easy solution deducing, as above, that there is nothing the electrical installer can do.

 

[JohnW2]

there will often be paths to earth from internal metal gas pipework due to mechanical+electrical continuity with water pipework.

I agree but, if not extraneous, need not be main bonded. It could require supplementary bonding in certain locations.

Whilst I agree with your latter comments, I'm not sure (in context) what point you're trying to make here. My point was that those paths to earth via water pipework might mean that you could only do your 'tests' (to determine whether the gas supply pipe constituted an extraneous-conductive part) by dismantling pipework.

Seemingly - but I'm not sure I would call that 'main bonding'...

What else would it be? As, from the outside, the internal pipes are extraneous and should be connected to the outside main earth - the ground.

You've somewhat lost me here. Are you really suggesting that you would be trying to create an outdoor equipoential zone (the one, and only, purpose of bonding)?

So, there is no solution. ... That sounds like an easy solution deducing, as above, that there is nothing the electrical installer can do.

Oh, there could be a solution (for the meter reader) and the electrical installer could certainly implement it, IF the regualtions so required. It would simply require the regs to insist on an insulating segment between an external meter and interior pipework and for bonding of anything within the premises to anything on the meter side of that insulating segment to be forbidden.

Kind Regards, John

 

[EFLImpudence]

there will often be paths to earth from internal metal gas pipework due to mechanical+electrical continuity with water pipework.

I agree but, if not extraneous, need not be main bonded. It could require supplementary bonding in certain locations.

Whilst I agree with your latter comments, I'm not sure (in context) what point you're trying to make here.My point was that those paths to earth via water pipework might mean that you could only do your 'tests' (to determine whether the gas supply pipe constituted an extraneous-conductive part) by dismantling pipework.

Oh right. I thought you were referring to the final situation.

You've somewhat lost me here. Are you really suggesting that you would be trying to create an outdoor equipoential zone (the one, and only, purpose of bonding)?

But that is what is needed to protect the meter reader.

So, there is no solution. ... That sounds like an easy solution deducing, as above, that there is nothing the electrical installer can do.

Oh, there could be a solution (for the meter reader) and the electrical installer could certainly implement it, IF the regualtions so required. It would simply require the regs to insist on an insulating segment between an external meter and interior pipework and for bonding of anything within the premises to anything on the meter side of that insulating segment to be forbidden.

Good point. I hadn't thought of that - simple.

 

[JohnW2]

You've somewhat lost me here. Are you really suggesting that you would be trying to create an outdoor equipoential zone (the one, and only, purpose of bonding)?

But that is what is needed to protect the meter reader.

In a literal sense, I suppose you're right - but we're not used to talking/thinking about trying to create an equipotential zone in a garden - and we would probably have to modify our usual definition of 'main bonding' a bit to encompass that situation, given that gardens don't come with a MET!

Oh, there could be a solution (for the meter reader) and the electrical installer could certainly implement it, IF the regualtions so required. It would simply require the regs to insist on an insulating segment between an external meter and interior pipework and for bonding of anything within the premises to anything on the meter side of that insulating segment to be forbidden.

Good point. I hadn't thought of that - simple.

Indeed - and, of course, as ebee sugegsted, exactly the same could apply to outside taps, and probably a few other situations I haven't thought of. Outdoor, 'touchable' Class I light etc. fittings would present a bit of a problem - I guess one would have to insist that the installation's CPC was not connected to it and that, instead, its exposed-conductive-parts should be connected to an earth rod (and the circuit be RCD protected).

Kind Regards, John.

 

[Spark123]

It seems to me that in the original post the person talking about an earth rod does not know the difference between earthing and equipotential bonding.
Why would a meter reader touch the meter? I read mine by looking at it.
35v does seem a bit high to me for the MET to mother earth.
The purpose of bonding is to keep ECPs at or about the same potential in a fault, the risk of someone being in contact with an ECP and mother earth at the same time is considered low hence why we don't need to bother ourselves with bonding the outside world.

 

[EFLImpudence]

Department of the Bleedin' Obvious to the rescue.

 

[JohnW2]

It seems to me that in the original post the person talking about an earth rod does not know the difference between earthing and equipotential bonding.

Very likely, although EFLI has somewhat stirred that up by introducing the concept of outdoor equipotential bonding, to create an outdoor equipotential zone, at least in the vicinity of the meter (which is what matters to someone touching the meter)!

Why would a meter reader touch the meter? I read mine by looking at it.

Agreed. I think that whoever introduced the 'meter reader' got it slightly wrong. The debate usually relates to those 'working on the meter' (or, similarly, delivering oil to a tank).

Kind Regards, John.

 

[Spark123]

I'm still of the chain of thought that it is up to the electricity supplier, where they provide an earth, to ensure it is at or about the same potential to mother earth to reduce the risk.