Chris Pearson:......Well yes, 2 > 1 = multiple, but the reality is that we don't know where those electrodes (if any) are situated. What I don't quite understand is that if the water supply is through a metal pipe, why has it not pulled the surface potential towards the distributor's earth.
Well it will do, a bit, but presumably the ground there is not so conductive (or the DNO electrodes would also be better connected to terra-firma too. It may also be a plastic water main in the road, and only a few metres of copper underground. In this case bonding the pipes to the neutral at the MET makes plumbing indoors safer, but the outside tap more dangerous.
Actually when PME first came in there was a recommendation for outside taps to have an insulating break in the pipes, but I think plenty never did and even plenty of new ones still do not. It was mentioned in "PME Regulations", which was a statutory document (a companion to the Electricity Supply Regulations in the UK). The later Requirements for the Application of Protective Multiple Earthing to Low Voltage Networks diluted that advice to "An insulating insert may be incorporated in the pipe to an outside water tap."
Certainly in older versions of Guidance note 5, there is stronger advice,
"It is recommended, however, that a plastic insert be provided in the pipe to the outside water tap." near the bit about PME and caravans.
I presume they are reserving judgement that outside taps for anything other than water (oil, propane, milk, beer etc) may come under other rules.
If each DNO electrode is 30-50 ohms and the water pipe is similar then the PEN to terra-firma resistance will still be some tens of ohms.
(The Zs and the PEN, being all-copper will be fractions of an ohm, but that is not the loop we are looking at.)
Chris Pearson:......Well yes, 2 > 1 = multiple, but the reality is that we don't know where those electrodes (if any) are situated. What I don't quite understand is that if the water supply is through a metal pipe, why has it not pulled the surface potential towards the distributor's earth.
Well it will do, a bit, but presumably the ground there is not so conductive (or the DNO electrodes would also be better connected to terra-firma too. It may also be a plastic water main in the road, and only a few metres of copper underground. In this case bonding the pipes to the neutral at the MET makes plumbing indoors safer, but the outside tap more dangerous.
Actually when PME first came in there was a recommendation for outside taps to have an insulating break in the pipes, but I think plenty never did and even plenty of new ones still do not. It was mentioned in "PME Regulations", which was a statutory document (a companion to the Electricity Supply Regulations in the UK). The later Requirements for the Application of Protective Multiple Earthing to Low Voltage Networks diluted that advice to "An insulating insert may be incorporated in the pipe to an outside water tap."
Certainly in older versions of Guidance note 5, there is stronger advice,
"It is recommended, however, that a plastic insert be provided in the pipe to the outside water tap." near the bit about PME and caravans.
I presume they are reserving judgement that outside taps for anything other than water (oil, propane, milk, beer etc) may come under other rules.
If each DNO electrode is 30-50 ohms and the water pipe is similar then the PEN to terra-firma resistance will still be some tens of ohms.
(The Zs and the PEN, being all-copper will be fractions of an ohm, but that is not the loop we are looking at.)
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