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:

lyledunn:

You are correct Alan, the last house is next door at the end of line about 150m away from the house in question. On the line running left on my sketch the first section is overhead to a pole where it feeds another TT property. At the same pole the cable goes below ground and feeds 3 more domestic properties on a run of about 500m to the last.

How many of the three further properties are TN-C-S? Is it likely that after at least 650 m, Ze could be < 0.35Ω?

Very good point Chris, but I cannot answer that. The lengths I gave were all estimates based on how far I can hit a golf ball!. I had to get into my van and drive to the farm from the house. The overhead from the transformer near the farm went as the crow flies to the first property then went underground and along the road before being teed off to the house in question. I know where the tee is on the side of the road because the first DNO crew dug it up a week earlier as they suspected a neutral failure at that point. The front lawn is at least a 60m pitch shot with a good whack with my hybrid and perhaps another pitch to get back to the transformer, so maybe 300m .  What I can tell you is that when the fault was fixed Ze at the house was  0.28 ohms with P to N displaying the same. Relatively small transformer so I guess the conductors must have been at least 50mm2. The last house looked further away than it actually is on Google maps so maybe they are just on the threshold.  By the way, the incoming water service is MDPE with copper thereafter. And dont go doing resistivity and impedance sums as my golf shots often fly well over the green!

Alan Capon:

lyledunn:

The only change I would make to Lyle’s drawing, is to extend his TNC-S supply past the “TNC-S” house and add one more earth electrode (obviously with a further resistance) to the Supplier’s neutral conductor. 


Regards,


Alan. 

With another electrode placed beyond the house the diagram is starting to look a lot like a diagram of three pole earth resistance testing.


Andy B.