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P.M.E. Installation Earth Electrode. Historic.

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The Regulations for  the Electrical Equipment of Buildings, 13th Edition, 1955 regarding P.M.E. supplies required that provision be made for the installation neutral to be connected to earth continuity conductor at the origin of the installation by the supply undertaking. (Reg. 409). Where a P.M.E. supply was to be used the regulations said, compliance with Clause C was "desirable" as "an added precaution." This meant that the installation was to have its own earthing electrode. A solid metal to metal buried water pipe system was suggested as an earth electrode but metal gas pipes were prohibited for use as such. A note mentions the increasing use of non metallic water pipes.


So, back then with a P.M.E. system, an additional installation earth electrode was recommended as an added precaution..


What happened to that requirement?


Note. The 14th Edition (1966) forbade the use of public gas and water service pipes as the SOLE earth electrode of an installation. (D 35). But was the need for an approved consumer's independent earth electrode done away with in P.M.E. systems.? And why?


Z.
  • 0
    Presumably they thought that bonding to the water pipes would serve that purpose anyway.

       - Andy.
  • 0
    Perhaps. Although in that case you may have expected the electrode requirement to remain for buildings with plastic service pipes. I suspect that there was pressure to remove it as an extra cost at installation, and something else to maintain and test. that did not do much when all was working well.


    The regulations have never really grasped the nettle of interconnected pipes with the neighbours - the unspoken point is that the shared metal services case is actually quite different to an isolated electrode in the ground.

    If next door has PME too, and bonding is as it should be then the pipe is neutralled in two places, and the potential for the full neutral current to flow in the pipe, and no-one realise, the effective 'Zs' to the water pipe may be tens of milliohms, but if you measured it as an electrode, it could be a few factors of ten higher.

    Meanwhile the addition of domestic water meters with plastic bodies has now largely broken that assumption, even where the incoming main is metal,  and now we in effect have  a large no of horizontal electrodes  that are a few metres long.

    Mike.
  • 0
    Although in that case you may have expected the electrode requirement to remain for buildings with plastic service pipes.

    I was thinking that banging in a rod or similar is next to useless for coping with broken PEN conditions. Foundation electrodes might be a bit better (but possibly still not ideal especially in poor soil conditions given our relatively large single phase supplies and so potentially large N currents) - but are completely impractical anyway, especially for existing buildings. Whereas something extensive like a water distribution system might just work. So the choice boils down to service pipework or nothing - if the water is metallic you get that for free via the main bonding anyway, otherwise there's nothing that can be done. So no specific requirement need be written.


        - Andy.
  • 0
    Thanks both.


    Z.

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