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PME and floating neutral

Discussion in 'Electrics UK' started by bernardgreen, 9 Feb 2007.

  1. bernardgreen

    bernardgreen

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    This is a result of a conversation with a retired engineer who has recently moved from a house with TT supply to one with PME supply.

    He noticed a very slight tingle when connecting the hose to the outside water tap so went to check the earth rod was in wet ground and couldn't find one. When I told him the PME system "earth" was taken from the supply neutral he sort of went ballistic........and asked what precuations were taken to ensure the neutral would never float.

    He has a point. If the network neutral did float all CPCs would float with it and while inside the house would be "Faraday cage" safe external water pipes etc would not be at ground potential.

    Is there a maximum permitted neutral to ground and in the event of a fault ( open network neutral and un-balanced loading on the phases ) how are premises protected from thier CPC having a potential far from ground.
     
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  3. ricicle

    ricicle

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    The DNO's neutral should be earthed at various points along its path back to the transformer.That is why PME is only available on certain TT supplies when requested due to inadequate earthing on poles.
     
  4. mapj1

    mapj1

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    It is a very real weakness of PME systems, particularly those fed from overhead singles, that say a tree might take out the neutral and not the live. The short answer is nothing - the metalwork of the house or whatever can go up to poractically 240v until the fault is remedied, so for swimming pools, caravan supplies, radio stations with external masts, and a few other situations where real terra firma earth and PME earth may meet, TT is often preferred if only a 2 wire feed is available.

    However like many things electrical, not enough people are hurt by it to make it worth improving. Remember that in the UK less than 20 folk a year are killed by domestic electricity, (having risen rather since 2005, as more extension leads and adaptors are being used, and as DIY folk are doing less, and so are less aware of dangerous situations) and if that sounds a lot of accidents, set it against 3000 motor fatalities, or 30000 fatal heart attacks and then decide where the money is best spent (as less than one per year is a PME fault death).
    Different parts of the world see it differently. On the Isle of Mann for example, PME is not permitted at all.
     
  5. dingbat

    dingbat

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    Crikey! Does the ONS know you have this information?

    I bet they would pay handsomely to be able to draw such statistically significant conclusions from such a small temporal sample! ;)

    You are right, however, about the relative insignificance of electrical fatalaties when compared to the roads. Shame. We sparks could do with a big well-publicised, electrical fire at least once a week and a couple of hundred stiffs a year would do our cause no harm at all. :D
     
  6. JohnD

    JohnD

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    Quite apart from the ME part of PME, I can't help wondering if he has metal pipes, and if they are main-bonded correctly.
     
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  8. bernardgreen

    bernardgreen

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    All metal pipes and nicely bonded via green and yellow cables to one side of the incoming 230 volt supply.

    So if that side of the incoming 230 volts is not at ground potential then the outside water tap is not at ground potential.
     
  9. JohnD

    JohnD

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    yes... but I was thinking that the bonded metal pipes are buried in the (presumably quite damp) ground... maybe the pipe to the outside tap is, too (depending on isolating effects of DPC) if it just sticks through the house wall.

    At the point where the bonded pipes meet the earth, the earth, neutral and pipes are all going to be at (nearly) the same potential, and even though we're not supposed to use utilities pipes as earth spikes, they will still have that effect.

    Something wrong, surely?
     
  10. bernardgreen

    bernardgreen

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    The pipes are not buried, the outside tap is feed through the wall straight from the metal pipe work under the kitchen sink.

    No metal pipes leave the house. Water and gas incomers are plastic so other than via the water in the incoming water main there is no conductive route out of the house to ground ( other than the PME via the incoming neutral )

    Nothing wrong as far as the regulations require.
     
  11. JohnD

    JohnD

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    Ah... that explains it :(

    Maybe we ought to be using spikes again. I suppose the incoming cable sheath isn't going to help either.
     
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