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Discuss When PME is NOT recommended.....why?? in the Electrical Forum area at ElectrciansForums.co.uk.

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  1. Rockingit
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    Rockingit Electrician's Arms

    Location:
    Somerset
    Something that's always bugged me, and I've never fully understood WHY (just know that's it what the regs say) is why certain installations are prohibited from using TN earthing, only TT (caravan sites etc etc).

    Anyone care to enlighten me?
     
  2. WPD Lad
    Online

    WPD Lad Guest

    I can post something from work tomorrow on earthing arrangements for different developments, steel framed divided buildings, flats caravans etc all have regs. ESQC regs etc

    If there is external metal work on the caravan then no PME can be given, if it's going into a brick enclosure then we can provide it
     
  3. rich.250
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    rich.250 Electrician's Arms

    Location:
    Cornwall
    Business Name:
    Rich Williams Electrical
    I think caravan hook ups are classed
    As temporary supplies hence tt, I was told tt is used for
    Temp supplies, agriculture. Due to the increased risk and to limit the pefc.
     
  4. GLENNSPARK
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    GLENNSPARK Electrician's Arms

    Location:
    west yorks.
    well surely if theres exposed metalwork in contact with the ground and this is essentially acting like an extrainious conductive part and the install was PME..in an area that could be subject to disturbance...like a building site...then the risk of dangerous potentials would exist....so by TTing it all.....would keep potentials equal wouldn`t it??.....
     
  5. Rockingit
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    Rockingit Electrician's Arms

    Location:
    Somerset
    This is where it gets to the bit I've never figured out why it's safer: in a tn system (assuming that everything is bonded properly) you've effectively got everything that can conduct under a fault all tied together back to one place using a system of actual copper cable, the resistances are near irrelevant mathematically so you get a nice fast disconnection time under ADS.

    But in a TT, it's all a bit left to chance with much bigger resistances and slower disconnection times (hence why needing RCD backup).

    Maybe our expert friend from the Orient can assist?
     
  6. rich.250
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    rich.250 Electrician's Arms

    Location:
    Cornwall
    Business Name:
    Rich Williams Electrical
    Yes I am unsure.
    Although higher resistances does mean lower fault currents.
     
  7. rich.250
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    rich.250 Electrician's Arms

    Location:
    Cornwall
    Business Name:
    Rich Williams Electrical
    Well pefc
     
  8. telectrix
    Online

    telectrix Scouser and Proud of It Trusted Advisor

    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    it's only TNC-S that is banned from caravan sites and the like. think it's more to do with what would happen if the neutral were to be lost. as the earthing is connected directly to the neutral at the cut-out, you have lost the earth as well so .............
     
    Last edited: Feb 22, 2012
  9. rich.250
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    rich.250 Electrician's Arms

    Location:
    Cornwall
    Business Name:
    Rich Williams Electrical
    Good point tele.
     
  10. Rockingit
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    Rockingit Electrician's Arms

    Location:
    Somerset
    Hmmm....so basically it falls to us to safeguard against the DNO messing up. That's a first.
     
  11. GLENNSPARK
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    GLENNSPARK Electrician's Arms

    Location:
    west yorks.
    well...its only like the scenario my boss n I were faced with the other week....that job with the shared supply and i couldn`t get a Ze on it......bloody DNO not ensuring there was not adequate earthing for THEIR APPARATUS.....and a cutout with both the knockouts missing.....of course client was charged for a PME connection to be supplied and we were slated as "know nowts" by this obnoxious little bint on the end of the phone for pointing out these failings to em...
     
  12. WPD Lad
    Online

    WPD Lad Guest

    Has their ever been a DNO earth at the property? If not, they're not obliged to give you one, if there was one (legit one) then they're obligated to maintain it!
     
  13. Rockingit
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    Rockingit Electrician's Arms

    Location:
    Somerset
    OK then, here's a different angle to it: If for arguments sake we're talking about a caravan park, TNC-S provided to the supply, if we ignore the supplied earth but bang down our own rod and use that, does that a) still satisfy regs/EQSR and b) achieve the same end result? When is a TT not a TT? Given that it won't be that far (in theory) to the DNO's nearest PME point in the ground anyway.
     
  14. SPARTYKUS
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    SPARTYKUS Trusted Advisor

    Location:
    Shropshire
    Answer: anywhere where the IET don't manage to convince you that the laws of physics changed in the 70's & its ok to suddenly use your return path as your earth too.
     
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