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  1. South Spark
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    South Spark Regular EF Member

    Location:
    South East
    I'm installing a 32A socket on a PME system.

    The socket itself is indoors and is on its own RCBO.

    The intention is to use the socket for power for outdoor events.

    My technical help line says that I should disconnect the earth at the socket then TT it. It seems to me that that would make it more dangerous.
     
  2. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    your help line is talking through it's corporate arse. personally I'd install a IP55 socket outside. saves leaving a door or window open. and use the TNC-S earth.
     
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  3. South Spark
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    South Spark Regular EF Member

    Location:
    South East
    It was a specific request to install indoors to avoid vandalism and unauthorised use.
     
  4. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    What reason did they give.
     
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  5. South Spark
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    South Spark Regular EF Member

    Location:
    South East
    Their reasoning was to do with exporting PME. The difference between PME earth and true earth in the event of a failed neutral.
     
  6. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    What would the socket supply.
     
  7. stevethesparks
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    stevethesparks Electrician's Arms

    Location:
    Northampton
    What scheme are you with?
     
  8. South Spark
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    South Spark Regular EF Member

    Location:
    South East
    Niceic
     
  9. South Spark
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    South Spark Regular EF Member

    Location:
    South East
    A PA system apparently. However I suppose I've got to consider multiple possibilities.
     
  10. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    I can see why they are saying this and they could be taking Section 711 into consideration.
     
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  11. Rob
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    Rob Trusted Advisor

    Location:
    North west
    Personally I would just fit one and have done.

    However you could please everyone by installing a decent rod with a good Ra and connect it to the MET of the house.
     
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  12. South Spark
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    South Spark Regular EF Member

    Location:
    South East
    Could you expand?
     
  13. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    It is a bit tenuous but Section 711 deals with shows and stands etc. where PME should only be used if supervised or its effectiveness has been confirmed. Probably way off the mark though.
     
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  14. South Spark
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    South Spark Regular EF Member

    Location:
    South East
    Speaking to them again , they also talked about 711. However they're now suggesting that a risk assessment with a warning that only CLass 2 equipment should be used and a non standard socket fixed.

    How likely is it that the supply neutral would ever be lost?
     
  15. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Sounds like they are back tracking. One way you could look at it is you have been asked to install a socket, by good chance internally. Do this because how they choose to use the power from the socket is up to them. Some people lose hours of sleep worrying about lost neutrals, I do not however it is something that occurs.
     
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  16. Murdoch
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    Murdoch Electrician's Arms

    Location:
    Woking
    A 32 amp socket?
     
  17. happysteve
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    happysteve Electrician's Arms

    Location:
    Nottingham
    Business Name:
    Dovecote Electrical
    It would have to be a very good Ra. :)

    If max demand (for the installation, not just the outside socket) is, say, 30A, then you need an Ra less than 3.35Ω if you want a touch voltage less than 70V (using the formula in A722.3 for EV charging in single phase installations, which is just the potential divider formula rejigged).
     
  18. happysteve
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    happysteve Electrician's Arms

    Location:
    Nottingham
    Business Name:
    Dovecote Electrical
    Regarding the first suggestions (Class 2 only), this is a complete non-starter, unless there's a specifc item of equipment to be used. For general PA... no chance.

    Regarding the second question: there was a study done, "Charging of Electric Vehicles at Domestic Dwellings Using Protective Multiple Earthing (PME) - Risk Analysis PE01417/2012/1" (Commentary on the Wiring Regs mentions it). Increase in individual risk (seriously injured or killed) was 1.1x10^{-6} to 7.8x10^{-9} (sorry, not sure how to get superscript!) - worst case just more than 1 in a million. That's only for people in contact with EV charging systems (not the whole population). Even with risks that low, extra measures are recommended to reduce the risk "if reasonably practicable to do so" such as putting in an earth rod with low Ra.

    In the DPC of the 18th Edition, they are proposing to remove the "if reasonably practicable" option in 722.411.4.1.
     
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  19. South Spark
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    South Spark Regular EF Member

    Location:
    South East
    14976186328671430970548.jpg
     
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  20. Rob
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    Rob Trusted Advisor

    Location:
    North west
    It's RCD protected...

    It's solving the problem of a lost neutral on the DNO side.

    Even then an Ra of 3.35 isn't a huge effort.
     
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  21. happysteve
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    happysteve Electrician's Arms

    Location:
    Nottingham
    Business Name:
    Dovecote Electrical
    I'm confused. Surely an RCD won't help for additional (shock) protection when there's a missing PEN (on the DNO side)? Current flows out through the L, through the RCD, through the load, back through the N, same current back through the RCD (so, balanced), to the PME block (where earthing conductor is connected to PEN), to MET and through CPC to exposed conductive parts, through body of person, to earth.

    I agree that having an earth electrode with good Ra is better than not having one (for PME) to reduce the touch voltage in the event of loss of PEN on the supply side. To remove the shock risk requires a very low Ra, and something like 3.35Ω, depending on the local ground conditions, may be... challenging. :)
     
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  22. Rob
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    Rob Trusted Advisor

    Location:
    North west
    Draw it out Steve, the N>E link is before the RCD, so current flowing down the PE will not go through the RCD causing an imbalance.
     
  23. Midwest
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    Midwest Electrician's Arms

    Location:
    Oxfordshire
    Are you both not saying the same thing?
     
  24. hightower
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    hightower Electrician's Arms

    Location:
    Durham
    On quickly reading and passing through, I've got to agree with happy steve.

    Scenario: Class 1 appliance, no faults present anywhere on installation except broken neutral on PEN conductor. Current is balanced through RCD, earthed conductors rise to a dangerous voltage, link between Class 1 appliance and earth made via a person, RCD is going to say "hey, this looks alright to me"

    What am I missing?
     
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    Last edited: Jun 16, 2017
  25. Midwest
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    Midwest Electrician's Arms

    Location:
    Oxfordshire
    Don't know, my understanding, limited as it is, is broken PEN conductor, is your worst case scenario (don't have nightmares folks) RCD or no RCD.
     
  26. South Spark
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    South Spark Regular EF Member

    Location:
    South East
    Realistically the place has metal pipework disappearing underground that will have a lower resistance than any bit of copper I could bang into the ground.
     
  27. hightower
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    hightower Electrician's Arms

    Location:
    Durham
    Which you can't utilise in your design of this addition.
     
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  28. soulman
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    soulman Electrician's Arms

    Location:
    Durham
    Hi, An Ra of 3.35 is not easily achievable. I have had 6 rods in the ground & managed 20 ohms
     
  29. Midwest
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    Midwest Electrician's Arms

    Location:
    Oxfordshire
    It's remarkable how some threads go off on a upload_2017-6-16_16-47-36.png
     
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  30. soulman
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    soulman Electrician's Arms

    Location:
    Durham
    Apologies, I was just surprised at that comment.
     
  31. Midwest
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    Midwest Electrician's Arms

    Location:
    Oxfordshire

    Which one? :)
     
  32. soulman
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    soulman Electrician's Arms

    Location:
    Durham
    Achieving an Ra of 3.35
     
  33. happysteve
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    happysteve Electrician's Arms

    Location:
    Nottingham
    Business Name:
    Dovecote Electrical
    Okey dokey :) The squiggles inside the circle are meant to represent an RCD. In the event of a broken PEN (and no other fault), the same current passes through both poles of the RCD and there is no imbalance. The current then flows through the CPC, through the person, and to earth.
    broken PEN.jpg

    The picture below just shows that the formula A722.3 is correct.
    touch voltage Ra broken PEN.jpg

    So I still maintain that an RCD will not help in this situation.
     
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  34. Midwest
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    Midwest Electrician's Arms

    Location:
    Oxfordshire
    I'm cooking 'tea' at the mo, so not following the thread, but this vid (Pete's copyright) is often used;

     
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  35. South Spark
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    South Spark Regular EF Member

    Location:
    South East
    Ok long day so bear with me. Informative video but it seems to me that it only applies if there is no other path to earth than the affected person.

    If the PEN is broken then would the current not flow back through the bonding to true earth especially if I introduce earth rods into the system?

    Could I introduce an earth rod connected to the earth terminal in the socket?
     
  36. Midwest
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    Midwest Electrician's Arms

    Location:
    Oxfordshire
    It's a matter of timing :)
     
  37. Des 56
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    Des 56 Trusted Advisor

    Location:
    Gliese 581C
    Utilise the Pme for the socket,add a rod

    Don't employ any Horses,Donkeys or Cows to operate the sound system and every ones happy :)
     
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  38. happysteve
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    happysteve Electrician's Arms

    Location:
    Nottingham
    Business Name:
    Dovecote Electrical
    Sorry South Spark for meandering off a bit...

    Re your first question... yes it would. There has been some to-ing and fro-ing about the numbers, but essentially yes, having an earth rod would make the situation safer.

    Re your second question... If you mean, "connect the earth rod to the socket and still retain the CPC from the TN-C-S" I would advise against this, unless you have 10mm earthing conductor all the way back to the MET (for the same reason that bonding needs to be 10mm). If you mean, "Run the CPC to the socket but don't connect it to the earth pin (or anything exposed) and connect the earth pin to a local rod," then this sounds like "TT-ing your shed" without a sub-board. The protective device (and additional protection) would be on one earthing system, and the socket-outlet on another. I don't know if this is relevant or not, and I've reached the limit of my knowledge about whether this would be permitted or advisable.

    I suppose the way of complying to keep everyone (incluing your CPS advisors) happy would be to treat this 32A socket as a separate board and TT it. So install an RCD garage subboard with the socket nearby, put this sub-board on the earth rod, and follow the usual procedure for making sure exposed conductive parts from the two different earthing systems are not simultaneously accessible (411.3.1.1).
     
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  39. South Spark
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    South Spark Regular EF Member

    Location:
    South East
    Where are you talking about adding a rod? To the socket (disconnect PME earth) or to the MET?
     
  40. davesparks
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    davesparks Trusted Advisor

    Location:
    guildford
    If it is to be used for outdoor events then you'd do well to read up on the requirements of BS7909 'code of practice for temporary electrical systems for entertainment and related purposes'
    This is becoming more commonly recognised in the industry, and councils are increasingly requiring compliance as part of event licensing.

    As it stands you may find you have a grumpy customer on the phone when the event electrician comes along and refuses to use the socket as it is non-compliant.
     
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  41. Midwest
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    Midwest Electrician's Arms

    Location:
    Oxfordshire
    Just checking, that you agree an RCD will not afford additional protection, in the case of a broken PEN conduct?
     
  42. static zap
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    static zap Regular EF Member

    Location:
    west midlands
    Is there a way forward with this situation .
    Detecting under voltage at the supplied load ,
    Supply voltage being shared between Load, and unfortunate Human earth connection .
    (remebering some old RCD technology ran warm
    and needed a suppy to stay latched ON )
     
  43. Rob
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    Rob Trusted Advisor

    Location:
    North west
    Not without the addition of a local earth rod connected to the met.
     
  44. Midwest
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    Midwest Electrician's Arms

    Location:
    Oxfordshire
    Arrgh I see, your recommending installation of earth rod with every PME supply. I was reading your #22.
     
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