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Discuss Metal lamppost in garden... TNC-S in the Electrical Forum area at ElectrciansForums.co.uk.

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  1. weevilward
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    weevilward Active EF Member

    Location:
    United Kingdom
    Evening all,
    Need some opinions on this one please:
    TNC-S house, complete new install, all going to plan, nearly finished second fix, customer then arrives with a huge metal lamppost (bought from Frinton-on-sea promenade...) and wants it hooked up in the back garden.
    What is the general consensus on this - my concern is: in the event of loss is supply neutral, everything connected to MET, including that lamppost, will rise to a voltage, anyone touches it may get a shock?
    Or, will the fact it's planted in the earth keep any possible voltage down?
    Am I making something out of nothing here or would this be safer as a TT?

    Any thoughts welcome.
     
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  2. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    What is it with this neutral loss, in the house no one bothers but go outside and there are shouts of ROD IT.
     
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    Last edited: May 19, 2017
  3. weevilward
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    weevilward Active EF Member

    Location:
    United Kingdom
    Ok, so touching a radiator bonded in the house in no different to touching a lamppost bonded in the garden? Is that what you're getting at?
     
  4. weevilward
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    weevilward Active EF Member

    Location:
    United Kingdom
    Ok, so touching a radiator bonded in the house in no different to touching a lamppost bonded in the garden? Is that what you're getting at?
     
  5. Flanders
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    Flanders Regular EF Member

    Location:
    Tamworth
    Im think there is somthing in special location for lamp post but it may be relating to street furniture
     
  6. Leesparkykent
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    Leesparkykent You Rock Gmes Staff Member Trusted Advisor

    Location:
    Kent
    The post will propably act as one big rod.
     
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  7. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Most lighting columns on the highway are PME they seem okay.
     
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  8. weevilward
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    weevilward Active EF Member

    Location:
    United Kingdom
    So I'm reading too much into it.
    Thanks fellas. Never thought twice about domestic outside lighting before, but when something different comes along it makes you think twice...
     
  9. Richard Burns
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    Richard Burns Trusted Advisor

    Location:
    Cambridgeshire
    Business Name:
    Richard Burns
    This is probably a case of minimal risk. In general you are not constantly handling a lamppost, as opposed to hand held electrical equipment, and so the risk of neutral loss fault occurring and it being a risk due to contact are low.
    Bonding will have limited effect since the potential you are trying to eliminate is between earth and the supply and bonding will only limit the PD between the MET and the supply.
    Within a property there should be no potential differences because the bonding connects all sources of potential difference together. Outside a property without installing conductive mats in the ground you cannot avoid being in the presence of a nominal 0V from true earth and so the risk is increased.
     
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  10. Paul mccullagh
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    Paul mccullagh EF Member

    Location:
    Cambridgeshire
    Run swa to the lamp post (volt drop calculations) bonded on a 30MA rcd in the consumer unit job done!
     
  11. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Why would it require rcd protection.
     
  12. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    This was suggested many years ago by the ILE but was felt the column would be unreliable as an electrode due to corrosion, protection on the column to prevent corrosion and the means of bedding it in the ground.
     
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  13. weevilward
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    weevilward Active EF Member

    Location:
    United Kingdom
    It doesn't need RCD but it wouldn't hurt to have it.
    Yeah this one is pretty rusty!
     
  14. weevilward
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    weevilward Active EF Member

    Location:
    United Kingdom
    Thanks Richard, this makes sense.
     
  15. Paul mccullagh
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    Paul mccullagh EF Member

    Location:
    Cambridgeshire
    I would personally protect this with a rcd due to someone dismantling fixtures and changing a light bulb!
     
  16. wirepuller
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    wirepuller Trusted Advisor

    Location:
    south uk
    Well you cud'a knocked me down with a feather when I saw this, normally it's only sheds that bring people out in a cold sweat when TNCS is mentioned. I think I've pointed out before that those who come over all unnecessary at the mention of a shed never seem to get that queasy feeling with other outdoor installs.
    So good on'yer Weevilward, shows you are thinking about what you are doing rather than just following the herd and subscribing to myths.
     
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  17. static zap
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    static zap Regular EF Member

    Location:
    west midlands
    Is it the tallest piece of garden furniture , on a Hill Top !
    (that's the other kind of rod ! )
     
  18. weevilward
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    weevilward Active EF Member

    Location:
    United Kingdom
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  19. stevethesparks
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    stevethesparks Electrician's Arms

    Location:
    Northampton
    How many people think to make their outdoor socket TT, whats the difference?
     
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  20. dmxtothemax
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    dmxtothemax Regular EF Member

    Location:
    Australia
    Business Name:
    David Haddock Electronic Repairs
    If you have any concerns over safety,
    then simply fit an RCD inside the house
    this will not only protect the post but also the wiring to it.
    you can never be too safe
     
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  21. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    Whilst rcds do improve safety it won't protect against an open circuit neutral on whatever the earthing system is.
     
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  22. Midwest
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    Midwest Electrician's Arms

    Location:
    Oxfordshire
    I was in a conundrum some time ago, when a customer wanted me to connect up a previously installed metal lamp post at the bottom of their driveway. It wasn't a full lamppost, but one of the smaller ones mounted on brick wall. However, it was located adjacent to the public footpath. The property had a PME supply, and a swa cable had been previously installed. The CU didn't have a spare way for additional protection by RCD or compatible new RCBO's.

    I read chapter 714, BS7671, outdoor lighting installations. This chapter seems only applicable to outdoor lighting in the public domain, street lighting etc. Whilst a light in someone's garden, is more under control and constant supervision than a street light would be, it does seem strange to have some many regs and guidance specifically for street lights and none per say for domestic lighting situations?

    My conundrum was the additional protection. Advice from my scheme, in these circumstances, was to provide RCD protection.
     
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  23. Flanders
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    Flanders Regular EF Member

    Location:
    Tamworth
    This is what I was refering to in me post yesterday i did not have the byb with me at the time, just had a look now and as you say it refers more about street furniture but it does say it includes gardens under the scope 714.1 (i) so i would use this section of the regs for the ops installation 714.411.203 covers pme
     
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  24. Midwest
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    Midwest Electrician's Arms

    Location:
    Oxfordshire
    Not sure that 714 includes domestic gardens, more places accessible to the public. But I take your point, it doesn't include domestic situations in it's exclusions.

    Lots to cover, if it does though. :)
     
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  25. dmxtothemax
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    dmxtothemax Regular EF Member

    Location:
    Australia
    Business Name:
    David Haddock Electronic Repairs
    just because it is not mandatory
    doesn't mean you cannot do it !
    If someone wants to be extra carefull ?
    then fitting a RCD would do just that
    provide more safety over a non rcd post.
     
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  26. Wilko
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    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
    I'm struggling here - if neutral was o/c any current flow would unbalance the device and cause it to trip (?)
     
  27. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    No imbalance as the same amount of current will flow between live and neutral in the rcd but if in pme the earth would also rise to possible mains voltage if supply neutral is broken back to the source of supply (DNO cable)
    There's no imbalance so rcd won't operate
     
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    Last edited: May 21, 2017
  28. Selfmade
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    Selfmade Regular EF Member

    Location:
    Aberdeen
    Is the problem with o/c neutrals not more to do with overloading the 1.0 / 1.5mm2 cpc if all the neutral current (including from shower, cooker etc) is trying to get back to the supply neutral via said cpc, rather than anything to do with shock? And isn't that why we have to run 10mm2 main bonding wherever we take power from a PME supply (because this is the smallest that would be unlikely to be overloaded in these circumstances)?
     
  29. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    If the neutral is broken at the supply side on pme then the current will be unable to return to the source via the neutral and as there is a link in the cut out that only exists in pme between neutral and earth then the current will flow through the earthing conductor and through all connected cpcs and bonding conductors.

    Yes 10mm to deal with neutral currents.
     
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  30. Selfmade
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    Selfmade Regular EF Member

    Location:
    Aberdeen
    Potentially putting very high currents through very small CPCs if these are connected to earth (e.g. by this lamp post) and the PME still has connections to earth on the supply side of the break.

    So surely the OP should be putting 10mm2 bonding in place to this lamp. Or else somehow insulating it from earth.
     
  31. wirepuller
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    wirepuller Trusted Advisor

    Location:
    south uk
    The purpose of bonding is to create a zone of equal potential in the event of a fault to reduce shock risk. As the lamp post will be surrounded by the general mass of earth at or about 0v it is not possible to create a zone of equal potential so there is no point in main bonding the post.
     
  32. Selfmade
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    Selfmade Regular EF Member

    Location:
    Aberdeen
    So perhaps bonding is the wrong term. But we must surely do something to remove the risk of up to 100A going through a 1.0mm2 CPC?
     
  33. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    Why 100amp?
    I=V/R
     
  34. Selfmade
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    Selfmade Regular EF Member

    Location:
    Aberdeen
    Assuming it's domestic 100A is the max rating of the cutout.
     
  35. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    The amount of current depends on the voltage and any resistance in the circuit.
    I=V/R
     
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  36. Selfmade
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    Selfmade Regular EF Member

    Location:
    Aberdeen
    Yes, point taken, so in practice it would probably never reach 100A, but surely it is theoretically possible to overload the CPC, depending on what equipment is running while the neutral is open circuit. It cannot exceed 100A (at least not substantially for very long) because of the cutout.
     
  37. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    But how can anything run if there's no neutral
     
  38. Billythekid
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    Billythekid Regular EF Member

    Potential difference Ian.
     
  39. Selfmade
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    Selfmade Regular EF Member

    Location:
    Aberdeen
    The path I'm seeing is as follows: supply transformer, supply line conductors, cutout, meter, CU incl. MCBs, final circuit lines, loads, final circuit neutrals, CU, cutout neutral/earth, main earth conductor, CU earth bar, final circuit CPCs, earthed metal work also connected directly to earth (e.g. post lamp), mass of earth, PME earth connection on supply side of break, transformer. So we have the full installation current going through that one CPC.
     
  40. Billythekid
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    Billythekid Regular EF Member

    Depending where the break in "Neutral" (PEN) is, you also have your neighbours loads to consider. Also, if a certain Entertainment provider is added to the mix it makes a hell of a mess.
     
  41. Selfmade
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    Selfmade Regular EF Member

    Location:
    Aberdeen
    So I was wrong about the 100A limit. Even worse. Not sure who the entertainment provider is?
     
  42. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    potential difference is basically voltage.
    If you have no neutral return path then no current can flow to make a circuit operate as there is no potential difference between the 2 points to run a circuit so nothing will work.
     
  43. Selfmade
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    Selfmade Regular EF Member

    Location:
    Aberdeen
    But the neutral path is there - via the CPCs and earth mass to the PME earth links (see my previous post).
     
  44. Billythekid
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    Billythekid Regular EF Member

    Ian you're thinking single phase, if the street/area is supplied with a 3ph+n cable and the neutral (PEN) is lost, you now have an unbalanced three phase circuit with no neutral.

    HTH
     
  45. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    The neutral is the earth in pme after the cut out so if it's broken how do you have the general mass of earth?
    Unless there's an extraneous part which is bonded.
    Your lights aren't going to work put it that way.
    If someone is unfortunate to touch say a gas pipe then they will create a potential difference as they are standing on the general mass of earth and may get the full mains voltage
     
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