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Steel Clad Building and High Voltage Overhead Cables.

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Views invited please.


There is a modern sheet steel clad building nearby. It has what appears to be plastic covered steel sheets as exterior walls. It is used as a Scout Hut and can probably accommodate about 80 people for events. Running overhead are three high Voltage cables that feed local transformers, including the scout hut transformer which is on a pole nearby. The scout hut has two earth electrodes which connect to a part of the building and earth it. Although they are quite close together so may not really count as two in function.


Theoretically what would happen if one of the H.V. overhead cables came adrift and fell onto the roof of the building regarding risks?


This is a theoretical question relating to a real building but with a serious intention.


Thanks,


Z.


  • 0
    Personal observation here in my residual area: When tree branches are blown against the HV line it just arcs and the line remains active.


    Also squirrels when closing the circuit between the tree and the HV line just get fried and again the line stays active. 


    Peter Brooks MIET

    Palm Bay Florida USA
  • 0
    We should not cater for anything except U.K. practice on this forum. I would be most surprised if the roofing was not earthed.


    Jaymack










































































    mg except UK practice m,t
  • 0

    Peter Brooks:

    Personal observation here in my residual area: When tree branches are blown against the HV line it just arcs and the line remains active.


    Also squirrels when closing the circuit between the tree and the HV line just get fried and again the line stays active.




    Both effects are because most parts of the USA (outside of calif) use the distributed neutral on the HV side, so a fault to earth appears as a normal load to the supply.


    In the UK, any current flowing to the star point is fault current or leakage. (as mentioned upthread).


    The squirrel would still be dead however.

  • 0
    I agree!


    Over and out!


    Peter Brooks MIET

    Palm Bay Florida USA
  • 0
    Probably an Auto Recloser to burn off the wildlife.


    Jaymack
  • 0
    If it was my Scout Hut and the external walls were metallic I'd be concerned about touch potential - the voltage between the feet and the hand of somebody's child standing on hard ground and leaning against the building.  I'd address this with a buried earth tape bonded to the building steelwork run around the outside of the building about 1m out and 0.5m deep.
  • 0
    I do not see why we may not consider and discuss what is done in other countries - just because US wiring does not meet our rules, does not mean they are stepping out of the front door over dead bodies in the morning ,and alternate approaches are worth looking at sometimes. Personally I think we should be open to discuss any other situations too. In any case HV is not included BS7671, UK style HV or anyone else's.

    It may be important to be clear  where things are the same or different, to avoid mis-advice to the wrong person, but  the title of this forum is "wiring And  regulations BS7671", not "ONLY how to do wiring to the regulations",   actually there is already a book for that - available from this very website ?


    Bad luck about the squirrels though, a goner in all systems. Here the birds on the wires stand a better chance, though not normally seen on 33kV and above.


    (As an aside, my avatar if you can see it, is a single wire earth return substation on a pole - hence only one HV wire and one insulator - I know its not to BS7671, or the US method, I'm not changing it.)
  • 0
    I will make one more comment concerning birds and local HV lines here in our area.


    The Local Power Company places Nesting boxes on some of the wooden power poles near the river - They are located about a foot above the HV line and used by Osprey during breeding seasons (which has just finished).


    Peter Brooks MIET

    Palm Bay, Florida USA
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    mapj1:

    Downed bare HV cables onto hedges etc are normally dead soon after landing.




    The worrying word here is 'normally'. I am aware of a case, a number of years ago now near where my mother grew up (S. Glos), of the son of a farmer who was in his tractor in a field using the hedge trimmer attachment when it met a downed HV cable which unfortunately was not dead.....

  • 0
    On the 21st May 1950 (today as far as the day/month date goes) a violent tornado swept through southern England from Buckinghamshire to Cambridgeshire. It wrecked houses and other buildings, uprooted trees and lifted cars and livestock into the air. The tornado was followed by violent thunderstorms which claimed at least three lives.


    Z.

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