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Steel Clad Building and High Voltage Overhead Cables.

Zoomup
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
    Farmers have shown concerns about steel buildings and H.V. overheads. Wombat's comments about an exploding transformer is worrying. Wasn't somebody on this forum recently going to buy a house next to a transformer?

    https://thefarmingforum.co.uk/index.php?threads/shed-and-powerlines.17841/


    Z.
  • 0
    I would hope that the metal roof and wall cladding is bonded to earth. In such cases I would expect that contact between HV conductors and bonded metalwork would promptly operate the HV fuses or circuit breaker.

    If the metal cladding is not bonded, even accidently or fortuitously, then it would become live and a fire started. Consider that the non bonded metal work might have a resistance to earth of a few thousand ohms. That would pass about an amp, and dissipate several Kw as heat.

    This earth leakage might operate HV earth fault protection, but don't count on it.


    If it was my building, I would bond the metal cladding to both the MET and would add an earth rod.
  • 0
    Here in the US there is a trend away from shingles to Metal roofs on residual homes.


    Talking to the roof installers they do not ground the metal roof . It is not required by the current building codes.


    In some cases the high voltage power lines which feed power to the homes (through transformers mounted on the same wooden poles), are adjacent to the metal roofed houses.  It is not unusual  during hurricane strength winds for trees to take down the power cables.


    The other problem with metal roofs is for the potential  electrocution of maintenance roofers, when solar panels are directly mounted over the metal roofs. 


    Needless to say I will not put a metal roof on my house.


    Peter Brooks MIET

    Palm Bay Florida USA
  • Former Community Member
    0 Former Community Member

    Zoomup:



    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.



     




    Hi zoomup, what would you say would be the likely cause this theoretical situation? Bulldozer into adjacent pole, or hurricane strength winds as mentioned by Peter or similar? 

  • 0
    Hello WB,

                         I have no specific theory as to a likely cause of a 11kV cable coming down and falling onto the metal roof of the scout hut but it may happen. We discussed on this forum in the past a cable that fell so low from a rail bridge in the U.K. that it was caught by a passing train and injured someone on a train platform. That drooping was caused by slowly degrading plastic cable ties, affected by the sun.


    Do you remember the Michel Fish denial of a storm in 1987? That weather man said that no such storm was due, yet it came. We had 90 mile per hour winds in the south of England.

    https://www.youtube.com/watch?v=FA6yj8ewOu0


    Then there is the possibility of material fatigue of the cable or supports, or as you suggest a vehicle collision with the supporting cable pole.


    Personally I would not have constructed a building, metal or otherwise, directly under 11kV electric cables.


    Z.




  • 0
    You web reference to the Great Storm of 1987 highlights my current concern about the effect of Climate change on power systems and the loss of electricity for longer periods of time (example 5 or more days) on a wider geographical areas. 


    With climate change Hurricanes are getting bigger and slower depositing more and more rain and flooding many in-land locations.


    Do you know if this subject is being discussed in any IET blogs at this time!


    Peter Brooks MIET

    Palm Bay Florida USA


    P.S. Our annual hurricane season begins in about 3 weeks.
  • 0

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



    I would expect the HV supply would most likely trip on earth fault (rather than by fuse or overcurrent CB) - it's one of the advantages of the UK HV system over the system used in the US as we don't distribute the N on HV - so any current returning to the star point at the HV source can only be due to an earth fault/leakage -so it's (relatively) simple to detect an HV earth fault and trigger disconnection. Not something to bet your life on of course - there's always the chance that the resistance to true earth is too high, or protective equipment faulty,  but in general most downed UK HV lines will be dead, while most US ones would likely still be live.


      - Andy.
  • 0
    Regarding the "medium" HV line arrangement - it depends on if it is a main line with three (lines) phases or a residential line with a single HV (phase line) which feeds the residual pole transformer (providing 2 phase)  protected by a link type fuse. 


    In the later case the HV is the highest cable on the pole followed lower down with the two (110 volt) phases then the ground followed by cable and phone lines. 


    By the way depending on the distance from the  house to the HV line it provides shielding against lightening strokes, which being the lightening capital of the USA, there is usually hundreds a month, somewhere around our county .


    When the trees come down (usually oaks) they stretch the Al cables and they sometime snap -- it has happened twice to us, when wind speeds exceeded 100 mph.


    Peter Brooks MIET

    Palm Bay Florida USA
  • 0
    I am not sure of the building's frame construction wood or metal, or surface coating, it may be paint or plastic. Also I am not sure how electrically conducting the outer sheets are as they are plastic covered to prevent corrosion. Will the sheets be electrically connected? I do not know.


    Z.
  • 0
    They will be almost always be earthed well enough that 6000 odd volts to ground will fire the earth fault relay - any gaps of a few mm or thin layers of paint/corrosion  will just flash.

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

    Note however than the modern trend is to fit insulation covered cable  or over sleeving  when renewing/repairing  higher risk sections of 11kV line


    (example is 6.1.3 in THIS doc) note that

    Although the covered conductor used in CCC lines is not an insulated conductor, the covering does provide some measure of protection against serious electric shocks and burning. This feature of CCC shall be considered when constructing or refurbishing lines in high risk areas (refer to Section 6.2). For example, CCC shall be used for 11kV or 33kV lines in the vicinity of a recreational site (eg fishing site) if the alternatives of undergrounding or diverting are not viable and the danger cannot be adequately managed by the installation of warning notices.




     


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