"extraneous conductive parts" of a shed

  1. bigegg

    bigegg I drink and I know things. Its what I do.

    Messages:
    6,425
    Location:
    Leeds, West Yorkshire
    i've been reading... :rolleyes:

    no.1 shed is a concrete garage with a plywood + felt roof.
    above the roof is a second roof, made of steel profile supported on scaffolding boards.
    there are no metal rafters, or frames, or any way of touching the steel from inside.

    i'm intending to run a 32A supply from a separate CU into the shed using 6mm 3core SWA - a distance of around 10m altogether.

    however, upon reading, it looks like i need to bond to earth any "extraneous conductive parts" using 10mm copper cable.

    so...

    is the steel roof an "extraneous conductive part"?
    what about a planned 22mm copper line from the air compressor in a separate (all timber) shed?

    should i just go for 10mm² swa cable anyway?

    tn-c-s earth to the house, btw.
     
  2. gaz1

    gaz1 Forum Supporter

    Messages:
    8,463
    Location:
    westyorkshire
  3. bigegg

    bigegg I drink and I know things. Its what I do.

    Messages:
    6,425
    Location:
    Leeds, West Yorkshire
    Memmeddu likes this.
  4. mtt.tr

    mtt.tr Member

    Messages:
    956
    Essex England
    All you need to do is measure its resistance to ground, that will tell you. Cant remember were in the regs it is. I'll look this evening.

    Well done gaz thats a good article
     
    bigegg likes this.
  5. Memmeddu

    Memmeddu Member

    Messages:
    1,114
    Location:
    Italia Sardegna
    Any part that could be electrified must be grounded
    The roof I would like to avoid because if grounded could be a lightning catcher
    Just do the things in the best way as possible protect the cables so they won't be pinched and short to metal parts
    Air lines must be grounded because galvanic corrosion and electrostatic currents
     
    barking mat and bigegg like this.
  6. mtt.tr

    mtt.tr Member

    Messages:
    956
    Essex England
    No because you could be causing a risk should you loose your neutral and you are on a pen system
     
    WorkshopChris likes this.
  7. Memmeddu

    Memmeddu Member

    Messages:
    1,114
    Location:
    Italia Sardegna
    I think that there is a miss understanding
    "PEN" isn't related to your electrical system but related to the medium low voltage transformer
    So from 15000V to 415V/230V
    This main transformer has the windings in Y configuration ,the Star point of this transformer is grounded .
    It's that the cause why you could get electrical shock .
    Neutral is provided by the main , you on your electrical installation are supposed to have at least 3 earth dispersion poles , connect the rebars sunk in the concrete to the equal potential copper strip .
    This provides protection from electrical shock because doing that you are closing the circuit between the main power transformer and your PE in the house/workshop etc
    In case of fault to ground on grounded thing for example a washing machine , current flows from the failed part to the metal housing , through the PE lead ,to the ground ,to the main power transformer
    At this point when reach a certain amount of current that flows from the phase but this doesn't flows to neutral the breaker trips (I know that you call this in another way ,we call those potential difference breaker)
    Usually 0,010A if specialised for humid areas such bathroom etc ,0,030A for normal environment,0,3A for the line longer than 3m from the main line to the breakers panel

    It's not ended here
    You should connect to the equal potential strip every single metal parts you have that could be electrified...
    So ladders , alluminium window and doors , copper lines ,water brass manifolds ,fences , gates anything metal that could be both touched and electrified
    Some advantages are that you are also providing protection to the water lines from galvanic corrosion but theoricaly you are doing worst but 64-08 says that so
    Now the roofing is hard to touch so in my opinion doesn't requires to be grounded .
    The earth is like big capacitor so the ground is an armour the sky the other one
    I don't know which one is positive and which is negative , but essentially during the day current flows from the an armour to the other
    When there is a lightning storm this will happen much more frequently and with much more violence because the capacitor is charged and the humid air is a way better conductor
    Now current does always prefer the shortest travel possible .
    Between two metal roofs one grounded and one not the lightning will always fall down on the grounded one
    Idem if you are in the middle of a plain with no trees ,stay sure that the lightning will fall down on you
     
  8. mtt.tr

    mtt.tr Member

    Messages:
    956
    Essex England
    If you have a building with plastic services which then join copper then you earth them you have created a hazard that didnt exist.
    Read the link on this page that's the regs we work to,.
    Also it will depend if it is class 1 or class 2 appliance you do not earth double insulated tools


    You make your grounding decision my measuring its resistance to the met.

    Pen = protective earth neutral

    From said article
    "It is a common misconception that bonding such items won’t cause any harm even if it is not required by BS 7671:2018 so a “better to be safe than sorry” attitude is taken. However, it is important to remember that by connecting to the main earthing terminal, in some circumstances, fault currents can be exported throughout the installation which would not be there if protective bonding had not been applied. This can cause an electric shock risk for persons outside of the installation in contact with the general mass of Earth and earthed equipment such as pipework, for example an outside tap or metal (class 1) electrical equipment. "
     
    Last edited: Nov 11, 2019
    sparkysy and Memmeddu like this.
  9. Memmeddu

    Memmeddu Member

    Messages:
    1,114
    Location:
    Italia Sardegna
    I think that we may miss understood , but yep double insulated tools if grounded are dangerous,and you explain that clearly up here
     
    Westfield-builder likes this.
  10. mtt.tr

    mtt.tr Member

    Messages:
    956
    Essex England
    Goes for metal parts that have no connection at all to earth, if you join them to the met you are creating a Hazard.

    I'll do a diagram later
     
    Memmeddu likes this.
  11. dobbslc

    dobbslc Forum Supporter

    Messages:
    4,412
    Location:
    Hertfordshire UK
    We used to bond everything in the old days!
    I remember fitting earth clamps to every radiator, window frame, steel frame etc. We don't do that now. Lots of the regs change every few years, it's to help sell new books and refresher courses......:D

    @bigegg does the metal roof have any light fittings attached to it? Might make a difference.
    You'll need to make the shed a TT earthing system with RCD protection if you're on a TN-CS supply.
    :thumbup:
     
    Westfield-builder and Memmeddu like this.
  12. Robotstar5

    Robotstar5 Casanunda Staff Member

    Messages:
    17,781
    Location:
    Birmingham
    I think that was the 15th edition?
     
    Westfield-builder and mtt.tr like this.
  13. mtt.tr

    mtt.tr Member

    Messages:
    956
    Essex England
    Or pay for the pleasure of running 16mm2
    If dno even allows it.
     
  14. dobbslc

    dobbslc Forum Supporter

    Messages:
    4,412
    Location:
    Hertfordshire UK
    Time flies!
     
  15. brightspark

    brightspark Member

    Messages:
    26,846
    Location:
    yarm
    Last edited: Nov 11, 2019
    pressbrake1, sparkysy and mtt.tr like this.
  16. mtt.tr

    mtt.tr Member

    Messages:
    956
    Essex England
    Hopefuldave, dobbslc and brightspark like this.
  17. mtt.tr

    mtt.tr Member

    Messages:
    956
    Essex England
    https://www.voltimum.co.uk/articles/principles-protective-multiple-earthing

    Thats saves my effort , should clear up what i mean
     
  18. Memmeddu

    Memmeddu Member

    Messages:
    1,114
    Location:
    Italia Sardegna
    Unfortunately I have to register
    But essentially we are saying the same exact thing ,the problem is that I can't figure out which terms I to use to make you in condition to understand me
    But essentially pen it's what we call it
    The neutral is grounded ,and PE is a neutral that close the circuit in case of insulation failure to ground
    Double insulation load must not be grounded for Safety reasons because the PE can be cause of injury (current return)
     
  19. mtt.tr

    mtt.tr Member

    Messages:
    956
    Essex England
    if you loose the neutral before the installation the rcd will not see the fault but voltage potential will rise on all earth( bonded) connected metal work, which is why we dont earth (bond) metal work unless it already has a low resistance to earth, diagram in this article shows the current flow during a neutral failure

    https://talk.electricianforum.co.uk...-principles-protective-multiple-earthing-pme/
     
    dobbslc, pressbrake1 and Memmeddu like this.
  20. bigegg

    bigegg I drink and I know things. Its what I do.

    Messages:
    6,425
    Location:
    Leeds, West Yorkshire
    no.
    the metal roof is "outside" the original garage.
    its screwed into scaff board roof beams which in turn are fixed (with steel brackets) to the old 1.25in (3/4" +1/2") plywood roof, which is then fastened to 4x2 timber beams.
    the light fittings are screwed into the underside of the ply.



    as to the earth in the house, i need to investigate further, but it looks as though there is no earth connection on the incoming supply: there is an earth to the incoming water main (22mm copper) which splits in a henderson to the three CUs.
    i'm thinking this makes the house a TT?
     
    dobbslc likes this.
Advertisements