extraneous conductive part

Discussion in 'Electrics UK' started by nigelm, 19 Feb 2009.

  1. nigelm

    nigelm

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    I have a garden room fitted with aluminium doors, do these count as a extraneous conductive part from the point of view of earth bonding ?
     
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  3. holmslaw

    holmslaw

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  4. bernardgreen

    bernardgreen

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    I would leave un-bonded provided there was no possibility of any electrical conductor coming into contact with the frame work. Regulations may provide a different answer.

    My reasoning comes from some years when a set of historic metal doors to the outside area were bonded to the main electrical earth because there was a light fitting on the door frame. PME type supply with poor balancing on the phases.

    Standing outside on the damp ground in damp shoes and touching the door and sometimes a gentle "tingle" could be felt. Never more than a few volts but to some people it was un-pleasant and a sign that all was not OK.

    Using a TT type supply and bonding the electrical earth to the real ground was one option but in the end the light fitting was re-wired to be fed from an isolated supply and the door frame bonded to an earth spike.
     
  5. dodginess

    dodginess

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    My lecturer told me about some fad (a few years ago mind) about metal window frames being bonded - logic prevailed when people realised that the likelihood of someone touching a window frame and a socket/live appliance at the same time was fairly minimal (especially given the need for both to be within arm's length reach).

    I think this is similar to the kitchen sink issue IE if you bond it then you're providing another path for the current to flow through (or more technically, a point of equal potential I suppose.

    To answer the first part of the question, the doors are very much "extraneous" because they do not comprise part of the electrical installation.

    Neil
     
  6. Spark123

    Spark123

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    Can they "introduce a potential, generally earth potential?"
    I'm thinking not unless as stated above, hence they would not be considered an extraneous conductive part.
     
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  8. dodginess

    dodginess

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    As soon as you bond anything metal then it will automatically be at whatever potential the earthing system is at, correct? I think the true definition of "extraneous conductive part" is anything that isn't part of the installation that is capable of conducting an electric current (even if only momentary) but I agree that in this particular case the definition is ambiguous, in a way that it wouldn't be for a radiator or towel rail.

    That said, it's possible that the garden room might be at a different 'earth' potential if the earthing system is TN-C-S, because then the house earth will take whatever the neutral potential is and the garden room would be at a different potential if an earth stake is used (as is recommended if the garden room is outside of the equipotential zone).
     
  9. Spark123

    Spark123

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    The definition of an extraneous conductive part is:
    "A conductive part liable to introduce a potential, generally earth potential. and not forming part of the electrical installation."
    So even if it is conductive, if it cannot introduce a potential then it is not considered extraneous.
    Metal parts buried in the ground which can act like an electrode are considered dangerous as they can introduce earth potential into an equipotential zone.
    One way to tell if a part is extraneous is to conduct a measurement between it and the MET. If the resistance is less than 22K ohms then it can be considered extraneous.
    If the resistance is greater than 22K ohms, the current which can flow in the event of a fault with someone simultaneously in contact with it (and has a body resistance of 10K ohms) is 10mA or less, hence is not considered extraneous.
     
  10. Pensdown

    Pensdown

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    Due to the light fitting, the designer may have bonded the metal door frame because it could be classed as an Exposed Conductive Part as opposed to an Extraneous Conductive Part.
     
  11. dodginess

    dodginess

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    OK - I take your point :) Seems like my tuition wasn't up to scratch... One point I will say in my favour is that whoever used to write the C&G 16th Ed papers didn't understand this concept either - this was the only time that I remember questions about exposed/extraneous parts being asked and I always got them right...
     
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