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2391 question

Discussion in 'Electrical Forum' started by i=p/u, Feb 19, 2012.

Discuss 2391 question in the Electrical Forum area at ElectrciansForums.co.uk.

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  1. i=p/u
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    i=p/u Trusted Advisor

    have 400/230v supply tn-c-s system with 35mm supply conductors..(ze 0.1 ohms) there is an extension built which is a 30m run of 3 core swa with 25mm conductors, one core is used as circuit protective conductor

    Q , state giving reason wheather the copper cpc incorparated in distribution circuit is suitable as part of the main equpotential bonding to gas and water services installed to the new building from public supply

    Q, descibe a suitaqble test to confirm the continuity of cpc in distribution cable
     
    Last edited: Feb 19, 2012
  2. R4BOE
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    R4BOE Guest

    no because of the posibity of the heating of the internal core CPC due to circulating currents in the network.

    r1 + r2 or R2 test
     
  3. R4BOE
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    R4BOE Guest

    i might have the wrong answer for question 1 but this is my thinking from guidance note 8

    For installations forming part of a TN-C-S system in which the supply is earthed at multiple points (PME conditions applying), and it is intended to utilise the armouring of a cable or a core of an armoured cable as a main bonding conductor the designer needs to consider the effects of the currents that may flow in the conductor due to network conditions. The armouring or a core of such a cable is normally not used as a main equipotential bonding conductor where PME conditions apply unless the electrical installation designer determines that the heat produced in the armouring or core due to its use will not cause overheating of the live conductors of the cable when on full load
     
  4. i=p/u
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    i=p/u Trusted Advisor

    sounds good but i dont get it fully

    id saya no because there is water and gas which are extrANEOUS.... WHICH WOULD REQUIRE AND EARTHING ROD(TT)

    BUT THX
     
  5. R4BOE
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    R4BOE Guest

    why would you need to make it a TT ? u can export the PME Earth ! you would just have to run a separate core and use that as your Main bonding conductor
     
  6. i=p/u
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    i=p/u Trusted Advisor

    so we have a single phase 30m distribution ciruit from 3phase board to supply new board in extension

    well if we add the 0.1 ohms ze of board 1 to the 30m of board two it equals 0.12 ohms, dont know if any relevence, im probably reading question wrong
     
  7. ringer
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    ringer Electrician's Arms

    Location:
    Peterborough
    If it is an EXTENSION then it is classed as being the same property, with the same main supply of services including electric, gas and water.

    Is this a real question, or one you have made up?
     
  8. i=p/u
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    i=p/u Trusted Advisor

    sorry i knoww this, it does say where pme exisits main bonding conductor shall have not less than half csa of earthing conductor


    so if we using 35mm as earthing conductor and 25mm as cpc for dist circuit surely 10mm bonding conductor will be fine,, ahhhhh
     
  9. i=p/u
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    i=p/u Trusted Advisor

    real question from mock paper, would you like to write the full question out, i did try cut it short and dont swear cuse i did this
     
  10. ringer
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    ringer Electrician's Arms

    Location:
    Peterborough
    but the question is asking ... " state giving reason wheather the copper cpc incorparated in distribution circuit is suitable as part of the main equpotential bonding to gas and water services installed to the new building from public supply"

    First off, I would say no, because main bonding should exist before the distribution circuit.
    Secondly, what is the core of the distribution circuit to be used for - earthing or bonding? Surely you can't use one conductor for both? I refer here to RCBO's reply above re circulating currents heating line conductors.
     
  11. i=p/u
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    i=p/u Trusted Advisor

    so its a naff question, cheers for replying , ive got 2391 course tomorrow night after work, il let you know on outcome.. even tho you know the awnser

    trick question
     
  12. i=p/u
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    i=p/u Trusted Advisor

    they do say its 30m to dis board 2 and 25mm conductor will go 68m and still be under 50v
     
    Last edited: Feb 19, 2012
  13. R4BOE
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    R4BOE Guest


    hi ringer

    out of curiosity what make you think you can not use the conductor as both a a CPC and a bonding conductor ?
     
    Last edited by a moderator: Feb 20, 2012
  14. spinlondon
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    spinlondon Trusted Advisor

    Location:
    Harlow Essex
    Yes the core is suitable to be used as a bonding conductor, as the CSA is greater than that required by Table 54.8 for the 35mm² supply conductors.
    There is no prohibition against using a conductor as both a circuit protective and a bonding conductor.
    Otherwise we would not be allowed a Main earth.
    As for the circulating neutral currents, I don't think they care what the conductor is called, if it's there and it's connected, they'll use it.
     
  15. i=p/u
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    i=p/u Trusted Advisor

    oh here we go,, tthanks for input, il look forward too tomorrow night and read this thread
     
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