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Conduit as CPC, suspected corrosion

Discussion in 'Electrics UK' started by Adam_151, 3 Nov 2010.

  1. Adam_151

    Adam_151

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    Job I'm on at the moment has a fair bit of the lighting wired in steel conduit, which also provides the CPC for the circuit. I'm getting high results on continuity tests (typically above 10ohms, sometimes a few hundred) and soft non tripping loop tests (typically 50-100 ohms), but a high current loop test returns values which look much more sensible.

    My thinking at present is that the joints in the conduit are starting to corrode or form a layer of oxide and this affects the low current tests, but the full loop test manages to arc past it, and that while in the current situation if a fault were to occur it would probably have the same effect and would cause the circuit to be diconnected correctly (though we cant say that with certainty), is that its an unacceptable situation because it indicates the conduit is starting to break down and it could eventually progess to a stage where the continuity is totally gone, and therefore is not an acceptable situiation to be in

    Just basically want to ask if people generally agree with my assement of the situation, or if others would just bang a high current loop test on and be happy if it gave an acceptable value? ,just don't want to make recommendations and theh have someone else come along, contradict me and say 'its fine, zs checks out ok'
     
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  3. londonboy

    londonboy

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    Is this domestic? Could you pull a CPC in fairly easily?
     
  4. Adam_151

    Adam_151

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    Its not domestic, and I don't think it would be possible to all to draw in a CPC.

    Looking at one of the joints into the DB where the baord has been swapped recently doesn't inspire confidence as the conduit as the othering system, there are three conduits into the board, one of them seems to be slightly shorter than the others (perhaps it moved a bit), the hole into the DB is a bit oversized and flared inwards, the bush is tightened up on the flared edges, and I couldn't get any continuity between bush and the cpc bar with the low ohms meter :eek: (whether this is providing the sitution described above where continuity is high and zs is arcs past it, or whether there are other points somewhere without earth continuity (well there are, but I put that down to the fitting not being connected to the conduit property)

    My confidence in the conduit is very low, but I'm worried (maybe unnessescarily) about being contradicted when I say its knackered and past its self by date
     
  5. sparkwright

    sparkwright

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    Sounds like the dodgy joint at the fuse board should be your first port of call.

    If you can fix this, re-test.
     
  6. ricicle

    ricicle

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    Quite unusual to get high readings on a large metallic conduit install where there is plenty of trunking and other circuit conduits.
    I would imagine in this case that there is only one conduit leaving the DB for the circuit(s) in question ?

    They are not Federal boards by any chance are they ?
     
  7. Adam_151

    Adam_151

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    yeah, well its certainly not helping the issue, unfortunatly the issue is not just confined to this one board, there are others similarly affected nearby and as I'm only here to do an inspection and comment on the installation (and my feelings are that the integrity of the conduit is suspect and should not be relied on to provide an earth path and wanted to bounce it off others to see if they agreed with that or not).

    I'd be interested to hear from anyone whos had a situation with similar readings, and managed to gain access to the joints in the system, and what the condition of the conduit actually was at those joints
     
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  9. Adam_151

    Adam_151

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    Serveral conduits leaving the DB, but no trunking in the installation and I preseume the conduits are all separate and there is no further interconnection between them

    And no not federal boards, why do you ask?
     
  10. ricicle

    ricicle

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    If there were inter-connections then you would probably never discover any high reading measurements. Seeing as you have single conduits then you will obviously be picking them up.

    I had issues with Federal boards where the earth bars were poorly attached to the casing so readings on outgoing conduits were higher than expected (only probably being earthed backwards from the fitings on the circuits)
     
  11. Adam_151

    Adam_151

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    Yep, would be interested in your thoughst about high readings only being on low current tests, do you think I'm right in thinking that the conduit is just startung to fail and that the situation will progressivly get worse, or is that an over dramatic conclusion to reach
     
  12. ricicle

    ricicle

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    Either started to fail or poorly installed in the first place !
    Imagine a fault current in the conduit of even a few tens of amps passing across a joint of a couple of ohms :eek:

    Any flammable materials about ? Code 2 methinks (in the absence of further investigation)
     
  13. Adam_151

    Adam_151

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    My thoughts exactly, I just like to bounce things off people (and expessly those who might just have a few years on me :p :p - to make sure that I'm not letting my affliction with more modern installation methods cloud my judgement)

    haha, cheers ricicle
     
  14. ricicle

    ricicle

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    Thats what the 'thanks' button was invented for !!! ;)
     
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