Warmup RCD

Discussion in 'Electricians' Talk' started by carbidder10, Sep 2, 2012.

  1. J.P.

    J.P. New Member

    Lol there not the same Harmonic..flipping eck m8..:)
     
  2. J.P.

    J.P. New Member

    I spy with my little eye summit beginning with M
     
  3. harmonic

    harmonic Member

    my mistake thought it was still in the met can see now it is to one side
     
  4. spinlondon

    spinlondon Screwfix Select

    Yes Unphased, I agree, GN3 does not specifically state that CPCs need to be disconnected, it does only state that bonding conductors need to be disconnected.
    Perhaps it should.
    Then of course I never said that GN3 does state that the CPCs must be disconnected.
    However the method shown to measure Ze is with the Main Earth Conductor disconnected from the CU, by doing this, the CPCs are disconnected from the Main Earthing Conductor.
    However it does not necessarily mean that the bonding conductors are also disconnected.
    The bonding conductors may be terminated at the earth bar in the CU, or they may be connected elswhere at a Main Earth Terminal.
    Opening the main switch or isolating all the circuits, will not disconnect the CPCs.
    The reason these actions are taken, is to prevent danger whilst the Main Earth Conductor is disconnected.

    You indicate, that out of curiosity, you have conducted the Ze test first with all bonding and CPCs disconnected, then with the conductors connected and the main switch closed.
    That is good, because that is the method you should use to determine PFC.
    Is that what you do in all cases?

    No Unphased, GN3 does not consider parallel paths provided by CPCs to be insignificant.
    The method it describes for measuring Ze entails that the CPCs are disconnected.
    It draws your attention to the fact that the bonding conductors may still be connected at a point other than in the CU, and reminds you that they should also be disconnected.
    Remember that the whole point of measuring Ze is to determine that there is a main earth, and that the Ze is equal to or less than the figure used in the design of the installation.

    No you are incorrect, this is exactly the type of argument that I desire.
    Apart from your assumptions in your last paragraph, I have no issues with your post.

    You are mistaken in your belief that the CPCs do not need to be disconnected, for the reasons Lectrician and myself have stated.
    I am concerned that you may be conducting the PFC measurement incorrectly.

    JP, I hope that answers your question as well.
     
  5. J.P.

    J.P. New Member

    If I may say something with ref to PFC Spin (I did ask earlier but it doesn't matter) I test with all earthing connected this is because it will give a worst case scenario..;)
     
  6. spinlondon

    spinlondon Screwfix Select

    Yes JP measure PFC with all conductors re-connected (see previous post).
    In your pictures, you are showing the conductor to the CU disconnected at the MET, which is adjacent to the Main switch.
    Are you connecting your Line & Neutral test leads at the main switch or at the CU?
     
  7. J.P.

    J.P. New Member

    DB Isolator Main switch Spin - off the top - without pulling the cutout this is as near to origin as I can reasonably get tbqh.
     
  8. harmonic

    harmonic Member

    if you can visually see the earthing condutor going no were else but to cut out tncs or sheath tns or rod tt then unless something is buried or parralled up and hidden this is a good indication you are just measuring earthing conductor from board back to sub station and connect board back after test should give evidence of  externeal ze or re as zs should be lower
     
  9. J.P.

    J.P. New Member

    But I think you are going to say it should be at CU position - I'm not really bothered about that but can see possible points raised. So really I should strip my MET at CU position and leave the main earth going to that MET in (board earth still really)

    Fair nuff cant argue with that.
     
  10. J.P.

    J.P. New Member

    Thats right Harmonic - its just the earth going onto the lead sheath - nowt more or less really..nothing to distract it or anything. .I mean it is the origin and I take the word origin literally which may or may not be a good thing..ok for a diye'r though I would think.
     
  11. Lokkars Daisy

    Lokkars Daisy New Member

    I'm sure I didn't say that !    Here is what I do . Disconnect earth from cu and measure directly onto Earth conductor
     
  12. J.P.

    J.P. New Member

    Sorry Spin - yes it does help matters with respect to my question and thanks.
     
  13. spinlondon

    spinlondon Screwfix Select

    My appologies if that's the case Lok.
    By doing such, you are disconnecting the CPCs and as long as the bonding conductors are conected at the CU, you are also disconnecting them.
    However if the bonding conductors are connected to a MET elswhere, they would require disconnection before conducting the measurement.
     
  14. J.P.

    J.P. New Member

    This is interesting all this stuff. So if the norm is to measure the Ze at origin ,and say with setups like mine this is measured at remote DB1 position which is fair enough because you are bringing the extended tails into the equation as well as the extended board earth, then say you form another submain (not subcircuit off of DB 1) (which I will be doing for my extension) then surely you cant do two Ze at origin figures - cannot see where the zedb for DB2 materialises tbqh - so surely the only origin is cutout board origin? - I can see potential inaccuracies arising
     
  15. Bazza-spark

    Bazza-spark Screwfix Select

    Hi JP

    As you say this is the origin, where the service first terminates at the property.  Yes the correct way to measure is with all other erth connections removed.  The easiest way to achieve that is as you have shown in your picture, by removing either the earth wire to the CU or if there are other connections on the MET then to remove the main earth wire from the terminal an measure directly to that wire.

    The fewer wires you have to disconnect to isolate that 1 main earth the better as there is less risk of forgetting to reconnect them when you have finished.

    Kind regards

    BS
     
  16. Bazza-spark

    Bazza-spark Screwfix Select

    Spin - I have never stated that cpc's should remain connected.  However you would not disconnect all the cpc's and bonding conductors to leave only the main earth connected for testing.  You would disconnect the 1 main earth from the terminal and test directly to that.
     
  17. J.P.

    J.P. New Member

    Many thanks Baz and understand fully. Only a quick question : - ok thats where I measure but in my case should I really measure from the CU position? By this I mean strip my CU met or pull out the boards earth, and then measure off the boards earth which is extended as well as the tails, or is it ok to still measure at board position - bearing in mind its more common to have the CU near the cutout as opposed to away from the cutout?
     
  18. J.P.

    J.P. New Member

    Sorry Baz yes you have answered it with ''where the first service terminates in the property'' ok everything ok and thanks Baz.
     
  19. spinlondon

    spinlondon Screwfix Select

    No what you stated was:
    This appers to indicate that you believe the CPCs should be connected.
     
  20. Bazza-spark

    Bazza-spark Screwfix Select

    Ok I see where you got that.  What I meant was as I explained to JP.  You would not disconnect a load of cpc's and bonding conductors to leave only the main earth connected.  You would remove just the main earth and test directly to that.  This would mean all bonding conductors and cpc's would remain connected to the earthing terminal but be physically diconnected form the main earth.

    Kind regards

    BS
     

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