how can i tell the difference between TNCS & TNS ......?

Discussion in 'Electricians' Talk' started by 12benny, Jun 17, 2009.

  1. 12benny

    12benny New Member

    how can i tell the difference between TNCS & TNS where ( in different installations) the main Earth goes into the split head?
    In other words, where typically, one E. goes to a split head and the other example the E.is clamped to the supply.
     
  2. gerrin2owd

    gerrin2owd New Member

    Test the PSCC and the PEFC, if they are the same values its TNCS if different values its TNS.
     
  3. bluevelvet

    bluevelvet New Member

    TN-S Separate live and neutral in the supply cable and earth is from the cable metallic sheath
    TN-C-S combined neutral and earth in a single conductor
    so the main earth cable that goes into the split head would usually be TN-C-S (PME) and the earth cable attached to the cable sheath would be TN-S .
     
  4. 12benny

    12benny New Member

    thanks for the replies. But what i meant was that some Earth TNS installations go into the split head just like a Earth TNCS installation.Without testing, can i tell at "a glance" which is which?
     
  5. bluevelvet

    bluevelvet New Member

    If it`s not an obvious connection, then you have to do as gerrin said - check it out with the meter
     
  6. mikhail faradayski

    mikhail faradayski New Member

    I used to think the same about checking PFC's for TNCS

    That was until i was talking to a DNO guy and he said not true.

    Something called TNS with MEN, means results show TNCS but it may still not be suitable.
     
  7. gerrin2owd

    gerrin2owd New Member

    I used to think the same about checking PFC's for TNCS

    That was until i was talking to a DNO guy and he said not true.

    Something called TNS with MEN, means results show TNCS but it may still not be suitable.


    I would be interested to hear a more detailed explanation on this MF and as for being suitable I don't believe the OP commented on that aspect.
     
  8. mikhail faradayski

    mikhail faradayski New Member

    Was told that its possible to have a TNS supply with a 'multiply earthed neutral', something to do with earthing down unacceptably high neutral voltages on some distributions. But it was the DNO who would decide wether it was suitable for use as a PME ( some thing to do with distribution grid maps and number of earthing points )

    Are there any signs/labels to indicate if it is a PME supply?
     
  9. sinewave

    sinewave Screwfix Select

    Are there any signs/labels to indicate if it is a PME
    supply?



    Sometimes!


    Should believe them?


    Sometimes!


    Always check by testing.
     
  10. mikhail faradayski

    mikhail faradayski New Member

    Sorry sine, my question was about this instance, not in general, so 'sometimes' is unlikely. Weell, it was there on wednesday, but.....

    If there is a PME label, it is a PME supply.
    After all, it would have been the DNO who put it there, they should know.

    If there isn't a PME label, it might be PME, but the label has come off, but who can say from PFC results whether it is PME

    Always check by testing? What about enquiry?
     
  11. stranger101

    stranger101 New Member

    Its simple

    Cut the seal , yank the cut out , unscrew the cover and have a look its the only way.

    Then of course just say the seal was already cut.

    simple's


    Or you could try ringing the DNO and asking them that is if you have a spare few hours to waste

    S101
     
  12. sinewave

    sinewave Screwfix Select

    If there is a PME label, it is a PME supply.
    After all, it would have been the DNO who put it
    there, they should know.



    You'd think so wouldn't you? :)


    Lots of DNO's have used subbies over the last 15 years to refurb and upgrade old supplies.

    You'd be amazed at the labels they shove on willy nilly which are totaly misleading.

    My old DNO had a whole village refurbed by Balfour Beatty staff and they automaticaly PME'd every new LUCY Cutout despite the whole area being TT
     
  13. pcelectrics

    pcelectrics New Member

    I understand your dilema perfectly. One of my assessors told me that this is a very commom source of confusion. The only suggestion he had though, was to take the cover off the head and inspect it.
    This is a '60s TN-S head:
    http://www.photobox.co.uk/album/28385663
    at first glance it looks just like a TN-C-S head.

    The best way I can come up with to find out which it is, without taking it apart, is to measure the resistance between the neutral (at the head, meter or CU) and the earth terminal on the head.
    If you have a TN-S you will be measuring the resistance of the neutral back to the Tx and the armour from the TX back to the earth terminal. This reading should be roughly the same as Ze, about 0.1 to 0.3 ohms.
    If you have a TN-C-S, you will only be measuring the resistance of the neutral tail, which should be 0.00 ohms on your meter (less than 0.002 in reality).
     

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