RCD on a TN-C-S supply question.

Discussion in 'Electricians' Talk' started by collectors, Apr 23, 2015.

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  1. collectors

    collectors Member

    Hi, i live in a rural area with an overhead supply classed as a TN-C-S supply. Now at the main intake looks to be wired the same as a PME system.
    My question is, all boards are fitted with RCD's but when i cut a cable with the fuse for that circuit off but the main RCD on, it wont trip the RCD. IE:N to Earth
    Now! i know the RCD is working as i picked up the damp plug to the cement mixer one day & got a mild shock & the RCD tripped. (not the best way to find out)
    So is it the style of a TN-C-S supply that makes it not trip between N & earth?.
    Also, all gas & water supply's are in plastic, so no other earth's.

    Thanks

    Chris
     
    collectors, Apr 23, 2015
    #1
  2. Lectrician

    Lectrician Screwfix Select

    If there is little or no load in the installation (not your circuit), then no current will flow from N to E when you short them together, so nothing will trip.

    If you had no earth, it would do the same thing. Sometimes an electrician connects an earth to the service heads exposed and accessible earth terminal, when this terminal has not had the link installed, so no earth.
     
    Lectrician, Apr 23, 2015
    #2
    FatHands likes this.
  3. unphased

    unphased Screwfix Select

    Morning Chris

    Does the RCD trip when you press the test button? What tripping current is the RCD rated? 30mA or 100mA?
     
    unphased, Apr 23, 2015
    #3
  4. collectors

    collectors Member

    Hi, thanks for getting back, Yes, all RCD,s trip individually when each test button is pressed. There are 4 RCD's (1 split load & 2 smaller boards) & all 4 are 30mA.
    When we moved in it had a previous fire from a chimney & it looks like the head might of been updated. But! We also found an old eath rod with a old 6mm VIR cable attached that had been cut off.
    The bungalow had been rewired before we moved in due to the fire & wasn't sure if the system had been change as i notice that some TN-C-S have earth rods & some don't.
    Oh! the earth is connected/linked at the neutral bock.
    Thanks.
     
    collectors, Apr 23, 2015
    #4
  5. Cliffy T Brown

    Cliffy T Brown Member

    You do know tncs is pme , don't you pal?
     
    Cliffy T Brown, Apr 23, 2015
    #5
  6. peter palmer

    peter palmer Screwfix Select

    You do know tncs is pme , don't you pal?

    He probably doesn't know that because its not.
     
    peter palmer, Apr 23, 2015
    #6
    seneca likes this.
  7. Cliffy T Brown

    Cliffy T Brown Member

    It is so
     
    Cliffy T Brown, Apr 23, 2015
    #7
  8. seneca

    seneca Screwfix Select

    Not necessarily!
     
    seneca, Apr 23, 2015
    #8
  9. Cliffy T Brown

    Cliffy T Brown Member

    it is
     
    Cliffy T Brown, Apr 23, 2015
    #9
  10. Cliffy T Brown

    Cliffy T Brown Member

    I win, no need to apologise Peter.
     
    Cliffy T Brown, Apr 23, 2015
    #10
  11. stu1312

    stu1312 Member

    All PME supplies are tncs but not all tncs are PME
     
    stu1312, Apr 24, 2015
    #11
  12. Lectrician

    Lectrician Screwfix Select

    Exactly :)
     
    Lectrician, Apr 24, 2015
    #12
    seneca likes this.
  13. retiredsparks

    retiredsparks Super Member

    Your move Mr Brown.......it all seems to have gone quiet ?...lol
    RS
     
    retiredsparks, Apr 24, 2015
    #13
    seneca likes this.
  14. JP.

    JP. Screwfix Select

    Must admit don't understand the above with ref to all tncs are not pme tbqh
     
    JP., Apr 25, 2015
    #14
  15. Lectrician

    Lectrician Screwfix Select

    How about a transformer feeding a single property with an underground wavecon cable? The neutral is earthed at the star point or neutral of the transformer as you would expect, but more than likely the neutral is not tied down elsewhere. It's then not PME. You may find the service head has the neutral earthed down by a bare 50/70mm hard drawn copper cable laid in the trench, so perhaps providing a single earthed down point.

    BS7671 Certs refer to TN-C-S, not PME. PME is more a DNO term.
     
    Lectrician, Apr 26, 2015
    #15
    FatHands likes this.
  16. stu1312

    stu1312 Member

    As lectrician said a TNCS supply isn't always supplied as PME when it has multiple earth electrodes, it can be supplied as PNB but this is generally only large supplies
     
    stu1312, Apr 26, 2015
    #16
  17. retiredsparks

    retiredsparks Super Member

    JP........why should you be expected to understand ?
    Its quite techy and its not your trade ?(AFAIAA)
    From what I have seen you are a very very capable and knowledgeable guy who takes a great interest and pride in the projects that you do. Asks questions and makes decisions on the best advice available and information from other sources.
    I am happy to advise guys like you(and vias versa i hope) if i can, as i know you will be trying to get it right.
    RS
     
    retiredsparks, Apr 26, 2015
    #17
    FatHands and JD 6400 like this.
  18. JD 6400

    JD 6400 New Member

    Hear hear , this is what an electricians forum should be about .
    It is just a shame that for far to many that go on them , it just ends up being a appendage measuring contest ! And the best of it is , they tend to be the obvious clowns .
     
    JD 6400, Apr 26, 2015
    #18
  19. JP.

    JP. Screwfix Select

    Thanks guys for your replies on the PME/TNCS subject - I will delve further into it as have never given said a second thought tbqh..I can see roughly what you guys mean but roughly is not enough.
     
    JP., Apr 26, 2015
    #19
  20. retiredsparks

    retiredsparks Super Member

    Bye the way JP...have done most of the painting on new house(20 freaking doors, 4 coats, rubbed between etc...and finish is good...thanks for the advice and your experiences about the water based paints.
    RS
     
    retiredsparks, Apr 26, 2015
    #20
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