Zs Readings on TT systems

Discussion in 'Electricians' Talk' started by networkgeeks, Nov 7, 2007.

  1. networkgeeks

    networkgeeks New Member

    Hi there

    I did a fused on a ring circuit on a TT system today. The Zs was 188 ohms and the r1+r2 on the circuit was .045 Ohms. The Ze on the earth electrode was 188 ohms, so the Zs reading of 188 ohms is about right bearing in mind that the water service pipe is plastic and there is no gas.

    The Ze is on the high side but less han 200 ohms so OK.

    Zs=Ze + R1+R2, so the results are OK.

    On my minor works certificate, there is a box for maximum Zs permittable under BS7871 (1.2 ohms for a B32 circuit breaker as on this circuit). Should I put 200 ohms in this box which is the maximum permissable as palinly my 188 exceeds 1.2 which is the figute in the OSG.

    Cheers
  2. networkgeeks

    networkgeeks New Member

    r1+r2 was 0.45, sorry for typo
  3. unphased

    unphased Screwfix Select

    TT systems are unusual because the maximum Zs values are always exceeded. Its more important to have the correct RCD set-up on the circuits. On your test sheet you can only record the max. permissible Zs readings according to the tabulated values (80% rule-of-thumb is normally sufficient).

    It is generally accepted that 200 ohms is the maximum allowable on an earth rod BUT readings higher than 100 ohms are considered unstable so really you shouold be looking at ways of lowering your 188 ohms to get under 100 ohms.

    It is always worth enquiring as to the availability of PME for the property concerned because this can be a better way of improving the earth system than messing around with earth rods which are very difficult to work with in comparison.

    UP
  4. poheZ

    poheZ New Member

    whack it on an rcd, if its not on one already

    didnt you post about tt systems before?
  5. M107

    M107 New Member

    there is a box for
    maximum Zs permittable under BS7871 (1.2 ohms for a
    B32 circuit breaker as on this circuit

    You'll find it's 1.50 ohms.

    Put down the max allowed by 7671, & in the measured max Zs box put down your test result.
    If you have rcd on the circuit & it passes tests all is well.
  6. networkgeeks

    networkgeeks New Member

    All TT round here (hants and wilts) also it 1.2 which is 80% of the 7871 values which is the value to use according to NIC
  7. M107

    M107 New Member

    All TT round here (hants and wilts) also it 1.2 which
    is 80% of the 7871 values which is the value to use
    according to NIC

    You need to swap the 8 & 6 keys ;)
    Yes lots of TT here in Berkshire also.
    As for using the 80% value I'd say niceic are wrong....you compare your measured Zs result against the 80% value of 1.20 ohm(found in osg) but the cert asks for
    "the maximum Zs permitted by BS7671"
    which to me is the tabulated value found in BS7671 & for a 32amp Typ B 60898 mcb is 1.50 ohms
  8. Joelp1

    Joelp1 New Member

    so you're only permitted 80% of the permitted amount? That sounds suspiciously like my payslips every week, you only get 80% because you're not in london...
  9. X

    X New Member

    Yes lots of TT here in Berkshire also.

    And in Scotland..

    It really don't matter what you write (either the max or the rot value).. the odds are 100 to 1 that you will be the only person who ever looks at and understands the certificate
  10. M107

    M107 New Member

    Yes lots of TT here in Berkshire also.

    And in Scotland..

    It really don't matter what you write (either the max
    or the rot value).. the odds are 100 to 1 that you
    will be the only person who ever looks at and
    understands the certificate

    How true X
  11. unphased

    unphased Screwfix Select

    All TT round here (hants and wilts) also it 1.2
    which
    is 80% of the 7871 values which is the value to
    use
    according to NIC

    You need to swap the 8 & 6 keys ;)
    Yes lots of TT here in Berkshire also.
    As for using the 80% value I'd say niceic are
    wrong....you compare your measured Zs result against
    the 80% value of 1.20 ohm(found in osg) but the cert
    asks for
    "the maximum Zs permitted by BS7671"
    which to me is the tabulated value found in BS7671 &
    for a 32amp Typ B 60898 mcb is 1.50 ohms

    Blimey M107, can't believe you made a basic error with your max permitted values. Its 1.2 ohms for a 60898 type B because you either use the 80% rule of thumb value as given in OS guide or on published tables OR you use the corrected value from the regs. As its fairly cumbersome to go through all the time consuming process of correcting the value in regs for ambient temperature etc then the generally accepoted rule is 80% which saves all the hassle. btw 17th edition amendments will be 80% as used by NICEIC and NAPIT.

    UP
  12. stumpy987

    stumpy987 New Member

    On niceic cert you have to record the value from the regs not the 80% figure printed in the pads of certs
  13. unphased

    unphased Screwfix Select

    On niceic cert you have to record the value from the
    regs not the 80% figure printed in the pads of certs

    No you don't. Its the CORRECTED value you record. Why do you think NICEIC give you the printed tables. They ain't for fun. Can't believe it. This is basic stuff. C&G 2391 and 2381 teach it.

    UP
  14. sinewave

    sinewave Screwfix Select

    Agree with Stumpy as that is what we were taught.



    If no one is ever going to use the max tabs in BS7671 then why are they there rather than the ROT numbers?
  15. stumpy987

    stumpy987 New Member

    cert says Max. disconnection time permitted by BS7671

    Not 80% disconnection time permitted By BS7671

    whoever taught you IS wrong
  16. M107

    M107 New Member

    cert says Max. disconnection time permitted by
    BS7671

    Not 80% disconnection time permitted By BS7671

    whoever taught you IS wrong

    Thank you ! :)
  17. triplegang

    triplegang New Member

    Got pulled up by the NIC on my inspection for using 80% rule of thumb in max permissible column. Told in no uncertain terms that the left half of the form is design characteristics and so you use values straight from 7671. Eg 60898 32A = 1.50
  18. fooman

    fooman New Member

    cert says Max. disconnection time permitted by
    BS7671

    Not 80% disconnection time permitted By NICEIC :^O

    whoever taught you IS wrong

Share This Page