Maximum Ze on TT system

Discussion in 'Electricians' Talk' started by 63AMP, Mar 26, 2006.

  1. 63AMP

    63AMP New Member

    Hi what is the maximum Ze permissable on a TT system I have a reading on an install of 155 ohms (tested with all other earths disconnected). While testing each circit with all main bonding reconnected the Zs readings come out at 3.60 - 4.99 ohms. Whilst realising these readings are above the maximum permissable Zs readings I have a split load consumer unit with 100mA time delay RCD and a 30mA RCD. Any comments would be appreciated.
  2. sinewave

    sinewave Screwfix Select

    Your OK with that reading.
    Anything upto 500 ohms is OK on a 100mA and 1666 ohms on a 30mA.

    However any reading above 200 ohms is classed as 'unstable' by the IEE, so you need to be under this in general to be safe and compliant. :)
  3. 63AMP

    63AMP New Member

    Spot on have NICEIC inspection tommorrow at this job, you start to doubt yourself and your work!!
  4. dr watson

    dr watson New Member

    hi max is 200 ohms however they like under 100 ohms this is to be the new max for tat soon i belive as came up in 2391 course last month.
  5. gaz58

    gaz58 New Member

    Did you put 200 ohms on the 2391 as required by BS7671 or use the formula answer of 1666ohm?

    I put 200ohm but think it could have been either.

  6. dr watson

    dr watson New Member

    hi 200ohms checked after with lecturer when doing practical said that was right.
  7. squeaky

    squeaky New Member

    I qoute OSG, page 78

    For TT systems, the value of the earth electrode resistance Ra in ohms multiplied by the operating current in amperes of the protective device i^n shall not exceed 50v e.g. if Ra=200ohms, then the maximum RCD operating current should nto exceed 250mA.
    30mAX1660ohms = 49.8v
  8. TeesdaleSpark

    TeesdaleSpark New Member

    I agree with what's been said regarding BS 7671, the Onsite Guide and Guidance Note 3. However the NICEIC book on Inspection and Testing says "BS 7430: Code of Practice for Earthing, suggests that a value of earth electrode resistance exceeding 100ohms may be unstable.
  9. Bando

    Bando Member

    I got caught out on this on a NICEIC inspection. Had a rod measuring 180 ohms on the EIC. He took me back there and asked me to test it. Got a Ze of 600 Ohms :( It was a split board with 100mA TD. So I had an excessive touch current under fault conditions. I had to go back and drop a second rod at bottom of steps taking it less than 100 Ohms. They charged for the re-visit and made me re-test it.

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