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Zs reading high!

Discussion in 'Electrical Forum' started by sjm, Feb 25, 2012.

Discuss Zs reading high! in the Electrical Forum area at ElectrciansForums.co.uk.

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  1. sjm
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    sjm Guest

    Hi guys, what do you think of this?

    Ze reading of 0.13. Zs reading at the CU of 0.12 with bonding attached etc.
    Shower supply R1 + R2 reading of 0.16. Zs reading at the Shower 0.91.
    Using my new Megger LRCD220 meter on 2 lead Hi setting for Ze and Zs at the board.
    Then using it on No trip setting for Zs reading at Shower.
    Can anyone think of a reason for the incorrect reading, or is the meter US as I think.

    Thanks in advance.
    Steve.
     
  2. Murdoch
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    Murdoch Electrician's Arms

    Location:
    Woking
    Poor connection(s)
     
  3. Mark_Burgess
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    Mark_Burgess Electrician's Arms

    Location:
    ..........
    As above, good look in the isolator switch (if one fitted) and maybe turn it on and off a dozen times.
     
  4. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    try taking a reading either side of the MCB . it could be the MCB affecting the reading.
     
  5. needasparks
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    needasparks Electrician's Arms

    Location:
    Essex
    Well spotted, so many people just put readings down without any thought in what reading they are likely to expect to get.
    sounds to me more likely a poor conection when the test was made unless your testing Zs at a different place to R1&R2.
     
  6. sjm
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    sjm Guest

    I'm pretty damn sure about all the connections. New isolator fitted, Crabtree 50 amp ceiling switch.
    Should have said that I'm getting similarly high readings on the cooker circuit and radial socket circuit. I'm fairly sure that the meter is not working properly on the no trip setting, but just wanted to run it past you guys.

    Thanks
     
  7. Amp David
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    Amp David Trusted Advisor

    Location:
    Glossop
    Test ZE with bonding connected, you've already done that i think when you state Zs at CU. Then test on the incomming side of RCD with Hi and no trip, then on the supply side of the RCD on no trip,

    Bet its the RCD or no trip causing it. If you have 2 RCDs I bet you'll get a substantial difference between the 2
     
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  8. Murdoch
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    Murdoch Electrician's Arms

    Location:
    Woking
    How well charged was the battery in your tester?

    How old are your tester leads?

    Did you null out the leads before the tests?
     
  9. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    if he's using a megger, no need to null leads on EFLI readings. bet the readings differ across RCD and/or MCB.
     
  10. needasparks
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    needasparks Electrician's Arms

    Location:
    Essex
    It may be worth doing a long flylead test on R1 & R2 separately to a random circuit just to eliminate any mistakes in the test.
    Are there any major voltage fluctuations that you have spotted ? near factory area etc. as with the quite low Ze it sounds like you are near to the transformer and you don't mention what earthing supply this is.
     
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  11. tony mc
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    tony mc Electrician's Arms

    IMO I would be inclined to say that this is the resistance of the RCD putting up the readings when you are useing the no trip on the meter.

    Putting only a 15mA test current from the line to cpc as opposed to 25A on the Hi loop test does affect the readings.

    Try linking out the line and cpc at the shower and doing a Hi 2 wire test at the CU without it going through the MBC or RCD and see what reading you get!

    As mentioned its good to question the values that you get !
     
  12. vernam616
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    vernam616 Regular EF Member

    Location:
    North
    Do a zs at the DB on no trip and see how it differs
     
  13. malcolmsanford
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    malcolmsanford Trusted Advisor

    I often think we sometimes over think things to be honest.

    Though with the tests for Ze and R1+R2 the OP expected results of 0.25 but in fact got 0.91.

    Well to be honest I think I could live with a 0.91 reading for what seems likely to be a 30mA RCD.

    Remember we have resistances in the protection devices, and in 40 yrs I don't think I have ever measured anything that on paper it should have been. I do applaud conscientiousness, but only to a point.

    0.91 ohm .......next please
     
  14. needasparks
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    needasparks Electrician's Arms

    Location:
    Essex
    I think the point being made is how reliable is that reading of 0.91 if the earthing has tripled, is it going to become non-existant soon. I would want to know why it has leaped so much. I understand what you are saying though but i would be worried i had missed something obvious and possibly major later.
     
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  15. malcolmsanford
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    malcolmsanford Trusted Advisor

    What we got to remember is the tolerances we are working with here, on MFT that really are not designed to be that accurate when down to 0.1 of ohms as we had with the Ze and the R1 + R2.

    We don't know about lengths of cables, the size of cable, protection devices influences, or even what difference the voltage RMS was on when taking the Ze and the Zs, there are so many factors but a Zs of 0.91 compared to a "calculated" 0.25, at these tolerances, well put it this way I wouldn't be losing much sleep.

    If I got a measured Zs of 2 point something, then yes I would be a little concerned.
     
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