Zs - Saturating RCD

Discussion in 'Electricians' Talk' started by bundyrocca, Feb 26, 2010.

  1. bundyrocca

    bundyrocca New Member

    When you measure Zs, and the test meter 'bypasses' the RCD, does it have any effect on the reading.

    Only Reason i ask.. consistently - measured R1 + R2 (with zero'd leads) + Ze is always way below what i get at the socket or wherever.

    My tester is on the cheaper side - Di-Log - but i've just had it calibrated and it came back with no adjustments needed at all.

  2. dale76uk

    dale76uk Member

    The Zs test does on most MFTs by-pass the RCD by injecting small fault currents small enough not to trip the RCD.

    I'm having difficulty in understanding what your are asking, is the (R1R2)Ze lower than the loop reading you are taking with the socket adapter (Zs)???

  3. bundyrocca

    bundyrocca New Member

    Basically - measured R1 + R2 plus Measured Ze..

    Is consistently lower - often by as much as half an ohm than measured Zs at the socket outlet, or light fitting or wherever.
  4. dale76uk

    dale76uk Member

    It could be parallel paths to earth??
  5. seneca2

    seneca2 New Member

    It could be parallel paths to earth??

    Parallel paths cause the measured Zs to be lower than R1R2Ze.
  6. dale76uk

    dale76uk Member

    yes because the Ze is done with the EC disconnected from the MET and thus does not include any other paths to earth.

    Zs readings taken at the socket outlet will include Water/Gas main protective bonding conductors and thus give a lower reading.
  7. JP.

    JP. Screwfix Select

    I don't understand how you measure Ze at a socket?..at origin yes, but at socket no.
  8. dale76uk

    dale76uk Member

    JP - i struggled to understand the question in the first post so haven't been able to clarify things clearly.

    I think he's taken a Ze at the origin for example 0.35ohms
    taken R1+R2 from the ring final circuit, i.e 0.55ohms
    worked the Zs=Ze(R1R2)
    :. Zs=0.35+0.55
    Zs=0.90ohms (Calculated)

    He's taken a Zs measurement at the sockets and they appear to be lower.
    But 0.5ohm lower doesn't sound right to me.
  9. losingit

    losingit New Member

    At a megger presentation this week,we were informed that noise on the cct than throw these low current loop testers out on the reading,unlike the robin D lock,which seems to be more consistent when I,ve used it.Hope this helps,
  10. JP.

    JP. Screwfix Select

    Only being a DIYer Dale then I can only guess that being 0.5 ohm lower taken by measurement is pretty much normal (as long as it falls within accepted measurements) For instance you can take a measurement and note it down, and then take another measurement with the instrument and it will not be exactly the same. Why this is I don't know, but can only guess that it is due to fluctuating circuit capacitance. As said the above is only conjecture and not necessarily correct.

    Anyway thanks for your reply Dale.
  11. seneca2

    seneca2 New Member

    This is why the Fluke is the only reliable meter for loop testing where an rcd is in circuit. The Fluke doesn't use a low current test, instead it uses a very short test time, something less than one half of one cycle, this is how it avoids tripping the rcd, others use the low current method which gives erroneous Ze results
  12. dale76uk

    dale76uk Member

    JP - your not a DIY'er ??
  13. bundyrocca

    bundyrocca New Member

    No -

    The reading at the socket is always consistently HIGHER than measured R1 plus R2 plus Measured Ze at origin...

    So Measured as above might give me 0.6 or 0.7..

    Wheras i can easily get over an Ohm when loop testing at the socket..
  14. bundyrocca

    bundyrocca New Member

    I asked my assessor at the last meeting and he didn't know - even asked him if it was the make of test equipment - but he thought not, and made a not on his pad to get me an answer...
  15. seneca2

    seneca2 New Member

    Bundy, does the di-log use a low current test? if so this is why you're getting odd readings. Can you try it on a non-rcd circuit, or by-pass the rcd just as a test?
  16. Oakey dokey

    Oakey dokey New Member

    Had my NIC inspection about a week ago-he asked why my Zs reading on the day were different to the readings on my test sheet(taken with the same tester).

    I have no technical answer but whenever I use my kit which has the rcd `bypass` facility the Zs Always adds up to more than the ZeR1R2 and on that day it decided to throw the spanner in thanks test kit.
  17. Sutton Spark

    Sutton Spark New Member

    who's your nic guy??? mine seems to have changed again this year
  18. Bazza-spark

    Bazza-spark Screwfix Select

    Measure the supply voltage before you start. Variations in supply voltage will give variations in the measured result because of the low resistance values concerned. Temperature will also have an effect.

    As Seneca pointed out the Fluke use a very short duration test so can be consistent and repeatable. Low current tests can have a load resistor that warms up and varies with the ttime the current is applied.

    Kind regards

  19. Learner mate

    Learner mate New Member

    I think this is due to the live and cpc rings or radial you are testing are different cross sectional areas. Dont worry about this its normal it can make a difference of 0.05 ohm. It all depends on how the circuit is wired and how the sockets are spread around it.
  20. gas monkey

    gas monkey Well-Known Member

    1.67 rings a bell

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