testing earth fault loop impedance

Discussion in 'Electrics UK' started by Steve, 30 Aug 2005.

  1. Steve

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    how is this done? Is expensive kit required? If so, how does this kit work? Just curious as so how one finds the resistance of something (earth rod) without passing a current through it and measuring the other side :confused:
     
  2. andy

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    get an earth fault tester. put the earth lead on earht, and live on live. wait a few secs and there you have it - the earth fault impedance

    as for price - i got mine for £70 from ebay!
     
  3. Steve

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    dont give much away, do you? How exactly does this thing work? And what exactly are you measuring the resistance (ohms) between?
     
  4. plugwash

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    its the loop impedance between live and earth.

    i'm not sure exactly how they measure it, my guess is they use a varying load and measure how it affects the voltage but i don't know for sure. Sparkys just treat the things as black boxes.

    theres also meters for testing rods theese are used to measure the resistance of a rod in isolation and involve several temporary test electrodes a long distance from the rod. Afaict theese meters are only generally used when putting in a rod for a generator install or similar where you can't just measure loop impedance as your generator is on the same rod as your load.
     
  5. mikely

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  6. pdcelec

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    The meter injects a current into the circuit (25A, or 15mA for a non RCD trip type) and measures the volt drop. V= IR seems to cover this one.

    The 25A are more accurate since the current also tests joints in conduit etc.
     
  7. plugwash

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    right but there is already voltage in the cuircuit so the forumulae is actually

    R=dV/dI

    however what i'm not sure of is how they compensate for other voltage variations that are not linked to the variation in test current (e.g. caused by other loads on the supply switching on and off etc).


    as for rod testing from the TLC site it seems its done with a 4 wire ohmmeter type test arrangement with one of the pairs connected to the rod under test and the other pair connected to ground through temporary electrodes. provided the electrodes are far enough apart with this system the resistance of the temporary test electrodes to earth should have negligable effect on the overall resistance reading.
     
  8. Lectrician

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    The loop tester puts a set impedance across L and E.

    It measures the volts, mesaures the current flow throught its impedance, hence, it can calculate the Ze in ohms.

    All loop testers work the same, and the impedance inside (probably just a resistor or a summet!) will get hot, and after to much work, will shut down.

    You are measureing the resistance in ohms between live and earth - this is down the live all the way to the transformer, through the transformer coil, out the neutral and onto the earth. Then up the earth and back to the tester. A big loop!

    The transformers internal resistance will be very low, and extreamly low on larger transformers (think of the cable sizes exiting the tranny, the internal coils will need to be sized according to current carrying capacity aswell! (although, they are allowed to run hotter).
     
  9. ban-all-sheds

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    Not too hot though:

    http://205.243.100.155/frames/longarc.htm#Blowup
     
  10. comms

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    Just took a look at the link to tlc for measuring efli it shows diagram of direct measurment, then states dissconnect equipotential bonding before conducting test, GN3 states the opposite(2.7.14) for direct measurement of Zs
     
  11. pdcelec

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  12. mits2hils

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    felling :oops: about not knowing,, do what i did ,, goto ebay and type in this ,,Digital Multi Function Loop Tester (Socket and See) all the instructions are there it will tell you whats wrong and the fault,, then goto electriciandiy.co.uk and there you will proberbly find the right way to resolve your problem,, now you might just feel like :LOL: good luck
     
  13. JohnD

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    if you seriously want to try it out of interest, not professionally, you can often buy a single-function old model, certification expired, very cheaply on fleabay.

    beware of touching the earth probe (if it has one) or anything it is touching during testing as there is a momentary connection to L

    edit: damn, a 5-year old post walks again :oops:
     
  14. ban-all-sheds

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    I don't know whether to laugh or cry....
     
  15. Grizzly

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    Cry, I reckon.
    Apart from the ropey spelling, grammar & punctuation, and the resurrecting of a 5 year old thread, it doesn't look like 'electriciandiy.co.uk' even exists!
    A great effort for a first post.
     

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