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Discuss High Zs values (Just) in the Electrical Forum area at ElectrciansForums.co.uk.

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  1. wirepuller
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    wirepuller Forum Mentor

    Location:
    south uk
    Given that Amd3 reduced max Zs values slightly what is everyone coding if a measured reading falls between the pre Amd3 value and the post Amd3 value. Failure to meet disconnection times would normally be a code 2, but a code 3 seems appropriate here......maybe a note to explain? On E-certs it will show a measured reading exceeding the maximum value given in the schedule.
    Just seems silly that not long back a reading met disconnection times and now it doesn't, nothing has actually changed.
     
  2. telectrix
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    telectrix Scouser and Proud of It Respected Member

    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    hmm. if you consider the load on the circuit as opposed to the In of the OCPD, then you can assume that the cable may never reach 70 deg.C and so no need to apply the 0.8 temp. correction. tin hat glued in place.
     
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  3. Ian1981
    Online

    Ian1981 Forum Mentor

    Location:
    North east
    I’m not sure we have any wiggle room to be honest when using max zs values.
    It is what it is for the British standard to what it complies to or what the manufacturers state if asked.
    I would apply a C2 if you can’t meet the stated disconnection times stated in bs7671 ie 0.4 or 5 seconds for TN arrangements.
     
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  4. Flanders
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    Flanders Regular EF Member

    Location:
    Tamworth
    This is what frustrates me about coding .It was fine last year no code but this year its dangerous ( according the regs), No saying that I would give it a code 2 as it was poorly designed in the first place if it was so close the the maximum permitted value, My reasoning for this is we are finding things all the time that were fine years ago but now with more knowledge are finding out things are not so safe ( asbestos for example) .I have always given code 3 for no RCD protection for cables burred in wall , but the other day a contractor got an electric shock because someone had screwed up a metal display unit and one if the screws went straight into the live conductor of the twin and earth cable making the unit live (No rcd protection ) so now i`m thinking should I be giving cables burred in walls less that 50mm a code 2
     
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  5. Wilko
    Online

    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
    Perhaps it's low current (no trip) earth loop measurement inaccuracy? And you could mark it for Further Investigation?
     
  6. westward10
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    westward10 In echoed steps I walk across an empty dream. Staff Member Moderator

    Top Poster Of Month

    Location:
    Northamptonshire
    You have to draw the line, either dither about thinking it was okay x amount of years ago and flip a coin or follow the new values blindly, regardless. I am with #3 on this.
     
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  7. wirepuller
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    wirepuller Forum Mentor

    Location:
    south uk
    Asking because this exact scenario has cropped up today, going with a code 3 and a note because I cant bring myself to state an installation that was compliant 3 years ago is now potentially dangerous. A further complication is that on E-certs the measured value will exceed the stated max value hence the need for a note I feel. Where this crops up in future I'll need to refer to pre Amd3 regs as well as the new if I take the code3 approach.
    It's a bloody minefield.
     
  8. wirepuller
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    wirepuller Forum Mentor

    Location:
    south uk
    No mate it's a good reading (high current) and pretty well matches the value on the previous EICR.
     
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  9. westward10
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    westward10 In echoed steps I walk across an empty dream. Staff Member Moderator

    Top Poster Of Month

    Location:
    Northamptonshire
    Have you measured Uo and calculated what the max Zs would be although voltages can change.
     
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  10. Ian1981
    Online

    Ian1981 Forum Mentor

    Location:
    North east
    T
    Thats your call but the cmin factor was added in to provide compliance with the scenario of a drop in supply voltage.
    I hear you about changes that now make installations potentially dangerous, just look at the wiring in escape routes headache and now the 18th will introduce metal supports against premature collapse to the entire installation
     
  11. plugsandsparks
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    plugsandsparks Electrician's Arms

    Location:
    Chesterfield
    Hasn't happened to me yet but the EICR i am currently doing (industrial) has always been at the ragged edge as i cannot convince the local electricians to go up a size in cable for long runs, however they produce their own LV on-site with auto tap changes so i wont be coding anything just because of the AMD3 changes to a voltage i have yet to actual see (ever , anywhere) concrete hat on, lol
     
  12. essex
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    essex Forum Mentor

    Location:
    Wiltshire
    Picking and choosing which Reg to follow is a slipery slope in my opinion.
     
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  13. Ian1981
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    Ian1981 Forum Mentor

    Location:
    North east
    I think the lowest voltage I’ve ever measured is about 220 volts
     
  14. essex
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    essex Forum Mentor

    Location:
    Wiltshire
    This issue is solved relatively easily and cheaply by installing an RCD on the circuit in wuestion.

    Code it appropriatly.
     
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  15. Paignton pete
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    Paignton pete Regular EF Member

    Location:
    Over the rainbow
    Shirley the multi function tester takes into consideration the measured voltage when giving the Zs value.
     
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