EICR high Ze reading query

Discussion in 'Electricians' Talk' started by pasty_girl, Apr 27, 2021.

  1. pasty_girl

    pasty_girl New Member


    I have had a EICR done on a rental property and feeling quite frustrated about the lack on information provided in the report (I will not be using the same company again).

    I've got a C3 rating, which I know we don't have to action but would anyone be willing to explain to me what it actually means? The report says:

    High Ze reading (from DB, not cutout)

    The extra information I got was:

    High ZE readings essentially refer to earthing arrangements in the property. High readings could be due to a wiring fault on the earth cable.

    And I have attached what I think is the relevant part of the EICR as a picture file. But:

    Earthing Arrangements TN-S

    External loop impedance, Ze (2) 0.65

    I was trying to look online and looks like this could be a DNO issue?

    Very grateful for anyone who is willing to help, but no worries if not.

    Attached Files:

  2. The Happy Builder

    The Happy Builder Screwfix Select

    The Ze and Zs measurement is used to determine if the main fuse will blow within 5 seconds.

    For that 80 amp fuse the required measurement is around 0.575 to 0.6 ohms, yours is 0.65 ohms.

    This is not a precise measurement as the test result depends on network conditions and meter accuracy, so it’s fair to say the measurement is on the limit of what is acceptable.

    If some connections are cleaned up or cables increased in size before the consumer unit it might be improved upon, but an electrician cannot take the suppliers equipment apart to check it out and there might not be anything that can be done other than reduce the size of the main fuse to 60 amps.

    It is something you need to be aware of and monitor, if it gets worse then action needs to be taken.

  3. pasty_girl

    pasty_girl New Member

    Thank you so much The Happy Builder! Much more sense than I can get out of the company!

    Is it worth me contacting the supplier do you think to get them to come and check their equipment? Can I do that or does an electrician need to?

    Many thanks again.
  4. The Happy Builder

    The Happy Builder Screwfix Select

    If there’s other remedial work to do ask the electrician to investigate whiles do that, it may just need the main earth tidying up.

    I doubt the electric supplier would attend unless it gets worse.
  5. The Happy Builder

    The Happy Builder Screwfix Select

    Having said that the Prospective Short Circuit Current test result is low, I suspect they have not tested it correctly and have taken it from the Ze test between live and earth rather than between live and neutral.

    If the PSC test result is correct then it could be a bad live connection on the DNO side or the property could be a long way from the transformer.

    Is the suppliers main fuse in a damp cellar or somewhere similar?

    As it's borderline then no further action may be required.
  6. pasty_girl

    pasty_girl New Member

    Thank you again.

    I do need to get someone out to change the bathroom spotlights... so I'll ask them to re-check the Ze reading and the PSC test result.

    Not sure what exactly I was paying the original guy for... I will go back if they are found to be inaccurate.

    But if they are low (PSC) and high (Ze) then I'll ask the electrician to take a further look. Thank you!
  7. The Happy Builder

    The Happy Builder Screwfix Select

    It's the joys of testing, you press the button and get a bit of an odd test result then have to work out why.

    Basically the Ze test is to ensure the fuse will blow and the PSC test is to ensure the whole consumer unit won't blow off the wall.

  8. pasty_girl

    pasty_girl New Member

    Haha! Both do sound fairly important.....
  9. spinlondon

    spinlondon Screwfix Select

    TN-S means that the Neutral and Earth are Separate.
    As such, you would expect there to be different values for PEFC and PSSC.
    This is because the Earth cable will be a smaller CSA than the Neutral.
    Also because the Earth with TN-S is usually formed by the sheath of the supply cable, there can be corrosion which would reduce the the conductivity.

    Maximum resistance allowed for TN-S is 0.8 Ohms, 0.65 is pretty good, and probably means a lot of the network has been converted to PME (TN-C-S).

    you can ask the supplier to convert your supply to PME.
    Might be free, might be a charge.
    rogerk101 likes this.
  10. pasty_girl

    pasty_girl New Member

    Thank you spinlondon, very kind of people to give their time.

    More things to ask the electrician about! Do you mean the Ze of 0.65 is not high if it's TN-S?
  11. The Happy Builder

    The Happy Builder Screwfix Select

    The note in the front of the IET Onsite Guide says the typical values quoted by the electricity distributors are TN-C-S 0.35 ohms and TN-S 0.8 ohms.

    Whilst you could have a TN-S supply with a Ze of 0.8 ohms it would mean you could only have a supply fused at 45 amps if you are relying on the suppliers fuse for earth fault protection without an upfront RCD.

    A bit of tinkering tidying up connections may reduce the test result, but so long as the meter tails are short and there's a tidy consumer unit I wouldn't code it anymore than code 3 so as to bring it to your attention.
  12. pasty_girl

    pasty_girl New Member

    Thank you Happy Builder - you've been very kind.
  13. The Happy Builder

    The Happy Builder Screwfix Select

    The fuse needs to blow within 5 seconds, it takes Michael Caine around 9 seconds to do that count down in "The Italian Job", 5 seconds is actually quite a long time because as I said in another post recently:

    Generally in the UK the suppliers' main fuse is used to provide both overload and fault protection to:
    1. The meter tails and metering equipment.
    2. Between live parts and the consumer unit metal enclosure.
    3. The main switch.
    4. The internal tails.
    5. The RCDs.
    6. The surge protection device and its pig tails.
    7. The busbars.
    So you don't want a particularly slow blowing fuse.

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