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Can Zs at DB ever be less than the Zs of the feeding circuit?

Former Community Member
Former Community Member
I am reviewing an EICR recently issued for a building with several Distribution Boards feeding sub-Distribution Boards.

I have noted that in some instances, the figure recorded for 'Zs at this board' is significantly less than the Maximum Measured Zs for the circuit recorded on the feeding DB.

e.g. DB FF4 is recorded as being fed from DB FF1.  The feeding circuit to DB FF4 is recorded as having a Maximum measured Zs of 0.4 Ohm, but the 'Zs at this board' for FF4 is recorded as 0.05 Ohm - which is less than the 'Zs at this board' recorded for FF1 (0.08 Ohm) - and which, is in fact, in turn itself less than the 'Maximum measured Zs' for the circuit feeding it.  Can this be true or are there errors in the report?  I thought that cascaded Zs can only get larger due to the added impedance of the feeding circuits? This is not my primary area of expertise, but I am concerned that the EICR is being used to justify the upgrade of several circuits which have passed previous inspections with no problem (hope the resolution of the extracts from the EICR below are sufficient resolution to read)...

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Many thanks if anyone is able to confirm my concerns or otherwise put me straight...
  • 0
    Was a copy of the 2015 EICR made available to the electrician who completed the recent EICR?
  • 0
    Date testing carried out: 24th June 2020. (first page)

    Date of calibration of Fluke 1664: 26th June 2020. (last page)


    We don’t know whether the test equipment was calibrated when the testing was carried out. . . Oh dear . . . If they can’t prove the test kit was calibrated, all bets are off. 


    Regards,


    Alan.
  • 0
    Peter S3:

    Hi Weirdbeard.  Thank you for your confirmation regarding the report

    Hopefully you will find the full report attached for your entertainment!

     


    Oh dear.


    Curious the inspector thinks that 0.2s disconnection times apply to a TN-C-S system and 0.4s for distribution circuits. Also that the entire inspection schedule can be N/A'd - likewise the rating of some protective devices. I find some of the observations equally puzzling. It also seems rather unlikely that a distribution board would contain a mix of BS EN 60898 MCBs and BS EN 60947 MCCBs as outgoing devices. Or that the feed to FF4 really was lacking a c.p.c. - especially as he does seem to have obtained a Zs value for that circuit. As others have previously mentioned, PFC figures don't tally with earth loop impedance readings for a 3-phase system.


       - Andy.


     


  • Former Community Member
    0 Former Community Member
    Sparkingchip: I don't know if the 2015 EICR was made available to the new inspector before their inspection.  I presume not, as if it had, they probably would have been able to locate their 'untraceable' circuits - and maybe to query themselves a bit more closely whether a previous NICEIC-approved inspector would really have missed such a major claimed error as a 63A 3ph DB being fed from a 32A 1Ph MCB...

    I am not sure if it is normal practice to provide the previous EICR to a new inspector, or to ensure/require the new inspector to carry out their inspection 'unbiased'? regards, Peter.

  • 0
    (1) What we normally call "calibration" is simply a calibration check, so provided that the machine was not re-calibrated, we must assume that it was performing satisfactorily during the whole interval between checks.


    (2) Comparison of successive EICRs is an important part of the process: even though a circuit may be within spec, any deterioration needs to be considered. It may justify earlier re-inspection. However, I do take the point that if previous EICRs are made available at the outset, they may introduce bias.
  • Former Community Member
    0 Former Community Member
    Peter S3:

    Hi Weirdbeard.  Thank you for your confirmation regarding the report

    Hopefully you will find the full report attached for your entertainment!


    Edited to remove link by WB


    Thank you for posting the reports, they make an interesting comparison!


    Do you have any idea why the 2015 inspection company was not employed for the 2020 inspection?


  • Former Community Member
    0 Former Community Member
    weirdbeard:
    Thank you for posting the reports, they make an interesting comparison!


    Do you have any idea why the 2015 inspection company was not employed for the 2020 inspection?



    I do not know this for sure.  I have been told that an unsuccessful attempt was made to contact them.  I will be trying again myself.

     


  • 0
    I assume it’s the rubbish software used for this forum, both reports have disappeared.


    With hindsight I should have been able to answer my own question as to whether the existing report was made available to the latest inspector, because it’s a question that is asked and should be answered on the new report. Of course existing reports should be made available to inspectors to allow them to work more quickly and also be able to spot additions, alterations and deterioration within the installation.


    However the latest inspector is not very good at answering questions as can be seen on the schedule of inspections with every item being mark non-applicable.


    I would not get hung up on the matter of how accurate the test meter being used is, because it’s a hell of lot more accurate than the person using it.


    If that calibration certificate dated two days after the testing says it’s accurate and did not need any adjustments it is more valid proof of the accuracy of the tester used than the previous calibration certificate.


     Andy B.
  • Former Community Member
    0 Former Community Member


    "I assume it’s the rubbish software used for this forum, both reports have disappeared.


    No, sorry, I removed the attachments as I realised that the redacting was not 100%.  Since it is looking like the trustees will need to be seeking resolution on this, I need to be careful.

    I may re-post once I have double-checked the redaction.


    Which raises the question: what would folks advise should be my next step?  The trustees have no confidence to invite the original contractor to repeat/correct the report, and it does not appear that he is NICEIC approved (I was not involved in his initial selection).  I am considering to ask whether he wishes to retract his report or allow us to submit it for independent review - with potential resulting claim through legal channels.



  • 0
    Its always wise to know when to stop giving advice. Legal channels now mentioned, you can picture the scenario, the boys on here who have been giving their best receive notes through the post, "please attend court in the matter of the disputed Earth loop impedance and others" 


    Of course. if the contract is offered perhaps one of the members will step forward. Anyone with Church experience?


    Regards, UKPNZap

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