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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...
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    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.


     


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  • 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.


     


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