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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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  • 0
    mapj1:

    Advising improved surge protection, or better earth bonding and more RCDs are improvements I'd sort of expect in an old building when met with new regs, 




    Surge protection and bonding the lighting protection system to the church electrical installation requires particular attention to detail and specific knowledge, GK told me not to do it without getting a specification from the lighting protection engineer.


    So I expect there should be an observation saying consult the lighting protection engineer about installing surge protection and bonding the LPS- code FI.


    I should add I went on an unexpected 125 mile round trip this afternoon, which gave me three hours to ponder life, work and the universe and I went by a church for which I quoted a rewire and these matters came under discussion, which set me thinking.


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  • 0
    mapj1:

    Advising improved surge protection, or better earth bonding and more RCDs are improvements I'd sort of expect in an old building when met with new regs, 




    Surge protection and bonding the lighting protection system to the church electrical installation requires particular attention to detail and specific knowledge, GK told me not to do it without getting a specification from the lighting protection engineer.


    So I expect there should be an observation saying consult the lighting protection engineer about installing surge protection and bonding the LPS- code FI.


    I should add I went on an unexpected 125 mile round trip this afternoon, which gave me three hours to ponder life, work and the universe and I went by a church for which I quoted a rewire and these matters came under discussion, which set me thinking.


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