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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)...

248ee514524cf5398885518b2007a96a-huge-image.png


b952bae4d3b1f32d959d675c6ede9a16-huge-image.png
05733e3016557d58306811936bac5e50-huge-image.png


Many thanks if anyone is able to confirm my concerns or otherwise put me straight...
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  • 0
    Sparkingchip:

    For an installation that appears to have been installed around the 1960's the EICR from five years ago was remarkably free of observations, whilst the latest EICR has some unconvincing observations. 


    Quite so!


    Observation 4: "37 power circuits have an absence of RCD (411.4.9; 411.5.2; 531.2)". That may be so, but 411.4.9 disappeared with 18th Edn; 411.5.2 concerns TT installations; and Chapter 53 is very different. However, there are plenty of socket circuits without RCD protection.


    Observation 10: "FF3/C17 untraceable (Left isolated for further investigation)." Yes but what about the other unidentified circuits?


    More to the point, why did the 2015 sparks not comment on these issues?


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  • 0
    Sparkingchip:

    For an installation that appears to have been installed around the 1960's the EICR from five years ago was remarkably free of observations, whilst the latest EICR has some unconvincing observations. 


    Quite so!


    Observation 4: "37 power circuits have an absence of RCD (411.4.9; 411.5.2; 531.2)". That may be so, but 411.4.9 disappeared with 18th Edn; 411.5.2 concerns TT installations; and Chapter 53 is very different. However, there are plenty of socket circuits without RCD protection.


    Observation 10: "FF3/C17 untraceable (Left isolated for further investigation)." Yes but what about the other unidentified circuits?


    More to the point, why did the 2015 sparks not comment on these issues?


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