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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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  • Former Community Member
    0 Former Community Member
    Well spotted!  You are absolutely correct.,  MP1 is in fact a 3-phase DB.  The inspector simply has not made any care in filling out the individual DB details.  The previous report (different inspector, 5 years ago) noted whether each circuit was 3ph or 1ph, but in this report, other than occasional mention (sometimes incorrect) that the DB is 1 or 3ph, the individual circuits are not clarified at all.  I took a quick visual look at the installation myself last night; MP1 is a 3ph DB, fitted with a mix of 3ph and 1ph MCCB, depending on which circuit is being fed.  The 32A MCCB that he claims (in his recommendations but not in his EICR) is feeding 3-phase DB GF2 is, in fact, a single phase MCB and definitely not feeding the 3ph DB GF2!  I am only advising technically as a lay person, but I think I will have to tell those authorising the work that the whole inspection and therefore the entirety of all recommended works have to be seriously held in question, even if some of them do appear to be justified.
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  • Former Community Member
    0 Former Community Member
    Well spotted!  You are absolutely correct.,  MP1 is in fact a 3-phase DB.  The inspector simply has not made any care in filling out the individual DB details.  The previous report (different inspector, 5 years ago) noted whether each circuit was 3ph or 1ph, but in this report, other than occasional mention (sometimes incorrect) that the DB is 1 or 3ph, the individual circuits are not clarified at all.  I took a quick visual look at the installation myself last night; MP1 is a 3ph DB, fitted with a mix of 3ph and 1ph MCCB, depending on which circuit is being fed.  The 32A MCCB that he claims (in his recommendations but not in his EICR) is feeding 3-phase DB GF2 is, in fact, a single phase MCB and definitely not feeding the 3ph DB GF2!  I am only advising technically as a lay person, but I think I will have to tell those authorising the work that the whole inspection and therefore the entirety of all recommended works have to be seriously held in question, even if some of them do appear to be justified.
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