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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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    Zs at DB should be the value of Ze plus the total R1+R2 of the total circuit feeding the DB right? R2 might be reduced at a particular DB by local bonding, but there will always be some R1 which will generally act to increase the total Zs at DB as you add more DBs into the chain? Zs at DB can be measured by having all feeding circuits live so that you can measure the total Zs back to the feeding transformer (using a calibrated suitable multi-tester), or you can do it by adding your measured maximum R1+R2 values to a measured or known Ze? Maximum measured Zs for a circuit fed from a DB should, in theory (allowing for slight measurement tolerance), always be more than the Zs at DB + R1 for that circuit? (possibly also +R2, but local bonding in the circuit may reduce the effect of the measured R2). Have I understood that correctly?

    Zs will increase by R1 certainly, but parallel paths could possibly reduce the effective R2 - if they reduce it enough it is theoretically possible it might outweigh the increase provided by R1 and so give a lower overall measured Zs.

       - Andy.
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  • 0
    Zs at DB should be the value of Ze plus the total R1+R2 of the total circuit feeding the DB right? R2 might be reduced at a particular DB by local bonding, but there will always be some R1 which will generally act to increase the total Zs at DB as you add more DBs into the chain? Zs at DB can be measured by having all feeding circuits live so that you can measure the total Zs back to the feeding transformer (using a calibrated suitable multi-tester), or you can do it by adding your measured maximum R1+R2 values to a measured or known Ze? Maximum measured Zs for a circuit fed from a DB should, in theory (allowing for slight measurement tolerance), always be more than the Zs at DB + R1 for that circuit? (possibly also +R2, but local bonding in the circuit may reduce the effect of the measured R2). Have I understood that correctly?

    Zs will increase by R1 certainly, but parallel paths could possibly reduce the effective R2 - if they reduce it enough it is theoretically possible it might outweigh the increase provided by R1 and so give a lower overall measured Zs.

       - Andy.
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