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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:

    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?


    See mapj1's posting above - the 4th response..


    I think that you are correct.


    I always think that it helps to draw out the installation.


    My algebra isn't what it used to be, but call the impedance of the conductors to the final DB R1a and R2a; and the impedance of the final circuit conductors R1b and R2b. Zs db = Ze + R1a + R2a. Zs at let's say a luminaire = Ze + R1a + R2a + R1b + R2b. Bolt that luminaire to the steel frame of a building which is bonded to the MET. Now assume that the parallel path through the building has a trivial impedance so R2a and R2b may be discounted. Zs now = Ze + R1a + R1b and Zs db = Ze + R1a + R2a. If Zs < Zs db, Ze + R1a + R1b < Ze + R1a + R2a i.e., cancelling the terms which are on both sides, R1b < R2a.


    So Zs of the final circuit will be lower than Zs db only if the parallel pathway has a trivial impedance and the impedance of the line conductor of the final circuit is less than that of the CPC of the distribution circuit. That would be most unusual!


    I hope that this makes sense - I could have done better 40 years ago!


     


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  • 0
    Peter S3:

    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?


    See mapj1's posting above - the 4th response..


    I think that you are correct.


    I always think that it helps to draw out the installation.


    My algebra isn't what it used to be, but call the impedance of the conductors to the final DB R1a and R2a; and the impedance of the final circuit conductors R1b and R2b. Zs db = Ze + R1a + R2a. Zs at let's say a luminaire = Ze + R1a + R2a + R1b + R2b. Bolt that luminaire to the steel frame of a building which is bonded to the MET. Now assume that the parallel path through the building has a trivial impedance so R2a and R2b may be discounted. Zs now = Ze + R1a + R1b and Zs db = Ze + R1a + R2a. If Zs < Zs db, Ze + R1a + R1b < Ze + R1a + R2a i.e., cancelling the terms which are on both sides, R1b < R2a.


    So Zs of the final circuit will be lower than Zs db only if the parallel pathway has a trivial impedance and the impedance of the line conductor of the final circuit is less than that of the CPC of the distribution circuit. That would be most unusual!


    I hope that this makes sense - I could have done better 40 years ago!


     


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