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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...
Parents
  • 0
    Another one of those EICRs that has the insulation resistance of every circuit at +500Mohms! I suspect those straight away! However, when Zs is very low, accuracy is an issue and becomes problematic when trying to assess disconnection times. As others have pointed out, there are many issues in the Sections of the report you have posted. By the way, I have 2 really  good guys who work for me, specifically carrying out I and T every day and I have often had to buttonhole them on some quite serious mistakes they have made. I hope that by calling their attention to them and requiring them to return to the installation they will learn and operate with a little more care. I may have years of experience under my belt but I do miss things occasionally. Only last week NICEIC inspecting engineer pulled me on a really stupid mistake I had made!

    Do the industry a favour and get the guy back to address the issues.
Reply
  • 0
    Another one of those EICRs that has the insulation resistance of every circuit at +500Mohms! I suspect those straight away! However, when Zs is very low, accuracy is an issue and becomes problematic when trying to assess disconnection times. As others have pointed out, there are many issues in the Sections of the report you have posted. By the way, I have 2 really  good guys who work for me, specifically carrying out I and T every day and I have often had to buttonhole them on some quite serious mistakes they have made. I hope that by calling their attention to them and requiring them to return to the installation they will learn and operate with a little more care. I may have years of experience under my belt but I do miss things occasionally. Only last week NICEIC inspecting engineer pulled me on a really stupid mistake I had made!

    Do the industry a favour and get the guy back to address the issues.
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