Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 82 replies
  • Subscribers 32 subscribers
  • Views 9479 views
  • Users 0 members are here
Options
You may also be interested in
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

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...
Parents
  • 0
    Quite right - a Zs reading will always exceed R1 unless something very odd is happening.  What varies with location a is how much of of the R2 contribution  is  by the wire you think it is, and how much is bypassed back to somewhere nearer the source earth by conduit, girders, plumbing etc.

    And you can always get a high Zs at any point along  the line if that box of tricks is not as well earthed.as it should be, or if the 'live' chosen for probing purposes was a bit tarnished or scratchy (a plug in test at a slightly corroded wall socket perhaps)

    At the 10 milliohms level you do not need much to throw the readings quite a long way out.

    But in summary before paying for any re-work, get the inspector to correct the errors - there may well be more than you have shown us.
Reply
  • 0
    Quite right - a Zs reading will always exceed R1 unless something very odd is happening.  What varies with location a is how much of of the R2 contribution  is  by the wire you think it is, and how much is bypassed back to somewhere nearer the source earth by conduit, girders, plumbing etc.

    And you can always get a high Zs at any point along  the line if that box of tricks is not as well earthed.as it should be, or if the 'live' chosen for probing purposes was a bit tarnished or scratchy (a plug in test at a slightly corroded wall socket perhaps)

    At the 10 milliohms level you do not need much to throw the readings quite a long way out.

    But in summary before paying for any re-work, get the inspector to correct the errors - there may well be more than you have shown us.
Children
No Data

Community Rules & Guidelines