Changes to electric circuit characteristics

Oct 03, 2019 Last reply: 6 years ago 6 Replies
C

I have had 3 extra lights and a fan added to a single lighting circuit.



The base circuit was installed completely new less than two years ago as part of a CU change and came with an Electrical Installation Certificate.



The minor works certificate shows that the extra lights and fan have had the following effect on the particular circuit characteristics



Circuit Impedance R1+R2 reduced to 0.77 Ohms from 1.31 Earth Fault Loop Impedance reduced to 0.98 from 1.62 RCD Operation Time measured at 24 ms (was 33 ms @I and 16ms @5I)


These changes look quite significant to me but I would like expert opinion are they


1) Typical for what might be expected following this sort of a mod.
2) Typical variation between test meters as two different electricians were involved.
3) Some combination of 1&2
4) A fault in the installation which requires further investigation
5) Some other issue.

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A

loose connection somewhere was tightened? Or original tester had about

0.5 ohm not nulled-out in test leads?
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J

I would not worry about the RCD time since its a pass in either case.

If measuring like for like, then no.

Possibly.

Unlikely IMHO

Its possible that the original measurements were made at the furthest lighting point on the circuit, while the new ones were made either at the newly installed positions, or at a different "furthest" point on the circuit[1]. If the new readings were made closer to the CU, then that might account for the difference.

It could also be the original readings were artificially high for some reason, e.g. test lead resistance not accounted for, or meter calibration difference.

Less likely, but possible, if could also be there was a lose connection that was corrected during the new installation work.

[1] Remember that lighting circuits are typically radials, and might have any number of branches rather than being one sequential run of cable from each light to the next. So you would need a table of the R1+R2 or ELI at all the lighting positions to get the full picture.
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C

As its over two months since the work was done and I have only just managed to extract the certificate I was wondering if it is possible that testing was never carried out and they might just be random numbers that happen to be compliant?

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B

Power factor and all that complex stuff does my head in. Does it work? Anything getting over warm or tripping? OK no issue! grin. Brian

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A

The minor works certificate is not a test of the full circuit but only a test of the work that was done.

If the new lights are closer to the CU than the ones tested in the installation results (which would in most cases be the furthest ones) then you would expect lower earth loop readings.

RCDs often give different readings to suit themselves on different testers or indeed on the same tester.

And we no longer record the 5x testing.

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J

Could be - Why not measure them yourself and find out?

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