On 03/10/2019 13:51, John Rumm wrote:
On 03/10/2019 11:53, Chris B wrote:
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 anÂ* 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)

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

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.

If measuring like for like, then no.

2) Typical variation between test meters as two different electricians
were involved.

Possibly.

3) Some combination of 1&2

4) A fault in the installation which requires further investigation

Unlikely IMHO

5) Some other issue.

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.

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?




[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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Chris B (News)