measured zs too high vs calculated

[darren bergin]

sce4nario

measured zs with meter too high! above max permissible zs

but calculated value is well within specification

what gives??
is the circuit ok?
what do i do on test results??
thanks

 

[Spark123]

Give us some figures!
Above max Zs is a fail so you can't issue an EIC for it
Could be a loose connection.

 

[darren bergin]

it is ring circuit
Ze 0.08
readings taken from socket outlet(easier)
L-L 0.35
N-N 0.35
E - E 0.60

CROSS L-N 0.15
CROSS L-E 0.24

From this now the calculated value of zs = 0.32
is this correct yes ze +(r1+r2)

just looked at my regs book again and it is within permissable max zs

but do these figures look right
why are calculated and measured so different

 

[Adam_151]

What Zs did you measure?

 

[ericmark]

Ze on a TN-C-S supply is taken as 0.35 ohms prior to the supply being connected when we would be using R1 + R2 figures.
However Ze measured could be well over 0.35 ohms in reality and this could of course give measured reading for Zs also over the permitted level.
With any RCD of course Ze only has to be better than 200 ohms and it is unlikely to ever get that high so would be rather unusual with a new installation.
But where measured Zs is higher than calculated Zs I would be worried often the other way around where we have parallel earths. I would be re-testing R1 + R2 and seeing if any wires had come lose and also plugging the ELI tester into the selected socket at home to see if the calibration seemed to still be within limits.
I know socket near my consumer unit is 0.34 and in kitchen the furtherest socket is 0.56 and I regularly test them so to get some warning if the meter starts to drift off calibration.
With a PIR I would not normally measure R1 + R2 unless something seemed wrong I would calculate from Ze and Zs if required in form I was using.

As already said what are the figures and what is the circuit and when was the ELI tester calibrated and do you use a designated socket for weekly test?

 

[darren bergin]

zs measures 0.60

 

[Spark123]

If the Ze on a TN-CS is over 0.35 then the DNO gets a kick up the backside!

You have to be careful when using small case and capital letters - they mean two different things!

r1 = end to end resistance of the phase conductor of a RFC
r2 = end to end resistance of the protective conductor of a RFC
rn = end to end resistance of the neutral conductor of a RFC

Ze = Zs + (R1+R2)

R1+R2 for all sockets on the ring will be roughly (r1+r2)/4 but R1+R2 will be higher on any spurs. The highest R1+R2 which you measure with your interconnections in is the one you record.

Also you might get lower figures if you pop the sockets forward and measure off the back.

 

[ericmark]

Just seen figures Zs = 0.3175 is quite low for a socket on B32A MCB with no RCD you are allowed 1.44 ohms so well within.
Did you leave anything plugged in when you tested? Get a couple of items left plugged in it can really mess up L-L reading.

 

[darren bergin]

ze is fine
its just the zs calculated and zs measured that looked odd
it is 32a type protected by rcd circuit

 

[Spark123]

Was it measured on a spur? Where the sockets all new? Sometimes the contacts in sockets are not brilliant and measuring off the socket back gives better results.

 

[darren bergin]

Just foundthis
seems to resemble my situation
but would zs calculated vs measured be 0.30 apart??
>>>>>>>>>>>>
it is a possibility that your measured values of Zs were obtained using an earth fault loop impedance test instrument employing some form of ‘soft test’ facility that does not operate the RCD when the test is carried out.

In the NICEIC’s experience, values of Zs measured using such a facility can be significantly less accurate than would be possible if an earth fault loop impedance test instrument using a ‘conventional’ test facility were used and an RCD were not in circuit. Such reduced accuracy could explain for the apparent disparity between your measured Zs values for these two circuits and the corresponding values of Zs obtained by the formula Zs = Ze+ R1+R2.
For circuits having conductors of up to 25 mm2 operating at a nominal frequency of 50 Hz, inductance may be ignored and it is acceptable to determine Zs using the above formula, rather than by measurement using an earth fault loop impedance test instrument.

 

[darren bergin]

i would say they are perfect results apart from zs

 

[Lectrician]

The NIC has long recommended that Zs values be calculated on new circuits. The R1 + R2 (and ring ends etc) needs to be done BEFORE energising anyway, and then you do a Ze test. With these two results, you have the Zs, so why measure?!?

 

[sparkiedave]

As suggested allready, try taking a reading or two from the backs of sockets, there is sometimes resistance on the switches of sockets, with old sockets I often flick the switch off and on a few times then measure again, often makes a difference, have you tried zs from the output terminals of the RCD? shows up bad contacts in the RCD

 

[17thman]

as spark 123 says, are you measuring off a spur?