Zs, to test or calculate?

Pencils can be truly wonderfull things ?

I know an electrician, now retired, who didn’t have his NICEIC renewal signed of by his assessor around fifteen years ago after a few clashes.


One clash was when his assessor suggested he bought a non-trip loop tester to get more accurate loop test results on RCD protected circuits and he replied saying he had a pencil.


On a personal note I still have my original Robin multifunction installation tester, the main reason I “upgraded” by buying a Megger non-trip loop tester to accompany it was the increasing number of RCDs being installed, now it seems all these years later I could have stuck with the Robin and a pencil.


 Andy Betteridge

It seems that nowadays it is acceptable across the board to calculate Zs.

As an aside, I don't think of adding dead R1+R2 results to Ze as being the calculation approach - it's still all based on measurement even if done in parts. For me, calculation would be something like working out the impedance based on the cable length and tabulated resistance per metre - which might have been the approach in the old days when earth continuity would be verified with nothing more than a set of test lamps.

   - Andy.

A loop impedance test of the kind that uses the  50Hz AC supply and switches in a load and looks at the voltage drop will necessarily include all the same impedances, both real and imaginary, that  will appear in a real fault, as that too uses the AC supply.

The accuracy problem is because   the test current is much lower than a likely fault current  (on a TN system anyway) the voltage drop caused during the test is fractions of a volt, and  if the mains is a bit bouncy for other reasons, as it often is if the supply is shared with loads that are switching, then the meter may mistake some of that voltage drop for the result of its own test load, so the results can be a bit 'fruit machine'.

  By keying the test load in and out in a psuedo-random sequence, and then looking at the average voltage droop and rise for each short part of that test, it is possible to cancel out and mitigate this noise problem, at least to a degree. Also when you are looking at PSSC of kA, then every milliohm starts to count, and the condition of test leads and their connectors starts to be a significant part of the reading, and beyond a certain point 'normal' meter  probes are not really good enough.


Also I  have seen the effects of RCD cores that look inductive for small currents, but for  large fault would saturate and not current limit, give an over pessimistic impedance on a very low current (no trip) test. The clue is the Zs readings before and after the RCD are far more different than the DC resistance of the contacts would suggest. Still only tens of milliohms of change, but in some cases that may be misinterpreted as the difference between additional work needed, and not.

I am not convinced that the loop test does not measure impedance. Yes, the current used is DC but, as I understand it, a very short pulse is used so inductive effects should be measured too.

I have also had great difficulty measuring the earth loop impedance of a main incomer, say 400A, because the dip in voltage caused by the brief current pulse was so small that it was masked by the normal spikes on a supply. I tried averaging several successive readings but as some of these were negative, I didn't trust the results. 

I suspect that, to get a proper result, I needed a much beefier bit of test kit, the sort that would take 2 people to carry it.

As far as I know, the reactive component of impedance is not measured by a loop “impedance” tester anyway. Further, the accuracy is always questionable for various reasons. Having that margin of 0.8 Zs7671 as a maximum test value perhaps addresses most issues. A loop impedance on the wrong side of an mcb is worse than a fuse.

For circuits below say about 63 A, we don't worry about the reactance of the cable - see tables at the back of BS 7671. Most of the reactance is in the external loop, and that's why we would normally measure at least Z_e or Z_DB.


Appendix 14 of BS 7671 advises that, under many circumstances, measurement prospective fault current in domestic installations is not necessary.


For larger circuits, I agree there's perhaps something missing, but it's worth remembering that, due to potentially larger prospective fault currents / lower loop impedances, and especially where you are close to the transformer, the loop impedance or prospective fault current measurement itself may be inaccurate. For larger loads, and even final circuit disboards, there is no reason at all why a test socket-outlet specifically for prospective fault current and loop impedance measurement, with suitable backup protection could not be provided, just below the DB or built into MCCs etc. - this is part of the CDM process.


Don't forget that Appendix 14 of BS 7671 advises that fused leads alone may not provide protection if backup protection at the point of measurement is not present.

Method 1 (R1+R2) in the OSG and GN3 doesn't show the cpc being disconnected.

Very good point Graham. I've certainly seen the c.p.c. and line 'choc blocked' together suggested somewhere (I'll have to rack my brains as to where now) - and was under the impression that it was frequently taught that way too - I've certainly seen it been done that way in practice. Good news if that's officially incorrect.

 

Why would the impact of parallel paths differ between Method 1 to Method 2?

I mean to suggest it would - just that disconnecting the c.p.c. from the Earth bar would eliminate some parallel paths (which I understood was the reason for the disconnection approach in the first place).


  - Andy.