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Zs, to test or calculate?

Paul Brewerton
A large contractor working on our site have told me yesterday that it is their policy not to live test final circuits where they cannot use a plug? In order to reduce risk, they will now only calculate Zs, on circuits where they would have to open an enclosure, such as FCU's and light fittings.
The control measures we insist are in place, are IP2X equipment, GS38 leads, two man rule with second man having resus training, among others. So I feel the risk has been reduced to as low as reasonably practicable. An d my instinct tells me that a measured Zs reading must be more accurate than a calculated one, since it will include all parallel paths under test.
The contractor is happy to live test distribution circuits, so it seems they want to pick and choose.
They also state that this is how things are now, and have worked at many different sites, Cross Rail, Heathrow, various MOD sites etc, and that they al accept this as common practice.

I like some opinions to find out what's going on out there on other sites. 
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    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.
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    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.
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