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Zs, to test or calculate?

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. 
Parents
  • That reversal is a class act and a good teaching example for why not live testing is a bad idea.  With a global LN reversal as we have here everything will work, even the test buttons. assuming neutral is afforded the same insulation as live, as it should be, then nothing may be noticed for ages. 

    It will presumably also pass all the dead tests, at least if you switch off at the double pole mains switch to perform them.

    Of course  an LE fault will may trip the old VOELCB,  and maybe the RCBOs but as they only open one pole they are not that useful in terms of making anyone safe.

    The double pole Wylex/Crabtree ones may be a better bet, but nothing beats not swapping the meter tails. Clearly brown blue polarity was not reversed at the CU main switch , which would have been wrong too, but safer and consistent.

    I presume the Bulgarian builder said whatever the Bulgarian is for 'goodness me' if he touched the black wires.

    But it also shows you need to be able to work live, as you may be doing so inadvertently.

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  • That reversal is a class act and a good teaching example for why not live testing is a bad idea.  With a global LN reversal as we have here everything will work, even the test buttons. assuming neutral is afforded the same insulation as live, as it should be, then nothing may be noticed for ages. 

    It will presumably also pass all the dead tests, at least if you switch off at the double pole mains switch to perform them.

    Of course  an LE fault will may trip the old VOELCB,  and maybe the RCBOs but as they only open one pole they are not that useful in terms of making anyone safe.

    The double pole Wylex/Crabtree ones may be a better bet, but nothing beats not swapping the meter tails. Clearly brown blue polarity was not reversed at the CU main switch , which would have been wrong too, but safer and consistent.

    I presume the Bulgarian builder said whatever the Bulgarian is for 'goodness me' if he touched the black wires.

    But it also shows you need to be able to work live, as you may be doing so inadvertently.

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