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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
  • 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 Ze or ZDB.


    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.
Reply
  • 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 Ze or ZDB.


    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.
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