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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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    Harry Macdonald:

    "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 all accept this as common practice."

    This seems to me to indicate that various, mainly large, organisations are now laying down their own rules which would imply that the methods described in BS7671 are widely seen as unsafe or impractical.

    Have we now reached the stage where the BS says one thing but everybody knows that they mean another? If so, that is a very worrying and dangerous state of affairs and exactly what a BS is designed to stop happening.

    I hope I am wrong, but with the number of luminaires where you can't easily remove the lamp, switch wire - neutral testing has gone. Live loop impedance seem to have gone, except for sockets, electronics connected to circuits means some don't even do insulation tests.

    I hope I am wrong, but I see a dangerous divergence between what the standards say and what electricians now actually do.


    BS 7671 does NOT require a loop impedance measurement on every circuit - or in fact, on any circuit!



    643.7.3 Earth fault loop impedance

    Where protective measures are used which require a knowledge of earth fault loop impedance, the relevant impedances shall be measured, or determined by an alternative method.





    Nor does BS 7671 require a measurement of prospective fault current.



    643.7.3.201 Prospective fault current

    The prospective short-circuit current and prospective earth fault current shall be measured, calculated or determined by another method, at the origin and at other relevant points in the installation.





    In fact, according to Appendix 14, in domestic (household) premises where the distributor declares a 16 kA value, you can assume your prospective fault current is 16 kA (although it's likely to be less than this) and not take a measurement at all.



     


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    Harry Macdonald:

    "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 all accept this as common practice."

    This seems to me to indicate that various, mainly large, organisations are now laying down their own rules which would imply that the methods described in BS7671 are widely seen as unsafe or impractical.

    Have we now reached the stage where the BS says one thing but everybody knows that they mean another? If so, that is a very worrying and dangerous state of affairs and exactly what a BS is designed to stop happening.

    I hope I am wrong, but with the number of luminaires where you can't easily remove the lamp, switch wire - neutral testing has gone. Live loop impedance seem to have gone, except for sockets, electronics connected to circuits means some don't even do insulation tests.

    I hope I am wrong, but I see a dangerous divergence between what the standards say and what electricians now actually do.


    BS 7671 does NOT require a loop impedance measurement on every circuit - or in fact, on any circuit!



    643.7.3 Earth fault loop impedance

    Where protective measures are used which require a knowledge of earth fault loop impedance, the relevant impedances shall be measured, or determined by an alternative method.





    Nor does BS 7671 require a measurement of prospective fault current.



    643.7.3.201 Prospective fault current

    The prospective short-circuit current and prospective earth fault current shall be measured, calculated or determined by another method, at the origin and at other relevant points in the installation.





    In fact, according to Appendix 14, in domestic (household) premises where the distributor declares a 16 kA value, you can assume your prospective fault current is 16 kA (although it's likely to be less than this) and not take a measurement at all.



     


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