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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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    No one has said that electricians are not competent, and I'm not trying to insinuate that.


    The argument used is simply based on the hierarchy of control for safety management, and nothing else.


    From experience (on some of these sites), I would say the arguments presented by Mike and David would not demonstrate that carrying out the loop impedance testing on every final circuit would provides a sufficient increase in safety to permit the electrical inspector to be exposed to the hazards of carrying out loop tests at IP2X terminals when an alternative method might be available.


    Unfortunately, David's method would leave exposed live IP2X terminals accessible in the period between re-energizing and returning to take the reading. There would need to be a means of control to prevent that access. In any case, you have to follow a method. If the measurement is being carried out at the point of isolation, you have that control, provided you follow the method.


    I've italicised method because the health & safety managers and their health & safety advisors would call this an administrative control in the hierarchy of controls. The hierarchy of controls is considered to be the following list, most effective at the top, least effective at the bottom;
    1. Eliminate the hazard

    • Substitute the hazard

    • Engineering controls protective devices, interlocks, guards, etc. - effectively, separate people from the hazard

    • Administrative controls changing the method of work to avoid hazards and/or reduce the severity

    • Use PPE



    For this reason, putting in place administrative controls (safe method of work) is seen to be trumped hands down by Eliminate and Substitute.


    So, and electrician on one of those sites has an uphill battle to get loop testing on all final circuits permitted.
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  • 0
    No one has said that electricians are not competent, and I'm not trying to insinuate that.


    The argument used is simply based on the hierarchy of control for safety management, and nothing else.


    From experience (on some of these sites), I would say the arguments presented by Mike and David would not demonstrate that carrying out the loop impedance testing on every final circuit would provides a sufficient increase in safety to permit the electrical inspector to be exposed to the hazards of carrying out loop tests at IP2X terminals when an alternative method might be available.


    Unfortunately, David's method would leave exposed live IP2X terminals accessible in the period between re-energizing and returning to take the reading. There would need to be a means of control to prevent that access. In any case, you have to follow a method. If the measurement is being carried out at the point of isolation, you have that control, provided you follow the method.


    I've italicised method because the health & safety managers and their health & safety advisors would call this an administrative control in the hierarchy of controls. The hierarchy of controls is considered to be the following list, most effective at the top, least effective at the bottom;
    1. Eliminate the hazard

    • Substitute the hazard

    • Engineering controls protective devices, interlocks, guards, etc. - effectively, separate people from the hazard

    • Administrative controls changing the method of work to avoid hazards and/or reduce the severity

    • Use PPE



    For this reason, putting in place administrative controls (safe method of work) is seen to be trumped hands down by Eliminate and Substitute.


    So, and electrician on one of those sites has an uphill battle to get loop testing on all final circuits permitted.
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