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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. 
  • 0
    gkenyon:
    Sparkingchip:

    I cannot think of any installation dead tests that cross wired installation would not pass.


    Andy B


    So a Ze or ZDB test could check for a supply wiring error? I think that's back to where we were - discussing live Zs tests on ALL final circuits.




    You have to do a live test somewhere along the line.


    Three phase you will be checking the phase sequence as well as polarity, I had an interesting discussion with two electricians in a pub one night, one of them works for a generator hire company and makes great use of a long wander lead to ensure he is hooking up temporary supplies correctly.


    Andy B.


  • 0
    Sparkingchip:

    I cannot think of any installation dead tests that cross wired installation would not pass.


    Andy B


    So a Ze or ZDB test could check for a supply wiring error? I think that's back to where we were - discussing live Zs tests on ALL final circuits.


  • 0
    lyledunn:
    Sparkingchip:

    I cannot think of any installation dead tests that cross wired installation would not pass.


    Andy B


    Indeed Andy. It looks like the tester you are using is of the type that would alert you to a live line conductor even  if another potential was not available. Some testers do not offer that facility and one needs to be careful as say a polarity reversal on a TNCS system would not be apparent when tested at a single-phase intake.


     




    It self tests as well.


    Andy B.


  • 0
    Sparkingchip:

    I cannot think of any installation dead tests that cross wired installation would not pass.


    Andy B


    Indeed Andy. It looks like the tester you are using is of the type that would alert you to a live line conductor even  if another potential was not available. Some testers do not offer that facility and one needs to be careful as say a polarity reversal on a TNCS system would not be apparent when tested at a single-phase intake.


     


  • 0
    I cannot think of any installation dead tests that cross wired installation would not pass.


    Andy B
  • 0
    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.

  • 0
    Looks to me like a cross on the outgoing side of the VOELCB. The incoming phase (red) is on the left side, and there is a bit of blue “inner” insulation on the left side outgoing tail. This is what I am basing my decision on. 


    Regards,


    Alan.
  • 0
    LOL. put your thinking cap on, I have posted these pictures before in a discussion.


    The RCBOs trip when tested, so the lights go out, the telly stops working and so so, IT APPEARS that everything in the house is isolated but in reality everything in the house is live.


    So nobody knows there is anything wrong, it is probably one of the most dangerous mistakes an electrician can make.


    Andy B.
  • 0
    Sparkingchip:
    Chris Pearson:

    I'll bet you any money that those RCBOs are not tested monthly!


    The single pole RCBOs are all in the off position, remember the bit about single pole devices having to be in the live conductor, that's not much use when there is reverse polarity on the entire installation.




    OK, so the tenant diligently presses the test buttons and nothing happens; but the lights come on, the telly works, heaters get hot, and motors go round and round. Does he (or she):


    (a) notice?

    (b) think that it is a bit odd, but they are all the same?

    (c) ask a question in here?

    (d) panic?

    (e) isolate and request an emergency call out from the nearest qualified sparks?


  • 0
    Chris Pearson:

    I'll bet you any money that those RCBOs are not tested monthly!


    The single pole RCBOs are all in the off position, remember the bit about single pole devices having to be in the live conductor, that's not much use when there is reverse polarity on the entire installation.


    Andy B.


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