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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
    Just to throw in a curved ball, how do you do a dead test on an existing installation? 


    You obviously must start with “proving dead”, which must itself be a live test. Even if you believe you have isolated it, there is still the chance that the isolation has failed for some reason. Until you have proved dead with your approved test lamp (including proving the test lamp before and after use), there must be the very real expectation that the circuit under test is still energised when you first expose the terminals. 


    Regards,


    Alan.
  • 0
    Sparkingchip:

    From experience I would say two people on a circuit is more dangerous than one, I have seen other electricians working in pairs energise circuits whilst one of them is still working on it, one pair did it three times in one day.


    In which case they have not undertaken safe isolation. Lock off with a hasp and one padlock each.


  • 0
    gkenyon:
    Sparkingchip:

    Is opening the ceiling rose of a pendant, which does not require any tools to test anymore dangerous than removing the cover of the consumer unit to test Ze, etc?


    Andy B


     




    Polarity Test can be a dead test, and in that respect is far less dangerous than removing the cover of the consumer unit to conduct live tests.




    Connecting a loop tester performed more than one test at the same time. 


    To be honest when changing a light fitting I will connect the tester to check polarity,  loop impedance and then trip the RCD if there is one before taking down the old fitting,  then walked back to the consumer unit to isolate,  then confirm the fitting is dead and so on and do forth.  Bearing in mind there's lots shoddy wiring out there to catch out the unwary, like the Bulgarian builder who dismantled this socket fitting and the bathroom fitter who pulled the cable into the back of it.

    3f59e353d703862c9abfa793d31c1e0d-original-20200311_0940570.jpga1e4572ccee6829c9ff8f11483404ebe-original-20200311_095810.jpgb3a65e27b89fa5c902044e530bed83e2-original-20200311_095403.jpg


  • 0
    Sparkingchip:

    Connecting a loop tester performed more than one test at the same time. 


    To be honest when changing a light fitting I will connect the tester to check polarity,  loop impedance and then trip the RCD if there is one before taking down the old fitting,  then walked back to the consumer unit to isolate,  then confirm the fitting is dead and so on and do forth. 




    Agreed ... but ... Polarity test should be conducted BEFORE energization, see Regulation 643.1



    And regards the other situation ... yes, I'm sure we get too many of those.


  • 0
    I'll bet you any money that those RCBOs are not tested monthly!
  • 0
    The point I made about 2 people is that they are in communication at all times. That is the safety link which appears to be important for proper safety. Safety problems often happen due to lack of communication, a point which is very difficult for H&S people to understand. I am amazed at some of the rules which occur, and the stupidity expected of everyone on site as a complete inability to see dangers themselves. Why would anyone not directly involved in the testing fiddle with anything, particularly open accessory boxes. Very few people are that stupid, and if the painter is about to paint the wall and sees something like this wouldn't he be at least rather careful because putting it back is likely to spoil his work?


    The comment about the 21st century from GK shows a curious view of History Graham. The world in general is probably much safer than it has ever been, and safety awareness is much higher. However the 21st century people have a different problem and that is not having any exposure to dangers growing up now do not understand it, and therefore do the more serious stupid things. All possibilities of accident are given the same value, which is very bad indeed. I have seen a carpenter stopped from using a nail gun, who then seriously damaged his hand by trying to use a hammer. I have suffered the "no ladders" thing on housing sites where a 3 step ladder is ideal for lights. I have been taught to use a pair of steps for two hours when I use one all the time and have done for many years. I have several scaffolding certificates, more days of training. Did I learn anything new, probably not. I have been known to fall out of a tree as a child. I now know how to avoid doing so, or from anything else for that matter. Modern children are simply not allowed to climb trees, so they have no sense of dealing with danger.
  • 0
    The statement about 21st Century is, quite simply, a statement of fact ... that we are soon likely need a new method of determining loop impedance and prospective fault currents, as the existing test methods will yield erroneous results. This is purely because inverters that are kicking out current below their current limit or crowbar threshold may appear as a constant current source - certainly if they are held up by battery storage.


    I also grew up falling out of trees (and similar), and have not discouraged my children (son and daughter) from doing so either. I think this is exemplified with the sport they both prefer to play.


  • 0
    You could be testing for a minor works certificate for alterations to an existing installation, an installation certificate for a new installation or circuit, or it may be an electrical installation condition report for an existing installation.


    The original poster has a quandary, site cabins have been delivered and connected to a new installation. If these cabins have been delivered with a full set of dead test results they can be used along with the dead test results from the new installation to do some calculations to determine the Zs. But you cannot the RCD test times, so unless you are going to accept pressing the test button on the RCD device as the only RCD testing required you are going to have to get someone to do some live testing somewhere.


    So the first question is what certification is required from the contractor? Obviously they need to provide a installation certificate for the supply to the cabins, but what about the cabins themselves? Did the cabin come with a test certificate? If so, the certificate can only have dead test results, so what, if any live tests need to be carried out by the contractor on site within the cabins and what certification should be provided by whom? The whom may be the cabin manufacturer, the cabin owner such as a hire company or the contractor powering them up.


    Andy Betteridge.
  • 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.


  • 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?


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