Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 58 replies
  • Subscribers 32 subscribers
  • Views 11511 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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
    I think I am with DZ, an electrician who cannot do live testing "for safety reasons"  is being unnecessarily  limited, and is likely to miss a whole slew of faults that are only obvious after re-assembly, and may actually be faults that are introduced by disconnecting things to do the -dead tests.


    At the risk of tracking off a bit,  in the tester  there are 2 test states, a near open circuit and the test resistor . If it only looks at the voltage i nthe 2 states  I agree you measure |Z| i.e.   the modulus of the impedance, and are unsure o how much f it is real or imaginary.

    You need another piece of information to separate that, and traditionally it is to connect an L or C and remeasure  but if you can count the cycle periods and and the small phase shifts between the load on and load off cases then you can say something.  (and a modern microprocessor clocked at MHz can measure short times)

    The simple instruments do not. as all you need is the fault current, and that is driven by |Z| . But it is in principle possible to sort out R from X, so long as they are comparable.



    IT would be possible to make an RCD that looked at the phase of the imbalance current against the voltage and could distinguish capacitors from a human, but so far no-one seems to think it is worth doing.
  • 0
    This pdf (archived:pdf) is an effort to explain in more detail why |Z| is not what's measured — and so short-circuit current estimates can be grossly wrong — when working only with voltage magnitudes, light resistive test-load, and reactive source.  Due to the situations in which a source has high X/R, I do not see this as a particularly practical issue (?).  I simply mention it as a possible interest.  The crux is the following relation, where Rt is the test-load and the other parts are in the Thevenin-model of a linear ac source:

    3dbee84fef5c6f95bcb990f2661ead52-original-zest_isc.png


    Regarding capacitors and RCDs .. the probable first objection would be "what if someone touches one end of a capacitor with wet hands?" (with its other end connecting to a phase-conductor).  In a situation with low body resistance, one could get a dangerous current even with a series capacitive reactance of serveral times the body resistance, so the total would look very close to a capacitor.  Perhaps the capacitive current could be a reason to delay a little more but still within the required operating time. 

  • 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.
  • 0
    With many light fittings now being LED without a removable lamp you cannot even fall back on some of the aids to safe working.


    See the source image
  • 0
    Well Graham, perhaps you have hit the nail on the head. I like to do testing with 2 people with radio communications, it is very much quicker, and overcomes your idea that an open accessory or whatever left with a meter and perhaps a warning notice is dangerous to anyone. The idea that one man tests a large installation by walking miles each day is simply arcane. We are in the 21st-century and need to use 21st-century tools. R1 + R2 testing takes very little time, insulation testing has someone close to the items being tested etc. You can see how this is so much safer and quicker but want to give a method statement and procedure which removes the value of the work. So be it, but I hope there is never a problem due to inadequate testing, it is likely one day. Personally I have no problem with live testing at all, but then I am of that age.
  • 0
    davezawadi (David Stone):

    Well Graham, perhaps you have hit the nail on the head. I like to do testing with 2 people with radio communications, it is very much quicker, and overcomes your idea that an open accessory or whatever left with a meter and perhaps a warning notice is dangerous to anyone. The idea that one man tests a large installation by walking miles each day is simply arcane.

    Hang on ... you mentioned the isolate ... connect ... renergize process?

    We are in the 21st-century and need to use 21st-century tools. R1 + R2 testing takes very little time, insulation testing has someone close to the items being tested etc. You can see how this is so much safer and quicker but want to give a method statement and procedure which removes the value of the work.

    Even with 2 people, it's still unfortunately a "process" or "method" vs "safety". As I said, it's not a matter of changing "what the electrician thinks is best" on these sites, but convincing the H&S Team - who might well be held at least partly to account if the maintenance checks on their electrical systems were deemed inadequate, so I guess after all it's their call.

    So be it, but I hope there is never a problem due to inadequate testing, it is likely one day. Personally I have no problem with live testing at all, but then I am of that age.

    So, you mentioned 21st century.


    I have a feeling that we are at a point where loop testing and prospective fault current testing might be a thing of the past ... because microgeneration with inverter output can provide a totally erroneous and meaningless reading.
  • 0
    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.


    Andy B
  • 0
    Sparkingchip:

    With many light fittings now being LED without a removable lamp you cannot even fall back on some of the aids to safe working.


    See the source image


    So for the EICR you need to confirm the polarity at the fitting, single pole switches and breakers are in the live phase, loop impedance and possibly the RCD trip time. 


    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


     


  • 0
    Sparkingchip:


    . . . There is a precedent.
    https://www.manchestereveningnews.co.uk/news/greater-manchester-news/dads-agony-after-apprentice-electrician-9085985 . . . 

     


    There may be a precedent, but as far as I can see, that is not it. The electrocution occurred whilst getting to the point of work, due to the travelling crane not being isolated, not as a result of tests being carried out at the point of work. I am of course assuming that this was an approved method of getting to the place of work. 


    Regards,


    Alan. 


  • 0
    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.


Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines