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 11527 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. 
Parents
  • 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.
Reply
  • 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.
Children
No Data

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