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 11533 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
    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. 

Reply
  • 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. 

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