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Zs, to test or calculate?

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.

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0 Paul Brewerton over 4 years ago

Thanks for everyone's replies. It's good to get other opinions. It seems that nowadays it is acceptable across the board to calculate Zs. When I used to do a lot of testing, the contractor I worked for insisted on measured readings. That was the company line and therefore that is how I used to work. At that time a calculated Zs was seen as "lazy", so that is my mindset still. Companies are a lot more wary of risk these days, whereas then, you were just told to get on with it.

One more nagging doubt though. If you measure with a loop tester, you are literally measuring the earth loop "Impedance". However, if you add r1+r2 which is "Resistance" to the Ze (or Zdb), then that calculation is actually "Impedance + Resistance"

Looking at the Impedance triangle, we have Z= √R²+(Xl-Xc)², so clearly Z is going to be greater than R.
This will only come into play if the circuit is actually operating and under load. For example a radial socket circuit with nothing plugged in, there will be no difference. But if you are testing a circuit feeding inductive or capactive loads, then the difference will be more pronounced. In this case wouldn’t a measured Zs be accurate whereas the calculated Zs less so?
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0 Paul Brewerton over 4 years ago

Thanks for everyone's replies. It's good to get other opinions. It seems that nowadays it is acceptable across the board to calculate Zs. When I used to do a lot of testing, the contractor I worked for insisted on measured readings. That was the company line and therefore that is how I used to work. At that time a calculated Zs was seen as "lazy", so that is my mindset still. Companies are a lot more wary of risk these days, whereas then, you were just told to get on with it.

One more nagging doubt though. If you measure with a loop tester, you are literally measuring the earth loop "Impedance". However, if you add r1+r2 which is "Resistance" to the Ze (or Zdb), then that calculation is actually "Impedance + Resistance"

Looking at the Impedance triangle, we have Z= √R²+(Xl-Xc)², so clearly Z is going to be greater than R.
This will only come into play if the circuit is actually operating and under load. For example a radial socket circuit with nothing plugged in, there will be no difference. But if you are testing a circuit feeding inductive or capactive loads, then the difference will be more pronounced. In this case wouldn’t a measured Zs be accurate whereas the calculated Zs less so?
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Zs, to test or calculate?

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