Zs, to test or calculate?

That's odd - I would have expected that conductors at 90°C to have a higher resistance than those at 20°C and hence to have a lower fault current.

The temperature of conductors directly affects the test results

Maximum 1-phase SC-current (temperature of conductors 90C) = 2.213 kA

Minimum 1-phase SC-current (temperature of conductors 20C) = 1.46 kA

Test results can confirm or not confirm the calculation results, nothing more.

Paul Brewerton:
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.

 

Hello Paul,

The main problem to compare results of testing Zs and calculating Zs (and their respective SC-currents) is temperature of conductors. The temperature of conductors directly affects the test results (real temperatures at the moment of the test) or the calculation results (theoretically given temperature or temperature defined by standards).

SC-current calculation.
Calculation is needed to obtain the theoretical values of the largest and smallest SC-currents. According to IEC60909 the calculations of the maximum SC-current and the minimum SC-current shall be performed for each checked point.
For example the MeteorSPEC software gives the following results for 3-phase Cu 50mm2 cable L=100m and for transformer 400kVA D/Y:

Maximum 1-phase SC-current (temperature of conductors 90C) = 2.213 kA
Minimum 1-phase SC-current (temperature of conductors 20C) = 1.46 kA (a fragment of the calculation report is shown at the end).

SC-current test.
In common case you don’t know the temperature of conductors at the moment of the test. Usually the tested circuit consists from many cable sections that are installed differently and were loaded differently before performing the test.
You get some result, but you can’t say anything - can the SC-current be in this place at another time more or less than what you measured now.

Test versus calculation
Testing and calculation results cannot be directly compared. Calculation gives the theoretical possible boundary. Test results can confirm or not confirm the calculation results, nothing more.

Thus, for the correct selection of the protective device, it is necessary to calculate the SC currents at the end points (usually enough minimum current) of a feeder and obtain the results of tests (which should be greater than the calculated minimum currents).


MeteorSpec-NETWORK  DETAILED REPORT  Minimum SC-current
Total length of cable = 100.0 m
Number of cables installed in parallel = 1
Phase Conductors = Cu 35.0 mm2
Resistance per unit length of phase conductor = 0.524 mOhm/m [20C]
Temperature factor of phase conductor metal = 0.00393 [1/.K]
Temperature of conductors = 90.0 C
Resistance per unit length of phase conductor = 0.6682 mOhm/m, [90C], IEC60909-0 [32]
Positive-sequence resistance of cable R ( 1 ) = 66.8215 mOhm, [90C]
Positive-sequence reactance per unit length = 0.0844 mOhm/m, [50Hz]
Positive-sequence reactance of cable X ( 1 ) = 8.4408 mOhm, [50Hz]
Return Conductor = Cu 35.0 mm2
Return by fourth conductor
Zero- and positive-sequence resistance ratio R ( 0 ) /R ( 1 ) = 4.0
Zero-sequence resistance of cable R ( 0 ) = 267.2861 mOhm, [90C]
Zero- and positive-sequence reactance ratio X ( 0 ) /X ( 1 ) = 3.8
Zero-sequence reactance of cable X ( 0 ) = 32.075 mOhm, [50Hz]
 Z ( 1 ) = 66.8215 + j8.4408 = 67.3525 mOhm, [90C][50Hz]

mapj1:  That said it does not feel sensible, and there are some potentially dangerous cases, like the total supply reversal, or whole building off-earth  that would go un-noticed perhaps for some years until the fault the ADS was meant to prevent occurred, perhaps with fatal consequences.

A fortunate feature for much of the 110 V world (US/Canada - not sure about how the others such as Japan do it) is their use of split-phase.  Similarly for countries with 3-phase supply.  Then total reversal, i.e. implementing the 'C-S' connection within TN-C-S to the wrong conductor, is a lot harder not to notice, as any swapping of the N with a L/hot would cause the wrong voltage to some of the equipment. 

 

Harry Macdonald:

"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 all accept this as common practice."

This seems to me to indicate that various, mainly large, organisations are now laying down their own rules which would imply that the methods described in BS7671 are widely seen as unsafe or impractical.

Have we now reached the stage where the BS says one thing but everybody knows that they mean another? If so, that is a very worrying and dangerous state of affairs and exactly what a BS is designed to stop happening.

I hope I am wrong, but with the number of luminaires where you can't easily remove the lamp, switch wire - neutral testing has gone. Live loop impedance seem to have gone, except for sockets, electronics connected to circuits means some don't even do insulation tests.

I hope I am wrong, but I see a dangerous divergence between what the standards say and what electricians now actually do.

BS 7671 does NOT require a loop impedance measurement on every circuit - or in fact, on any circuit!

643.7.3 Earth fault loop impedance

Where protective measures are used which require a knowledge of earth fault loop impedance, the relevant impedances shall be measured, or determined by an alternative method.

Nor does BS 7671 require a measurement of prospective fault current.

643.7.3.201 Prospective fault current

The prospective short-circuit current and prospective earth fault current shall be measured, calculated or determined by another method, at the origin and at other relevant points in the installation.

In fact, according to Appendix 14, in domestic (household) premises where the distributor declares a 16 kA value, you can assume your prospective fault current is 16 kA (although it's likely to be less than this) and not take a measurement at all.



 

Sparkingchip:

You have to do a live test somewhere along the line.


Three phase you will be checking the phase sequence as well as polarity, I had an interesting discussion with two electricians in a pub one night, one of them works for a generator hire company and makes great use of a long wander lead to ensure he is hooking up temporary supplies correctly.


Andy B.

Agreed - but perhaps we need to consider whether it's a loop impedance test when we have lots of microgeneration sources out there. The results using current test methods could well be largely meaningless.

Sparkingchip:

In the back of my van I have a telescopic fibreglass R2 testing pole, I used it to test a light on the gable of a house a couple of weeks ago, I don't intend to take an extension ladder to check a light such as that for an EICR, visual and continuity testing with both feet on the ground.


However when replacing a light fitting and access equipment is already in place then I would loop test, as these were site cabins being installed that were already being wired before delivery to site some testing is required, if need be I would test at a switch with two feet safely on the floor, but obviously that may require a two lead loop tester and I have a couple to use.


There is a precedent.

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


Andy B.


 

We can keep going around in circles with this, the simple truth is we need to choose the time and the place to do live testing, personally I avoid doing it on top of a ladder or the like, but you may still need to prove dead.


I remember going to look at a faulty outside light one wet winter evening, I climbed up the aluminium ladder thinking it may just need a new lamp in it and without touching the light fitting put I one probe of the voltage detector on the ladder and the other on the light fitting, it lit up like a Christmas tree so I climbed back down the ladder and started to put it back on the van, the lady customer enquired what I was doing, my reply was going home then coming back when it is light and dry as I wasn't going to mess about with it in the wet and the dark.