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Zs, to test or calculate?
Question
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. 
58 Replies
AJJewsbury 77361768
1611 Posts
It's certainly been talked about a lot - the thinking being that the legal requirement is that you need to reduce the risk as far as reasonably practical for each particular situation. So a certain level of risk that's acceptable in one situation because there's no reasonable alternative, might not be acceptable in another if a safer alternative exists there.

I think a lot of people now take the policy of dead testing (e.g. R1+R2 test) at every point - but as the normal procedure for R1+R2 involves disconnecting the c.p.c.from the mean of earthing, doing one live loop test on that circuit to prove that the c.p..c. has been properly re-connected afterwards.

Personally, I prefer to leave the c.p.c. connected and temporarily connect the outgoing line conductor to the earth bar to do a R1+R2 test - so that the c.p.c. connection is proved during the dead test. But that does have the perceived disadvantage of potentially including some parallel paths that would have been excluded by the original method (not that the original method would necessarily exclude all parallel paths either).

In theory, any testing should only be verifying that the actual Zs is broadly in line with the design Zs - i.e. no gross mistakes have been made during installation - rather than being the only means of ensuring that Zs is within permitted values. (Although I appreciate that in practice there aren't always proper designs to work from).

There's also the point that the resistance of a low current d.c. test perhaps isn't a good indication of the impedance of an a.c. circuit - for small copper conductors the difference is probably negligible but for larger conductors or where a lot of steel is involved the impedance might differ significantly from resistance.

The big advantage on a loop test is that it naturally tests the entire loop - final circuit, however many distribution circuits that supply it and the means of earthing all at one. To achieve the same using separate dead tests would mean a lot more care and co-ordination and perhaps extra tests to ensure that temporary disconnections have been properly reinstated. No good proving the distribution circuit is OK and the final circuit is also OK if the connection between the two is dubious.

   - Andy.
Sparkingchip 72796851
2474 Posts
If this is for periodic testing then testing R2 with a wander lead may suffice.

Andy Betteridge 
This is firstly for inspection of final circuits within site office/welfare cabins, but I feel if I allow this easy option now, they will want to continue in the same vein when they start on the main project. And then they will use the argument of "well it was alright last time..."
Sparkingchip 72796851
2474 Posts
I’m with you.
gkenyon 82239607
1038 Posts
I agree with the risk reduction measures, particularly where there is an alternative "dead test" method.

That is common-sense application of the hierarchy of control - "remove the hazard" being the first option in all cases.

To continue to conduct live tests where an alternative method is available - using figures you may already have from previous dead tests, say if you adopt (R1+R2) for a combined earth continuity and polarity test - leaves you wide open to greater scrutiny in the event someone gets hurt.

Because of this, I don't think there's an argument you could use that will convince the site management and their health & safety advisors, otherwise.
lyledunn 83218531
292 Posts
Totally agree with GK. The periodic inspection end of my business has very strict protocols of a similar nature. Test Zdb and other similar items like MCCs but R1+R2 Or R2 at most other things other than sockets. Not so long ago NICEIC expressly required a Zs reading for each circuit tested. I completely ignored that on the basis that it was me who was responsible for the safety of my employees and a perfectly reasonable alternative to live testing was available. As AJ points out there are pros and cons but they are not so stark that safety should be compromised. 
 
Sparkingchip 72796851
2474 Posts
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.

 
gkenyon 82239607
1038 Posts
Apologies for going back a few posts, but I thought I'd add some suggestions to Andy's post ... not to criticise, but provide some alternative views into the debate.
AJJewsbury:
It's certainly been talked about a lot - the thinking being that the legal requirement is that you need to reduce the risk as far as reasonably practical for each particular situation. So a certain level of risk that's acceptable in one situation because there's no reasonable alternative, might not be acceptable in another if a safer alternative exists there.

I think a lot of people now take the policy of dead testing (e.g. R1+R2 test) at every point - but as the normal procedure for R1+R2 involves disconnecting the c.p.c.from the mean of earthing, doing one live loop test on that circuit to prove that the c.p..c. has been properly re-connected afterwards.
Method 1 (R1+R2)  in the OSG and GN3 doesn't show the cpc being disconnected.

Nor does Method 2 (R2 measured with wander lead).

There is a note in GN3 about parallel paths, though.

Why would the impact of parallel paths differ between Method 1 to Method 2? Since the parallel paths are in parallel with R2 the answer is ... it wouldn't.

Parallel paths may also be cause by earthing/bonding downstream ... or by means put in place for high protective conductor currents, so disconnecting the cpc at the CU / DB might not help ...
 
Personally, I prefer to leave the c.p.c. connected and temporarily connect the outgoing line conductor to the earth bar to do a R1+R2 test - so that the c.p.c. connection is proved during the dead test. But that does have the perceived disadvantage of potentially including some parallel paths that would have been excluded by the original method (not that the original method would necessarily exclude all parallel paths either).
That's what's shown in GN3 and OSG - for CU's where the busbar is accessible during dead tests, it's also possible to croc-clip link the busbar to the earth bar, open the breakers of all circuits except the one under test, and check for high resistance in the mcb, which may well be pertinent in a construction site environment, or for mobile/transportable units.
In theory, any testing should only be verifying that the actual Zs is broadly in line with the design Zs - i.e. no gross mistakes have been made during installation - rather than being the only means of ensuring that Zs is within permitted values. (Although I appreciate that in practice there aren't always proper designs to work from).
Not necessarily. If it's a periodic on an installation in which the design was carried out before the voltage change to 230 V in the 16th Edition, or the minimum voltage factor Cmin was included in BS 7671:2008, the design Zs would not satisfy the Zs for the current edition of BS 7671.

Periodic verification is normally carried out to the current edition, although that does not mean the installation according to previous versions of the standard are unsafe.
 
There's also the point that the resistance of a low current d.c. test perhaps isn't a good indication of the impedance of an a.c. circuit - for small copper conductors the difference is probably negligible but for larger conductors or where a lot of steel is involved the impedance might differ significantly from resistance.
  1. That may be true, but we avoid high current testing now because of the potential for fire ... for those who are old enough to remember the "conduit ohmmeter" test ...
  2. The loop impedance test purposefully puts a fault on the circuit.
  3. If we are using a low current test to keep things safer, we know full well this also isn't accurate.
 
The big advantage on a loop test is that it naturally tests the entire loop - final circuit, however many distribution circuits that supply it and the means of earthing all at one. To achieve the same using separate dead tests would mean a lot more care and co-ordination and perhaps extra tests to ensure that temporary disconnections have been properly reinstated. No good proving the distribution circuit is OK and the final circuit is also OK if the connection between the two is dubious.

   - Andy.

Well, not quite.

No-one's mentioning parallel paths with loop tests... but they will be there. Why aren't they a problem for the loop test, but they appear to be a big issue in some minds for earth continuity testing? Perhaps an argument that "loop testing" doesn't provide a more reliable means of verification. Whilst in practice parallel paths are there, they may or may not change over time ...

AJJewsbury 77361768
1611 Posts
Method 1 (R1+R2) in the OSG and GN3 doesn't show the cpc being disconnected.
Very good point Graham. I've certainly seen the c.p.c. and line 'choc blocked' together suggested somewhere (I'll have to rack my brains as to where now) - and was under the impression that it was frequently taught that way too - I've certainly seen it been done that way in practice. Good news if that's officially incorrect.
 
Why would the impact of parallel paths differ between Method 1 to Method 2?
I mean to suggest it would - just that disconnecting the c.p.c. from the Earth bar would eliminate some parallel paths (which I understood was the reason for the disconnection approach in the first place).

  - Andy.
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?
 
gkenyon 82239607
1038 Posts
For circuits below say about 63 A, we don't worry about the reactance of the cable - see tables at the back of BS 7671. Most of the reactance is in the external loop, and that's why we would normally measure at least Ze or ZDB.

Appendix 14 of BS 7671 advises that, under many circumstances, measurement prospective fault current in domestic installations is not necessary.

For larger circuits, I agree there's perhaps something missing, but it's worth remembering that, due to potentially larger prospective fault currents / lower loop impedances, and especially where you are close to the transformer, the loop impedance or prospective fault current measurement itself may be inaccurate. For larger loads, and even final circuit disboards, there is no reason at all why a test socket-outlet specifically for prospective fault current and loop impedance measurement, with suitable backup protection could not be provided, just below the DB or built into MCCs etc. - this is part of the CDM process.

Don't forget that Appendix 14 of BS 7671 advises that fused leads alone may not provide protection if backup protection at the point of measurement is not present.
lyledunn 83218531
292 Posts
As far as I know, the reactive component of impedance is not measured by a loop “impedance” tester anyway. Further, the accuracy is always questionable for various reasons. Having that margin of 0.8 Zs7671 as a maximum test value perhaps addresses most issues. A loop impedance on the wrong side of an mcb is worse than a fuse.
I am not convinced that the loop test does not measure impedance. Yes, the current used is DC but, as I understand it, a very short pulse is used so inductive effects should be measured too.
I have also had great difficulty measuring the earth loop impedance of a main incomer, say 400A, because the dip in voltage caused by the brief current pulse was so small that it was masked by the normal spikes on a supply. I tried averaging several successive readings but as some of these were negative, I didn't trust the results. 
I suspect that, to get a proper result, I needed a much beefier bit of test kit, the sort that would take 2 people to carry it.
mapj1 80733779
2070 Posts
A loop impedance test of the kind that uses the  50Hz AC supply and switches in a load and looks at the voltage drop will necessarily include all the same impedances, both real and imaginary, that  will appear in a real fault, as that too uses the AC supply.
The accuracy problem is because   the test current is much lower than a likely fault current  (on a TN system anyway) the voltage drop caused during the test is fractions of a volt, and  if the mains is a bit bouncy for other reasons, as it often is if the supply is shared with loads that are switching, then the meter may mistake some of that voltage drop for the result of its own test load, so the results can be a bit 'fruit machine'.
  By keying the test load in and out in a psuedo-random sequence, and then looking at the average voltage droop and rise for each short part of that test, it is possible to cancel out and mitigate this noise problem, at least to a degree. Also when you are looking at PSSC of kA, then every milliohm starts to count, and the condition of test leads and their connectors starts to be a significant part of the reading, and beyond a certain point 'normal' meter  probes are not really good enough.

Also I  have seen the effects of RCD cores that look inductive for small currents, but for  large fault would saturate and not current limit, give an over pessimistic impedance on a very low current (no trip) test. The clue is the Zs readings before and after the RCD are far more different than the DC resistance of the contacts would suggest. Still only tens of milliohms of change, but in some cases that may be misinterpreted as the difference between additional work needed, and not.
AJJewsbury 77361768
1611 Posts
It seems that nowadays it is acceptable across the board to calculate Zs.
As an aside, I don't think of adding dead R1+R2 results to Ze as being the calculation approach - it's still all based on measurement even if done in parts. For me, calculation would be something like working out the impedance based on the cable length and tabulated resistance per metre - which might have been the approach in the old days when earth continuity would be verified with nothing more than a set of test lamps.
   - Andy.
Sparkingchip 72796851
2474 Posts
I know an electrician, now retired, who didn’t have his NICEIC renewal signed of by his assessor around fifteen years ago after a few clashes.

One clash was when his assessor suggested he bought a non-trip loop tester to get more accurate loop test results on RCD protected circuits and he replied saying he had a pencil.

On a personal note I still have my original Robin multifunction installation tester, the main reason I “upgraded” by buying a Megger non-trip loop tester to accompany it was the increasing number of RCDs being installed, now it seems all these years later I could have stuck with the Robin and a pencil.

 Andy Betteridge 
ebee 81966746
691 Posts
Pencils can be truly wonderfull things 😀
lyledunn 83218531
292 Posts
mapj1:
A loop impedance test of the kind that uses the  50Hz AC supply and switches in a load and looks at the voltage drop will necessarily include all the same impedances, both real and imaginary, that  will appear in a real fault, as that too uses the AC supply.
 

But how is that possible? The reactive component of voltage drop in the circuit being measured cannot be determined from an arithmetic subtraction of the voltage drop of the instrument test resistor from the supply voltage. As far as I understand it my MFT places a 59 ohm resistor as a dummy load, tries to establish the volt drop across it over a couple of half cycles and then determines the circuit “impedance” from the voltage difference between that and the supply voltage. Where the reactive component is pronounced surely that is not possible as the VD across the reactance is in quadrature.

Nathaniel 32483845
60 Posts
That's true (lyledunn).  A method that compares voltage magnitudes with and without a resistive load (where that load impedance is many times the source impedance, e.g. to take 20 A) is very insensitive to reactance in the source - it practically doesn't see it at all. 

One can make inferences based on curve fitting for a range of resistive loads, but that's more for amusement than practical ... I've tried it a few times. 

The following plot was my attempt some years ago to illustrate the relation between current and voltage when a varied resistive load (conductance increasing from open-circuit to short-circuit) is connected to a source with either purely resistive (blue) or purely reactive (red) impedance, or to a current-limited power-electronic source.
- With the resistive source and load the gradient is a nice straight line as with dc circuits, so any measurement that varies the load between (say) zero and 20 A in a system with 1 kA short-circuit current would make a good estimate of the short-circuit current. 
- But with the reactive source and resistive load the estimate based on this same pair of currents would be a much higher short circuit current than the actual value that's been chosen here to be the same as in the resistive case, because the voltage drop due to mainly resistive current flowing through the reactive source impedance is pretty well in quadrature with the larger source voltage, just as you say. 
- (And on the other hand, the current-limited inverter could regulate its voltage to look very stiff, and yet provide much lower short-circuit current: it is 'brittle' stiffness!)

v-i-relation

"In theory", using phasor calculation, one could calculate the source impedance easily for a Thevenin-style source with resistive and/or reactive impedance. But that would require knowing the angle relation between the voltage phasors at the source (assumed constant for the 0 A and 20 A case) and at the measurement point. The simple practical measurements don't have a way to know this, and just measure voltage magnitude. It is possible with analog methods (phase-locked loop) or digital methods (extrapolate a sine-wave for further cycles) to keep a memory of the phase of the source-voltage based on the times when the current isn't being drawn, and to assume this value continues in the few cycles afterwards when the test-current is being drawn. Then one should be able to get a better calculation, although the small change in voltage for a resistive perturbation of a reactive system would still make it more susceptible to noise. Using a test-inductor (or capacitor) as the load instead of a resistor would let the source reactance be measured while largely ignoring the source resistance. Or electronics could synthesise the currents in phase and quadrature. 
Getting away from phasors, one can take rapid pulses of current such that much of the voltage drop is caused by L*di/dt instead of R*i, as a way to assess inductance (but this is rather dependent on local shunt capacitance). 

There's a lot that can be done. Some has been in other applications of impedance estimation, such as inverters that use reactive power consumption to avoid excessive voltage rise with active power injection, or distance-relays that look at the L*di/dt rather than phasors.  Installation testers have got cleverer and more digital, but I'm not sure about the current state of art in implementations by the usual manufacturers of today's products.  They tend not to say anything very interesting about the fine details!  One has to test the tester.  

I don't remember the details of the setup, but the following are from a quick check a few years ago, using an oscilloscope to see what a simple MFT was doing during (I think) a low-current (RCD-friendly) loop test.  It correctly measured the oscilloscope's supposed input resistance, and the oscilloscope showed an interesting waveform that definitely wasn't sinusoidal. 
zest_meas_current  zest_result

 
I am not at all sure that this policy is in any way satisfactory. If a qualified electrician cannot safely test any part of a live circuit he is inadequately qualified for the testing job. This is simply a time saving idea, which may well deteriorate to no testing at all, because it is quicker. The proper completely safe procedure is as follows:

Isolate the circuit. Connect your loop tester using suitable connectors clips or insert the leads into the connector, accessory or whatever live terminal and the other to earth. Energise the circuit. Test Zs and note the reading. Reverse the procedure to disconnect. If anyone can see how this is unsafe I should love to hear.

You will never find faults by calculation. It is faults we are looking for, loose connections to earth conductors where the sleeving is trapped under the screw etc. One never knows the quality of the workmanship and such are common faults in new installations! Why calculate (presumably that is already done in the design) when a proper test is much better and will find real faults. Multiple paths cover up these faults and ARE NOT SAFE.



 
mapj1 80733779
2070 Posts
 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.
Nathaniel 32483845
60 Posts
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:

sc-current estimate and reality

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. 
 
gkenyon 82239607
1038 Posts
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
  2. Substitute the hazard
  3. Engineering controls protective devices, interlocks, guards, etc. - effectively, separate people from the hazard
  4. Administrative controls changing the method of work to avoid hazards and/or reduce the severity
  5. 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.
Sparkingchip 72796851
2474 Posts
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
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.

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