Zs, to test or calculate?

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.

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..."

I’m with you.

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.

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. 

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.

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 C_min was included in BS 7671:2008, the design Z_s would not satisfy the Z_s 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 ...

• The loop impedance test purposefully puts a fault on the circuit.

• 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 ...

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.