Zs, to test or calculate?

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 ...

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 ...