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.

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.