ZS on a new installation

[EFLImpudence]

Exactly the same consideration applies to TN installations, although the difference between Ze+R1+R2 and directly-measured Zs is obviously going to be much less than is usually the case with TT.

Agreed .

Do you therefore believe that one should always record Ze+R1+R2 and never record a directly-measured Zs

I would say yes because of what has already been said the form asks for maximum measured Zs.

(and, indeed, that the only reason for measuring it directly is as a 'double check' on the R1+R2)?

Yes.

I think you may be thinking too much about the reasons because you have a TT supply.
You are likely thinking far more than the designers of the forms.

Firstly, because there is only one form for all types of earthing methods.

Also, because the form asks for maximum measured Zs after already requiring (for Ring circuits) the Ze and R1, Rn and R2 to be recorded from which the Zs could be 'calculated'.
Indeed, so may the Zn and PSCC about which you asked although as Zn would usually be less than or equal to Zs it would satisfactorily operate the OPD.

Having said that, for radial circuits, it only requires R1+R2 or R2.
So R1, for some reason is not required. I don't know why.


As Bas said, it is probABly more to show that you have done the tests and, hopefully, rectified any noticeable discrepancies, than to record for posterity what the values actually were.

 

[JohnW2]

Exactly the same consideration applies to TN installations, although the difference between Ze+R1+R2 and directly-measured Zs is obviously going to be much less than is usually the case with TT.

Agreed.

Do you therefore believe that one should always record Ze+R1+R2 and never record a directly-measured Zs

I would say yes because of what has already been said the form asks for maximum measured Zs.

(and, indeed, that the only reason for measuring it directly is as a 'double check' on the R1+R2)?

Yes.

That's an awful lot of agreeing for us

I think you may be thinking too much about the reasons because you have a TT supply. You are likely thinking far more than the designers of the forms.

I don't know that I'm "thinking too much", since it's potentially important, but I'm clearly more aware of the difference between Ze+R1+R2 and directly-measured Zs because I've got a TT supply. The difference between, say 1.2Ω and 1.1Ω is hardly as likely to be 'noticed', or considered important,as the difference between 76.2Ω and 1.1Ω!

Firstly, because there is only one form for all types of earthing methods. Also, because the form asks for maximum measured Zs after already requiring (for Ring circuits) the Ze and R1, Rn and R2 to be recorded from which the Zs could be 'calculated'.

The question really is whether it is correct/acceptable to record a directly-measured Zs in that column, given that, as we are both agreed, that should not be used to determine whether the disconnection requirements are satisfied.

Having said that, for radial circuits, it only requires R1+R2 or R2. So R1, for some reason is not required. I don't know why.

That would,indeed, appear irrational.

As Bas said, it is probABly more to show that you have done the tests and, hopefully, rectified any noticeable discrepancies, than to record for posterity what the values actually were.

Yes, but only as 'belt and braces' - as I think we've been agreeing, if one has correctly measured and recorded Ze and R1+R2, then (apart from 'belt and braces' reassurance), recording a directly-measured Zs really adds nothing (apart from some potential confusion of interpretation, if the answer is appreciably lower than Ze+R1+R2).

Kind Regards, John

 

[Electrifying]

Again, everybody goes around the houses to explain something that really is quite simple.

Measuring R1 +R2 is one method of proving continuity of the circuit protective conductor within a circuit, before energising the installation.

Measurement of R2, (also acceptable), is another method of proving CPC continuity within a circuit.

Noting the value of these measurements -

1) Shows you've actually done the tests.
2) Allows comparison with design calculations/specs in order to identify any faults before energising.
3) Gives a reference point for future testing.

After performing the R1 + R2 test the installation is plated up, all conductors terminated, sockets screwed back on, light fittings screwed up etc etc.

In other words a lot could go wrong - e.g. slightly overtightening a cpc in CU after R1 + R2 and snapping the conductor in the terminal, giving a high resistance, (if any), connection...........

So, we come to Zs.

If you wish to calculate Zs by sitting in your van and adding R1 + R2 + Ze, then so be it......but you won't find out about the snapped conductor in the CU.

My point is that assuming one has measured the (true) Ze (and found it to be acceptable) and the R1+R2, then, unless the R1+R2 is so ludicrously high that it needs to be investigated and corrected in its own right - the actual value of the Zs is of no relevance or importance.

Kind Regards, John

It is of relevance and importance because you may have had CPC continuity back during the dead tests, but that doesn't mean something hasn't gone wrong since.
Zs is 'measured' to confirm the existence of an earth fault path in an energised, fully functional installation.

I would always measure Zs because :-

1) It confirms the circuit actually has a CPC during normal operation, and that it's impedance is sufficiently low to trip the protective device under fault conditions.

2) By comparison with the 'calculated' figure and the design specs, it confirms that everything is as it should be.

And I would hate to be the person who performs all his Zs by calculation, only to find at a later date that he forgot to put the 'earthing conductor' back in the MET, after measuring Ze.

Do you therefore believe that one should always record Ze+R1+R2 and never record a directly-measured Zs

I would say yes because of what has already been said the form asks for maximum measured Zs.

Yes, it asks for maximum measured Zs......which is a measurement of circuit impedance - NOT R1 + R2, a dead-test measurement of resistance to confirm CPC continuity.

The question really is whether it is correct/acceptable to record a directly-measured Zs in that column, given that, as we are both agreed, that should not be used to determine whether the disconnection requirements are satisfied.
Kind Regards, John

Disconnection times are determined, and indeed, satisfied at the 'Design' stage.
Testing is done to confirm that the installation complies with the design.

It will already have been determined as to whether a particular circuit's Zs will be low enough to trip the protective device, with testing you are confirming this............which is why I say MEASURING Zs is very important - to confirm that there actually is an 'Earth Fault Path', regardless of parallel paths etc etc..............any decent tester knows what to look for in these measurements and when comparing the figures.

 

[JohnW2]

Again, everybody goes around the houses to explain something that really is quite simple.

Maybe - but 'simplicity' is sometimes in the eyes or mind of the beholder. The points I have been making are all really based on the fact that direct Zs measurements are made with 'parallel paths' present.

Measuring R1 +R2 is one method of proving continuity of the circuit protective conductor within a circuit, before energising the installation. Measurement of R2, (also acceptable), is another method of proving CPC continuity within a circuit. Noting the value of these measurements -
1) Shows you've actually done the tests.
2) Allows comparison with design calculations/specs in order to identify any faults before energising.
3) Gives a reference point for future testing.

Totally agreed (assuming we are talking of reputable people**) - and I don't think anyone has suggested that such measurements should not be undertaken and their results recorded, for all the reasons you give. [** of course, the existence of recorded and credible 'resuts' does not show/prove that any tests have actually been undertaken. R1 and/or R2 could easily be estimated from knowledge of cable size and length, by a lazy and disreputable person!]

After performing the R1 + R2 test the installation is plated up, all conductors terminated, sockets screwed back on, light fittings screwed up etc etc. ... In other words a lot could go wrong - e.g. slightly overtightening a cpc in CU after R1 + R2 and snapping the conductor in the terminal, giving a high resistance, (if any), connection........... So, we come to Zs. If you wish to calculate Zs by sitting in your van and adding R1 + R2 + Ze, then so be it......but you won't find out about the snapped conductor in the CU.

True, but why the conceptual switch from circuit resistance to loop impedance? Wanting to confirm that nothing has 'gone wrong' since one's R1+R2 measurement is reasonable enough, but (unless one also measures 'Ze' with parallel paths connected) the only way of really being sure of that is to repeat the equivalent of the original resistance measurement - otherwise an actual increase in R1+R2 could go undetected if it were 'cancelled' by the parallel paths.

It is of relevance and importance because you may have had CPC continuity back during the dead tests, but that doesn't mean something hasn't gone wrong since.
Zs is 'measured' to confirm the existence of an earth fault path in an energised, fully functional installation.

See above. I agree that it will detect major deteriorations in the earth fault path between initial testing and energisation but, as above, small (but potentially important) deteriorations could go undetected, because of the effects of the 'parallel paths'.

I would always measure Zs because :-
1) It confirms the circuit actually has a CPC during normal operation, and that it's impedance is sufficiently low to trip the protective device under fault conditions.

Yes, but as EFLI has been saying, only in the presence of parallel paths, which could disappear one dark night if the man with a lorry full of plastic pipe is around. I have to agree with him that one really wants the reassurance of knowing that the loop impedance is low enough for protective devices to operate satisfactorily under fault conditions if the parallel paths were to be removed - i.e. ADS should not be reliant on the continued existance of unchanged parallel paths.

2) By comparison with the 'calculated' figure and the design specs, it confirms that everything is as it should be.

Only if one has a handle on the impedance of the parallel paths - e.g. if one measures 'Ze' with them connected.

And I would hate to be the person who performs all his Zs by calculation, only to find at a later date that he forgot to put the 'earthing conductor' back in the MET, after measuring Ze.

Yep, I've already repeatedly mentioned 'belt and braces'. My only real uncertainty is the appropriateness of recording Zs - since it may tempt some to assess the adequacy of ADS on the basis of measurements when parallel paths were present.

It will already have been determined as to whether a particular circuit's Zs will be low enough to trip the protective device, with testing you are confirming this............which is why I say MEASURING Zs is very important - to confirm that there actually is an 'Earth Fault Path', regardless of parallel paths etc etc...............

You've rather lost me there, since you appear to be saying exactly what I have said above, but seemingly drawing the opposite conclusion. One does, indeed, want to know that there is an adequate "earth fault path" "regardless of parallel paths etc." - but my whole point has been that a Zs measurement will not demonstrate that, since it is measured with parallel paths present. Indeed, it would be very possible to have perfectly satisfactory' Zs measurements which were due entirely to parallel paths (just disconnect the 'earthing conductor' from my MET and you'd see that) - paths which theoretically could disappear.

Kind Regards, John