ZS on a new installation

[amateurSparky]

Hi
Its a rewire am doing an EIC on. I was wondering does the ZS value have to be measured? Am short of time and zs for all circuit can be time consuming then doing by calculation. The LABC is coming TOMORROW!!

 

[RF Lighting]

I don't think taking shortcuts with your testing is the best way to get LABC on your side.

Calculated Zs is acceptable, but seriously, it shouldn't take more than 20 mins to test the Zs of every circuit in a standard domestic property.

 

[ban-all-sheds]

Zs doesn't, but R1+R2 does.

 

[SaladFingers]

Can be calculated by Ze + R1 R2, but to be honest either will take similar time, the former a little more.

 

[EFLImpudence]

In my opinion adding a measured R1+R2 to a measured Ze does not count as a 'calculation'.

R1+R2+Ze gives a better value for maximum measured Zs (as required on EIC) than does measuring it from a socket/lamp etc. - because it does not include parallel paths which may be removed at some time.

 

[JohnW2]

In my opinion adding a measured R1+R2 to a measured Ze does not count as a 'calculation'. R1+R2+Ze gives a better value for maximum measured Zs (as required on EIC) than does measuring it from a socket/lamp etc. - because it does not include parallel paths which may be removed at some time.

I agree, at least in a TN installation. Do you think that such a 'calculated' Zs (excluding parallel paths to earth) serves any useful purpose in relation to a TT installation? Although it again will represent the maximum Zs which would arise if the parallel paths were to disappear, will the (high) numbers actually have any useful meaning at all?

Kind Regards, John

 

[JohnW2]

Can be calculated by Ze + R1 R2, but to be honest either will take similar time, the former a little more.

Yes, but if you've already measured R1+R2 (and measured or 'assumed' the value of Ze) anyway, then adding them together to get Zs is going to be quicker than measuring Zs.

However, I have to say that I am somewhat of a 'belt and braces' man, so I would usually measure Zs as well, if not only as a check on the R1+R2 measurement.

Kind Regards, John

 

[EFLImpudence]

In my opinion adding a measured R1+R2 to a measured Ze does not count as a 'calculation'. R1+R2+Ze gives a better value for maximum measured Zs (as required on EIC) than does measuring it from a socket/lamp etc. - because it does not include parallel paths which may be removed at some time.

I agree, at least in a TN installation. Do you think that such a 'calculated' Zs (excluding parallel paths to earth) serves any useful purpose in relation to a TT installation? Although it again will represent the maximum Zs which would arise if the parallel paths were to disappear, will the (high) numbers actually have any useful meaning at all?

Firstly, it would have been better had I said
"In my opinion adding a measured R1+R2 to a measured Ze does count as a measurement"

Also, I would actually take the farthest measurement as well to ensure it was less or roughly equal.
A higher figure would raise suspicions.



However - I suppose the TT figures actually do not, as such, serve any 'useful' purpose other than they have to be recorded and without them you would have no idea what they were.

I am not really sure what you have in mind regarding this.
Are you saying there is no point testing the rod? Obviously not.

 

[plugwash]

However - I suppose the TT figures actually do not, as such, serve any 'useful' purpose other than they have to be recorded and without them you would have no idea what they were.

The "useful purpose" of a Zs figure is to determine whether the protection in place against faults to earth is adequate.

Are you saying there is no point testing the rod? Obviously not.

I don't think he is saying that.

What I think he is saying is that the R1+R2 value of the circuit is so small compared to the max allowed Zs in the circumstances that you really don't need to actually do the sums to know what the answer will be to the question of if it passed or not.

What I do wonder though is why there is so much focus on P-E fault loop impedances and yet noone seems to care about P-N fault loop impedances. IMO in an install where you are relying on RCDs to protect against earth faults you really should be doing seperate calculations to determine if the protection against P-N faults is adequate.

 

[JohnW2]

Firstly, it would have been better had I said ... "In my opinion adding a measured R1+R2 to a measured Ze does count as a measurement"

Yes, I agree - that is a measurement (albeit one with two components to it), not a 'calculation'.

Also, I would actually take the farthest measurement as well to ensure it was less or roughly equal. A higher figure would raise suspicions.

Exactly - that's the very 'belt and braces' to which I referred.

However - I suppose the TT figures actually do not, as such, serve any 'useful' purpose other than they have to be recorded and without them you would have no idea what they were.

That's the pragmatic answer.....

I am not really sure what you have in mind regarding this. Are you saying there is no point testing the rod? Obviously not.

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. There is also the confusion which results from the parallel paths - the Zs (measured with parallel paths present) in a TT installation will usually be very much lower (dramatically so in my house) than Ze(without parallel paths)+R1+R2. Which would you record?

Kind Regards, John

 

[JohnW2]

However - I suppose the TT figures actually do not, as such, serve any 'useful' purpose other than they have to be recorded and without them you would have no idea what they were.

The "useful purpose" of a Zs figure is to determine whether the protection in place against faults to earth is adequate.

In a literal sense, that's true. However, we know that, in a TT installation, Zs determined as Ze+R1+R2 will aways be too high for MCBs to provide that protection, and the R1+R2 would have to be so ludicrously high not to provide the protection via an RCD that fault protection would be the least of one's worries (the circuiot simply 'would not work'!), and the cause of the very high R1+R2 would have to be determined and corrected.

There is also the confusion resulting from parallel paths. In my house (TT), the directly measured Zs of every single final circult is well low enough for the OPDs to provide satisfactory disconnection times with L-E faults, whereas that obvioulsy is not true of Ze+R1+R2. So, as I just asked EFLI, which of the two (very different) Zs figures would you record as an indicator of whether "the protection in place for faults to earth is adequate"?

Are you saying there is no point testing the rod? Obviously not.

I don't think he is saying that. What I think he is saying is that the R1+R2 value of the circuit is so small compared to the max allowed Zs in the circumstances that you really don't need to actually do the sums to know what the answer will be to the question of if it passed or not.

Quite - or, as I put it above, if the Zs was so high as to be inadequate even for RCD protection, there would be a fundamental problem that needed to be sorted - and there would be no risk of 'missing' this situation, since the circuit simply would not 'work' in any meaningful sense. I guess one of the things that worries me is that some measurement-obsessed joker might measure a Zs of, say 1,500Ω in a TT installation (in the presence of an acceoptable Ze) and 'be content' that this satisfied the (RCD) fault protection requirements!!

What I do wonder though is why there is so much focus on P-E fault loop impedances and yet noone seems to care about P-N fault loop impedances. IMO in an install where you are relying on RCDs to protect against earth faults you really should be doing seperate calculations to determine if the protection against P-N faults is adequate.

Yes, I agree. When relying on an RCD for L-E fault protection, one really should look at L-N loop impedance to ensure it is satisfactory for OPD operation. I always meaure L-N loop impedance in my TT installation.

Kind Regards, John

 

[ban-all-sheds]

One of the reasons for being keen on recording R1+R2 is that is shows you have actually tested for, and got, cpc continuity.

 

[JohnW2]

One of the reasons for being keen on recording R1+R2 is that is shows you have actually tested for, and got, cpc continuity.

Indeed - and,unless I've misunderstood, no-one has suggested that one shouldn't measure R1+R2. The discussion was initially about whether one should get Zs by adding Ze to R1+R2 (or by direct measurement) and (subsequently, thanks to me!) whether Ze+R1+R2 has any particularly useful meaning (over and above that of R1+R2 alone) in an TT installation.

As I said, if R1+R2 is so high as to not satisfy fault protection requirements when reliance is on an RCD, there is something fundamentally wrong that needs to be sorted before one even starts thinking/worrying about what the Zs is!

Kind Regards, John

 

[plugwash]

There is also the confusion resulting from parallel paths. In my house (TT), the directly measured Zs of every single final circult is well low enough for the OPDs to provide satisfactory disconnection times with L-E faults, whereas that obvioulsy is not true of Ze+R1+R2. So, as I just asked EFLI, which of the two (very different) Zs figures would you record as an indicator of whether "the protection in place for faults to earth is adequate"?

The latter because the protection needs to remain adequate even if your gas and water pipes are later replaced with plastic.

 

[JohnW2]

The latter because the protection needs to remain adequate even if your gas and water pipes are later replaced with plastic.

Yes, I agree that is the safer (most conservative) approach, for the reason you give.

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. Do you therefore believe that one should always record Ze+R1+R2 and never record a directly-measured Zs (and, indeed, that the only reason for measuring it directly is as a 'double check' on the R1+R2)?

Kind Regards, John