Final Circuit EFLI - measurement vs calculation

[JohnW2]

[If your terminology is incorrect you will never learn. The poor understanding of practically every electrician in the land, of practically every important element of his work is often directly caused by the use of incorrect terms.

I agree totally, if the terminology used is significantly incorrect and/or introduces any uncertainty about meaning. However, I don't think there is anyone who knows anything about the subject would would not understand the meaning of 'main earthing conductor', 'main earth conductor', 'earth conductor' or anything like like (and similarly 'final ring circuit' etc.).

I've spent most of my life working in fields in which the general public very often don't get the technical terminology quite right and, provided their intended meaning is clear, it's often a case of life being too short to bother about correcting ('educating') them!

Kind Regards, John.

 

[dingbat]

... NAPIT requires a measured value for Zs on their EIC.

That's very irresponsible of them, in view of Regulation 14 of the EAWR 1989.
Have a chat with their helpline and ask them if they would accept two measurements added together.

I take that to indicate that you agree with me that directly measuring the value of Zs (with parallel paths to earth present) is essentially pointless...

Not at all.

However, you are doing what so many try to do an over-thinking it all out loud.

Go back to considering why you want to know Zs in the first place and the penny may drop.

 

[JohnW2]

I take that to indicate that you agree with me that directly measuring the value of Zs (with parallel paths to earth present) is essentially pointless...

Not at all.
However, you are doing what so many try to do an over-thinking it all out loud.
Go back to considering why you want to know Zs in the first place and the penny may drop.

I'm not sure that the 'why' is the same for me as it is for the regulations. The reason why I want to know Zs is so that I can be confident that the installation provides acceptable disconnection times with the installation 'as is' - i.e. with the paths to earth via service supply pipework present. Particularly given that the ancient water supply pipes travel a long distance under my own land (and some adjacent buildings!), I am totally confident that they are not going to cease to be effective connections to earth without my knowing all about it (and thereby able to review the Zs situation).

On the other hand, the regs appear to require that I don't rely on the water supply earth (i.e. they are considering the possibility that someone might dig up my garden and replace the pipes with plastic one dark night, without my knowing) - and therefore seeming want to know Zs so that they can be confident that the installation provides acceptable disconnection times even if all paths to earth other than my earth electrode were to disappear.

Those two different versions of 'why one wants to know Zs in the first place' can obvioulsy lead to very different answers - in the case of my installation, the difference between about 0.5Ω and about 80Ω.

Perhaps you can help me to understand your position by telling me what is your view of "why one wants to know Zs in the first place"?

I'm also a bit confused as to why you consider that I am 'over-thinking' about the issues, when you very recently wrote that "The poor understanding of practically every electrician in the land, of practically every important element of his work ...". The solution to that, for electricians and non-electricians alike, is sure to think very carefully?

Kind Regards, John.

 

[riveralt]

... NAPIT requires a measured value for Zs on their EIC.

That's very irresponsible of them, in view of Regulation 14 of the EAWR 1989.

Have a chat with their helpline and ask them if they would accept two measurements added together.

During the 2391 practical I asked the examiner if it was okay to calculate Zs rather than measure it. I was told to do both (probably to demonstrate the skill required and the H&S issues to consider).
I understand Reg 14 of the EAWR but your suggestion that NAPIT (NICEIC also require Zs measurement on their form) are irresponsible is wide of the mark.
While R1+R2 is obviously a dead test Ze is a live test - (unless you are advocating that we solely utilise the Ze by enquiry).
At some point we have to work live and provided Reg 14 a,b and c are covered should not be a problem.

 

[dingbat]

...your suggestion that NAPIT (NICEIC also require Zs measurement on their form) are irresponsible is wide of the mark.

Not at all.

It was your assertion that only a direct measurement was acceptable that I was questioning.

 

[riveralt]

...your suggestion that NAPIT (NICEIC also require Zs measurement on their form) are irresponsible is wide of the mark.

Not at all.

It was your assertion that only a direct measurement was acceptable that I was questioning.

If you have read the whole of the post you will see in the example given that I checked both calculated and measured values - however Zs measured values are required for the certificate.

 

[EFLImpudence]

As there is no such conductor, that's not surprising.

Noted and forever remembered.

... NAPIT requires a measured value for Zs on their EIC.

The point in my previous post was that, in my opinion, 'Ze + (R1+R2)' IS a measured value.
Employing pedantry you could argue that the addition of two measurements is a calculation but surely a calculation is more akin to what the designer of a circuit has to do to ensure compliance, namely, using the length of a cable and the resistance of the conductors, i.e. actually having to 'work out' these values.

I actually wonder if there is any point in measuring

On TN systems I would measure the values of Zs to confirm that the conductors had been installed/reconnected correctly and that these values were indeed as could be expected allowing for parallel paths and even maybe the higher values given by low current no-trip readings. As long as they are all below the maximum allowed then all's well.

On your TT system the readings of Zs, although vastly different, will be, at least, informative and, perhaps, comforting by knowing your installation is in fact hugely improved by the parallel paths even though you cannot rely on them.
Perhaps what this really shows is that the earth rod, in general, is not that effective albeit all there is and so acceptable.

 

[JohnW2]

... NAPIT requires a measured value for Zs on their EIC.

The point in my previous post was that, in my opinion, 'Ze + (R1+R2)' IS a measured value.
Employing pedantry you could argue that the addition of two measurements is a calculation but surely a calculation is more akin to what the designer of a circuit has to do to ensure compliance, namely, using the length of a cable and the resistance of the conductors, i.e. actually having to 'work out' these values.

I'm afraid that I probably started all this by the title I chose for the thread, and the distinction I was making between 'measured' and 'calculated' Zs. Maybe I was assuming too much, but I thought that those reading the thread would understand (from context) this shorthand I was using. I agree that both methods are based on measurements and, like you, would not call something 'calculated rather than measured' simply because the result I was declaring was arrived at by adding measurements together.

However, as I know you know, this was not the distinction I was making. To give the 'legend' of my shortcuts:

• 'By measurement' meant measuring Zs/EFLI with a meter on a live installation.
  and
  'By calculation' meant measuring Ze (or obtaining a value by enquiry) and (R1+R2) on a dead installation and then adding the two figures together.

I presume, but don't know for certain, that what riveraft was telling us was that NAPIT requires a 'measurement' as per that definition (i.e. measured on a live installation) and that they would not accept a figure resulting from the addition of two measurements taken on a dead installation (or addition of one such measurement with a figure determined by enquiry).

On TN systems I would measure the values of Zs to confirm that the conductors had been installed/reconnected correctly ....

I agree it serves that purpose (provided that Ze is appreciably less than the resistance of parallel paths - which it ought to be), but one such measurement on one final circuit would obviously be adequate to confirm that the conductors had been installed/reconnected correctly.

....and that these values were indeed as could be expected allowing for parallel paths and even maybe the higher values given by low current no-trip readings. As long as they are all below the maximum allowed then all's well.

Fair enough. Double-checking is never a bad thing, but (once one has verified that the earth conductor is connected and functioing correctly) I'm not sure that re-doing the determination of Zs figures can be justified any more than could re-doing of any of your other tests.

On your TT system the readings of Zs, although vastly different, will be, at least, informative and, perhaps, comforting by knowing your installation is in fact hugely improved by the parallel paths even though you cannot rely on them.

Yes, as I wrote before, it does provide comfort and is the measurement which really interests me - since I am very confident that those parallel paths are not going to disappear unexpectedly. However, in theory, the regs, and anyone (or any organisation) working to the regs, should not really 'be interested' in such figures.

Perhaps what this really shows is that the earth rod, in general, is not that effective albeit all there is and so acceptable.

Indeed - clearly nowhere near as effective as metal water supply pipes; I guess that means that one could get a pretty good earth with rods or suchlike if one tried hard enough.

I imagine that what I'm about to say will attract some flack, but I wonder how good a path to true earth is actually provided by TN 'earth' terminals. Admittedly, they are connected to earth rods (or whatever) somewhere, but what they actually represent (and what one measures if one attempts a Ze measurement, and what determines the fault current) is a low resistance return path to the neutral side of the generator. When one measures EFLI/Zs on such a system (or when there is a L-E fault in the installation), the entire fault current path is copper (or, at least, metal!), without any current flowing through the earth at all - hence the resistance/impedance of the path to earth is irrelevant to that measurement. That is obvioulsy totally different fromn a TT system, in which earth really matters, and carries fault currents.

Kind Regards, John.

 

[JohnW2]

Perhaps what this really shows is that the earth rod, in general, is not that effective albeit all there is and so acceptable.

Indeed - clearly nowhere near as effective as metal water supply pipes ....

I've been thinking about this (with apologies to those who don't like me thinking aloud!).

First, a question ... is it credible that (per my initial post) the water pipes coming to my house could, in themselves, be providing the path to earth of only around 0.25Ω ?

If not, then another possibility comes to mind. A few years back (following some incidents involving trees and overhead cables!), the supply to most of the properties in my village was changed from overhead to underground, but they stopped that process slightly short of my house, on the very edge of the village. It's never previously occurred to me to go and look at a neighbour's service head but, if they now have TN supplies, my neighbours' MPBs will be connecting the DNO-provided 'earth' to their water supply pipes and, hence, via the supply pipe network, to my water supply pipes. If that were the case, I'd have almost a TN supply via the pipes and my MPB.

Kind Regards, John.

 

[EFLImpudence]

is it credible that (per my initial post) the water pipes coming to my house could, in themselves, be providing the path to earth of only around 0.25Ω ?

I'm not an expert in this department, so would like to be corrected if necessary, but I would think 0.25Ω is too low using the supply pipes solely as an earth rod.

my neighbours' MPBs will be connecting the DNO-provided 'earth' to their water supply pipes and, hence, via the supply pipe network, to my water supply pipes. If that were the case, I'd have almost a TN supply via the pipes and my MPB.

This, to me, seems much more likely.
I have calculated with the 0.25Ω quoted -

Say a reasonable 0.12Ω for Ze on a TNC supply. This leaves an additional 0.13Ω to be found.
Calculating conservatively for a copper water pipe of 22mm diameter with a wall thickness of 0.6mm the length of pipe would be 298 metres. (A thick lead pipe could probably extend this distance but i don't have the figures for lead).
Is your nearest neighbour within this distance? (or whatever the figure for lead)
If so I would say you have found the answer.

 

[JohnW2]

I'm not an expert in this department, so would like to be corrected if necessary, but I would think 0.25Ω is too low using the supply pipes solely as an earth rod.

As you will have realised, that was also my suspicion.

my neighbours' MPBs will be connecting the DNO-provided 'earth' to their water supply pipes and, hence, via the supply pipe network, to my water supply pipes. If that were the case, I'd have almost a TN supply via the pipes and my MPB.

This, to me, seems much more likely.

It's certainly a credible possibility and, as you say, seems much more likely than such a low Ze coming from the pipes themselves. There are, of course, some uncertainties - not the least being whether my neighbours actually do now have TN supplies; I'll have a snoop when I next get a chance!

This puts a whole new perspective on 'extraneous conductive parts' and Main (Equipotential/Protective) Bonding. As you are aware, extraneous conductive parts are defined as conductors which are "liable to introduce a potential (normally earth potential)". Given that a supply-side fault can result in the DNO's 'earth terminal' rising to a significant potential above earth (a problem that I had hitherto been thinking my TT system was immune to!), it looks as if my situation may be bit different from that "(normally earth potential)", in that the water supply pipes may have the ability to introduce a potential well above earth potential. However, my MPB will still serve the intended purpose of attempting to make my property an equipotential zone, even if that 'equipotential' is elevated via the water pipes (my 80Ω earth rod would obvioulsy not make any significant impact if a non-earth potential was being introduced via some 0.25&#937

I have calculated with the 0.25Ω quoted - Say a reasonable 0.12Ω for Ze on a TNC supply. This leaves an additional 0.13Ω to be found.
Calculating conservatively for a copper water pipe of 22mm diameter with a wall thickness of 0.6mm the length of pipe would be 298 metres. (A thick lead pipe could probably extend this distance but i don't have the figures for lead).
Is your nearest neighbour within this distance? (or whatever the figure for lead). If so I would say you have found the answer.

It's (1 inch, I think) iron pipe which enters my property. The nearest neighbour who might have a TN supply is some 80-100 metres ('as the pipe flies') from where the pipe enters my property. I imagine the resistance/impedance arithmetic is pretty credible. I'll let you know when I've investigated the neighbours' supply!

Kind Regards, John.