Upgrading supply/dno earthing

[JohnW2]

.... No need to disturb the earth bar once you’ve done your R1 + R2 test ...

What about when you are doing the R1+R2 test (on a ring final)?

On a modern installation every circuit will have RCD protection so you can only carry out low current loop testing anyway so there’s nothing to be gained.

Many installations are, of course, not 'modern', in that many will have at least some circuits which are not RCD protected. However, do you not trust the results of "low current loop testing"?

You can record either measured or calculated figures on the certificate.

Is that not potentially confusing/misleading? The standard Schedule of Test Results has, for each circuit', columns for 'maximum Zs', (R1+R2) and Zs, but not one for (R1+R2+Ze). It is therefore possible that ('measured') Zs figures could be recorded which were below the recorded 'maximum Zs' figure for the circuit, even though (R1+R2+Ze) is actually above that 'maximum'. That would probably not be immediately obvious from looking at the recorded figures.

What is iffy about low ohms testing? I have a calibrated meter which gives consistant results, and can be verified by calculating the estimated length of the cable run multiplied by the resistance per meter values of said cable.

You must be familiar with the experience that 'wiggling' the clips, or moving the tip of a probe around ('scraping' whatever it is touching) can result in an appreciable change in displayed readings. Admittedly, this phenomenon errs on the side of safety - since, whilst it is easily possible (by 'wiggling' etc.) to get a reading which is higher than the actual resistance, it's impossible (within the accuracy of the instrument) to get a reading lower than the actual resistance.

Kind Regards, John

 

[EFLImpudence]

What about when you are doing the R1+R2 test (on a ring final)?

You have to measure it.

 

[JohnW2]

It is relevant as you were discussing measuring and calculating Zs. I would say a calculation would be knowing the length of a cable and the resistance per meter and working it out; anything done using meters is a measurement.

As I have said, you are (literally and) semantically/pedantically correct.

However, as I've also said, in this discussion we have been doing what most other people do, in using the phrase "by calculation" as 'shorthand'to indicate that (R1+R2) and Ze have been measured separately (by totally different methods) and then added together to get the total loop impedance (in contrast to measuring Zs with a single measurement).

As I said, I can calculate speed by measuring distance and time and then dividing one by another. That is 'a calculation' (performed on two measurements).

Kind Regards, John

 

[JohnW2]

What about when you are doing the R1+R2 test (on a ring final)?

You have to measure it.

The question is whether one (or RF) 'disturbs the earth bar' (e.g. by removing a conductor from it) when measuring R1+R2 on a ring final circuit.

Kind Regards, John

 

[Michaelsf90]

My thread has well been hijacked! The joys of inspection and testing. I rung the dno this morning. An engineer was coming today. Its half six and still no sign

 

[JohnW2]

My thread has well been hijacked! The joys of inspection and testing. I rung the dno this morning. An engineer was coming today. Its half six and still no sign

Sorry about the hijack! As I said, the DNO guys usually turn up pretty rapidly, but I suppose most things are not happening 'as usual' at the moment. Let us know when he does turn up, and what he has to say!

Kind Regards, John

 

[securespark]

WRT low current EFL readings, I have found that they are always higher.

When I was trained in inspection and testing, I was told to do both dead and live tests. The instructor's thinking was that a live loop test would confirm the dead tests and that any conductors displaced for dead testing were correctly reconnected.

 

[JohnW2]

WRT low current EFL readings, I have found that they are always higher.

Fair enough - although I have to say that, since both are just measuring current (at the same voltage), I find it hard to see why there should be any significant difference.

When I was trained in inspection and testing, I was told to do both dead and live tests. The instructor's thinking was that a live loop test would confirm the dead tests and that any conductors displaced for dead testing were correctly reconnected.

Thanks. As you will realise, that's exactly what I have been suggesting, and for the same reasons.

Kind Regards, John

 

[securespark]

Fair enough - although I have to say that, since both are just measuring current (at the same voltage), I find it hard to see why there should be any significant difference.

I can't explain it, those are my observations.
I wonder if other sparks here observe the same?

It could be the equipment I have used: various generations of Metrel multi-testers.

 

[JohnW2]

I can't explain it, those are my observations. I wonder if other sparks here observe the same?

An interesting question - but I suspect that (for obvious reasons) it's probably not all that common that they do 'high current' loop impedance tests on final circuits.

I may well be missing something but, as I said, at first sight I cannot think of any obvious reason why apparent loop impedance should differ with different tester currents (with the same voltage driving the current in all cases).

It could be the equipment I have used: various generations of Metrel multi-testers.

The equipment involved could obviosuly make some difference, but I thought what you meant was that you have observed the difference (between results with 'high current' and 'low current' tests) for the same circuit and with the same machine. Is that not what you meant?

Kind Regards, John

 

[securespark]

Yes, it was.

But I initially had not considered that the brand of tester may affect the result.

 

[RF Lighting]

My experience is the same as securespark, using various different metrel, fluke and megger MFTs over the years.

 

[JohnW2]

My experience is the same as securespark, using various different metrel, fluke and megger MFTs over the years.

Interesting. Any thoughts about a possible 'explanation'? What sort of degree of difference are we talking about? - and, indeed, do have have any idea as to which is closer to being 'correct'?

Kind Regards, John

 

[securespark]

I would say the HC test is closer to the real figure.

But I have worked on the principle that if the LC test reading is within spec, then the HC test reading would be lower, so no worries.

Only once or twice have I had a borderline reading; usually it is either a very obvious pass or a spectacular fail.

 

[JohnW2]

I would say the HC test is closer to the real figure.

Fair enough - but I would still love to hear thoughts (from anyone) as to why there should be a significant difference.

But I have worked on the principle that if the LC test reading is within spec, then the HC test reading would be lower, so no worries.

As I've been discussing with RF, whether or not that 'no worries' is necessarily justified depends entirely on one's view of the situation and the nature/extent of bonded extraneous-c-ps.

As I understand it, RF's view is that (R1+R2+Ze) must be below the 'maximum Zs' for the OPD in question, regardless of what ('measured' - whether HC or LC) Zs may be. As I've been discussing with him, since Ze is measured with bonding disconnected, but Zs is measured with it connected, it is possible for the measured Zs to be lower, perhaps considerably lower, than (R1+R2+Ze). Hence, a circuit which you 'passed' on the basis of a Zs (say 'low current') measurement could be a resounding 'fail' as far as RF was concerned.

Only once or twice have I had a borderline reading; usually it is either a very obvious pass or a spectacular fail.

In terms of 'faults' (in the everyday sense), I would expect that to be true. However, I imagine that there are plenty of circuits out there (particularly 'old' ones) which are long and/or have somewhat under-sized cable, such that the ('true') loop impedance is just a little above the 'maximum permitted' - in which case you might 'pass' the circuit (on the basis of a LC Zs measurement, reduced by extraneous-c-ps) whereas RF would 'fail' it (on the basis of R1+R2+Ze).

Kind Regards, John