Meauring an earth rod impedance

[JohnW2]

For anyone who was as confused as me with the mixture of terminology in this thread:

I fear that the situation is probably much more messy/confusing, with widespread misuse of terminoolgy, than you go on to suggest.

A rod install can be tested in two ways.
You can measure Ra (The resistance of the rod to earth) using an 'Earth Electrode Resistance Tester'.
This is a 'dead' test and is measured between the end of the (disconnected) earthing conductor/CPC, through the rod and then through 'earth' to the two 'additional' probes in the ground.,
This is resistance, (hence the 'R' in Ra), and is measured in ohms.

When one uses this method, whether one is measuring resistance or impedance depends primarily upon whether one's Tester uses AC or DC for the testing. I don't know about other makes, but Fluke MFTs like the one I have (e.g. 1653B/1654B) use AC (25V 128Hz) - hence, despite the fact that the documentation and labelling of the machines all talks in terms of 'resistance', I can but presume that what is actually being measured is impedance (or maybe I'm wrong, and it's somehow managing to extract the resistive component?).

The same is true if one uses the 'transformer method' of measuring an earth rod. In that case one definitely uses AC and simply measures the AC voltage applied and AC current flowing (with no consideration of phase angles/'power factor' etc.) - so one is definitely measuring impedance - although that is, again, almost always called 'resistance'.

In any event, what actually matters is earth path impedance (not that it will usually differ significantly from resistance), since any fault currents it carries will necessarily be AC.

For The other method is the same as your normal Ze test.
Measured with an 'Earth Loop Impedance tester', this measures the 'Earth Fault Loop Impedance' (which includes the 'rod'), and gives an impedance measurement in ohms.

Indeed - and that certainly is impedance that is being measured, since the test voltage then necessarily comes from the AC supply.

Kind Regards, John

 

[Electrifying]

Yeh, sorry Sparticus, your replies were pretty good - I think it was the third post by 'Bernardgreen' that puddled my brain.

 

[Electrifying]

When one uses this method, whether one is measuring resistance or impedance depends primarily upon whether one's Tester uses AC or DC for the testing.

No it doesn't - using an AC test meter won't introduce reactance of any type whilst measuring the Ra of the 'Rod', so the measurement will be purely resistive......that's what determines what you are measuring.

If you were measuring 'impedance', they would have called it Za, not Ra.
The clue is in the letters, you see, R1, R2, Rn, Ra!

When measuring Ze there is the chance of reactance, hence it's 'impedance'.

In any event, what actually matters is earth path impedance (not that it will usually differ significantly from resistance), since any fault currents it carries will necessarily be AC.

In some installs an impedance measurement might not differ from a resistance measurement (due too lack of reactance) BUT an Ra measurement will definitely differ from a Ze measurement..........for a start, one of them doesn't contain the impedance of the 'Line' conductor (Ra), whilst the other does (Ze).

 

[JohnW2]

No it doesn't - using an AC test meter won't introduce reactance of any type whilst measuring the Ra of the 'Rod', so the measurement will be purely resistive......that's what determines what you are measuring.

If you apply an AC voltage, measure the resulting magnitude (but not phase) of the resulting AC current, and then divide one by another, there is absolutely no doubt that what one has measured is impedance. I agree that the reactance will probably be negligible, so that resistance will be almost the same (albeit not exactly the same) as the measured impedance, but that doesn't alter the fact that what one has measured is impedance, not resistance.

If you were measuring 'impedance', they would have called it Za, not Ra.
The clue is in the letters, you see, R1, R2, Rn, Ra!

That's what I meant about the situation being messy/confusing - that applies to the 'standard terminology'which has evolved as well as everything else. When talking about AC circuitry, it would make sense to be consistent in always talking about impedance - as per standard Electrical Engineering practice. Sure, in some conexts, resistance will be almost (but never exactly) the same as impedance, but that doesn't really IMO justify sloppily talking about resistance when one should really be talking about impedence.

In some installs an impedance measurement might not differ from a resistance measurement (due too lack of reactance) BUT an Ra measurement will definitely differ from a Ze measurement..........for a start, one of them doesn't contain the impedance of the 'Line' conductor (Ra), whilst the other does (Ze).

No argument with any of that, but I'm not sure what it has to do with what I said. Also, as I wrote to Bernard earlier, Ze (measured at earth rod/conductor) is generally only goung to be very slightly higher than Ra,since the impedance of the supply L conductor is going to be extremely small in comparison with Ra.

Kind Regards, John.

 

[Electrifying]

To be totally clear, here is the Spec from a 'Megger' Earth Electrode Resistance Tester - you'll notice the voltage is AC, but all measurements are 'resistance'.

Instrument output:

Voltage: ±25 V or ±50 V at 128 Hz
Current: 4.5 mA or 0.45 mA
Resistance range:
0.01 ohms to 20 kohms
Earth voltage range:
0 - 100 V
Resistance accuracy
2P measurements: 2% ±3 digits
3P measurements: 2% ±3 digits
4P measurements: 2% ±3 digits
Maximum probe resistance:
Rp limit: 100 kohm (50 V output voltage)
Rc limit: 100 kohm (50 V output voltage)
Limits reduced to 50 kohms for 25 V output voltage
Limits reduced to 5 kohms for 0.01 ohm resolution
Ground voltage range:
0 to 100 V a.c.

 

[JohnW2]

To be totally clear, here is the Spec from a 'Megger' Earth Electrode Resistance Tester - you'll notice the voltage is AC, but all measurements are 'resistance'.

Sure, as I said, it's exactly the same with Fluke machines. You can't really blame them for talking about 'resistance' - since that's what the industry has decided to call these measurements. However, I frankly doubt (but have yet to manage to confirm) that they are actually presenting impedance results (since measuring phase angles and determining true resistance would be tedious/expensive!).

As I've just written in another message, it just seems a bit bizarre to me that (assuming this to be the case) an instrument measures one thing but chooses to call it something else, on the basis that the two are going to be nearly the same!

Kind Regards, John.

 

[Electrifying]

You really are amazing, John, - even when presented with it in Black & White, you still argue the point.

With the lack of capacitance or inductance, it is purely resistance.

It is a 'resistance' measurement if there is NO inductance or capacitance - regardless of whether the voltage used to measure it is AC or DC.

No phase angle, no current lagging/leading the voltage........resistance.

I know what you're going to argue now - that it's not Black & White, but a funny shade of grey ..........give us a break!!

 

[JohnW2]

You really are amazing, John, - even when presented with it in Black & White, you still argue the point.
With the lack of capacitance or inductance, it is purely resistance.
.... I know what you're going to argue now - that it's not Black & White, but a funny shade of grey

Nope, I'm not going to argue that. I'm going to argue that it is Black and White - but that my Black and White is different from yours

Do this thought experiment for me, if you would .... stick a couple of rods in the ground a few feet apart. Connect as high an AC voltage between them as you dare, hence producing a reasonably high current between them. Now take a length of insulated cable a bit shorter than the distance between the rods and lie it on the surface of the ground along the line between the rods. Connect a high input impedance/resistance (!) AC voltmeter to the ends of that cable. Do you find a measurable voltage?

If you answer 'no', then I guess I'm sunk - but if you answer 'yes', even if only a very small 'yes' (very small voltage), are you going to continue to say that there is no reactance?

There will even be some capacitance between the two rods, but that really will be negligible.

Although I've always felt that the terminolgy being used (universally) in this area is very sloppy, I would not have raised the issue if you hadn't. It was you who claimed to be very confused by people's use of 'resistance' and 'impedance', even though we are are agreed that the two are going to be nearly identical in the situation under discussion?

Kind Regards, John.

 

[sparkticus]

Well, as far as I know (and have experienced over 30+years) no circuit exhibits pure resistance and that includes the ground. Admittedly the reactive component of certain common damp soils is small compared to that of the purely resistive component but it still exists and can be observed as a general impedance increase as the frequency of an AC test current is increased.

During a loop impedance (external Ze) I suspect the most "reactive" component to be the sub-transformer winding(inductive), then possibly the cables(capacitive) then the earth(probably mostly inductive) The reactivity of soil is probably going to depend heavily on its mineral/metal content and to moisture level which could increase the "width" of the "conductive channel" and possibly include additional reactive contents in the soil.

I remember several years ago performing a DC loop resistance test and comparing it with an AC loop impedance test at 50Hz. The difference was probably about 10% (the impedance measurement at AC was higher than the resistive measurement at DC)

John's test is an interesting one, I have never tried it but strongly predict a reasonable PD between the ends of the wire.

 

[bernardgreen]

I recall trying to explain to someone why the DC insulation test gave a "pass" result but the same test using AC gave a "fail" result. He could not grasp the concept of AC current passing "through" insulation ( the capacitive effect ) when DC could not pass through it.

 

[JohnW2]

Well, as far as I know (and have experienced over 30+years) no circuit exhibits pure resistance and that includes the ground.

Quite. It's not only "as far as you know" - it's an inevitablity, since any conductor of finite length has a calculable inductance, and any two conductors (or points on a conductor) have a capacitance between them. Hence, with just one sort-of exception (in case there are any nitpickers around!), any circuit necessarily has at least some reactance. ((That one 'sort-of exception' arises when one measures impedance at the resonant frequency of the capacitance and inductance present - in which case the reactances will 'cancel' leaving only the resistive component.))

Admittedly the reactive component of certain common damp soils is small compared to that of the purely resistive component but it still exists and can be observed as a general impedance increase as the frequency of an AC test current is increased.

Exactly.

As I'm sure you understand, my point has been about the rather strange (and arguably incorrect) 'standard terminology' that has arisen in the industry. Even if others disagree, I hope you accept that if one measures with AC (and doesn't attempt to correct for phase angle) what one is measuring is impedance - which is simply defined as that property of a circute which impedes/resists the flow of AC current. Although (as above) in practice it is an inevitability, there is nothing about the definition of impedance which requires all three possible components to be present - so, even if it were possible to have a circuit that was purely resistive, it would still be correct and appropriate to talk about the impedance of that circuit (particularly if that's what one had measured!!) which would be the same as the resistance at all frequencies.

The way I was brought up, I would have been 'shot' if I had measured impedance and then described my result as a resistance, on the grounds that the reactive components were almost certainly very small!!

I remember several years ago performing a DC loop resistance test and comparing it with an AC loop impedance test at 50Hz. The difference was probably about 10% (the impedance measurement at AC was higher than the resistive measurement at DC)

That doesn't surprise me. I suspect that the inductive reactance of the transformer and, particularly if long, that of the the supply cables are the main (inductive) components.

John's test is an interesting one, I have never tried it but strongly predict a reasonable PD between the ends of the wire.

I've never tried it, either, but I feel sure that one would be able to measure a PD, even if one had to resort to a sensitive meter.

Kind Regards, John.

 

[JohnW2]

I recall trying to explain to someone why the DC insulation test gave a "pass" result but the same test using AC gave a "fail" result. He could not grasp the concept of AC current passing "through" insulation ( the capacitive effect ) when DC could not pass through it.

Indeed. Whilst I do believe that, in relation to AC circuits, we should always measure, speak and think about impedance, insulation resistance is a necessary exception. If one were interested in the AC 'leakage current' that would flow when the circuit was energised, then impedance would still be the thing to measure. However, since what we're usually really looking for is deterioration in insulation or emergence of a 'parallel path' (e.g. due to moisture), rather an an intrinsic property of an intact circuit, we sensibly measure just resistance, using DC - since we would otherwise have the problem of having to try to separate out the reactive components of measured impedance.

Kind Regards, John.

 

[ban-all-sheds]

You really are amazing, John, - even when presented with it in Black & White, you still argue the point.

With the lack of capacitance or inductance, it is purely resistance.

There cannot be lack of reactive components.

It is a 'resistance' measurement if there is NO inductance or capacitance - regardless of whether the voltage used to measure it is AC or DC.

No phase angle, no current lagging/leading the voltage........resistance.

It doesn't matter if the values are identical to as many places as you care to measure, if the current is direct the property you are measuring is resistance, and if the current is alternating the property you are measuring is impedance.

 

[ban-all-sheds]

If you are doing this to provide an earth for a generator etc, you need to use an earth electrode resistance tester

But you take the supply from the generator in a way which mimics a TT, TN-S, or TN-C-S distributor's supply, so why can't you use a loop tester?

 

[JohnW2]

You really are amazing, John, - even when presented with it in Black & White, you still argue the point....

There cannot be lack of reactive components. ....It doesn't matter if the values are identical to as many places as you care to measure, if the current is direct the property you are measuring is resistance, and if the current is alternating the property you are measuring is impedance.

Thank goodness I'm not the only person who subscribes to my version of 'Black and White'! Thanks.

Kind Regards, John.