# Meauring an earth rod impedance

#### bernardgreen

I know the approved method of two test probes driven into the ground with a known injected current and then measure voltages to calculate impedence of the rod being tested.

But that method requires the furthest test probe to be about 20 metres from the rod under test. For this installation that is the middle of a busy road, the lounge of the next door house or a solid concrete slab of the cake shop's terrace.

Is there another way to measure a ground rod's impedance that is acceptable to the "authorities" without having to provide them with a course in basic electrical theory to understand how the measured result is achieved

A calibrated EFLI tester will do the trick.

EDIT
I am assuming that you are installing an electrode for a system which has a return path to a distributors electrode. In that case a simple Ze test can be done.
If you are doing this to provide an earth for a generator etc, you need to use an earth electrode resistance tester

The rod is for a sub main to an outhouse being configured as TT. Supply is from a PME source in the house. SWA earthed to the PME earth at the house and insulated at the outhouse end.

Is there another way to measure a ground rod's impedance that is acceptable to the "authorities" without having to provide them with a course in basic electrical theory to understand how the measured result is achieved
Assuming that the installation is connected to a supply, a Ze measurement will give you the required figure - theoretically a slight overestimate, but the DNO component of the measured Ze should be insignificant compared with the rod's impedance.

Kind Regards, John.

The armouring will be providing earthed mechanical protection from the PME supply. This needs testing as a Zs at it's extremity to ensure disconnection in the event of a fault on the SWA.
The rod can be tested using the EFLI tester this time the result being the Zdb for the remote CU. (With as you say the armouring left totally insulated from the rod earth.)
Remember this test is done with the CU isolated and any MEBs/CPCs disconnected (or more simply the earthing conductor disconnected)

I can see that using the PME "earth" is in theory useable as the reference for true ground as it is connected to true ground at the substation far enough away that the "resistance areas" do not overlap. In theory it can be used as the remote test electrode for a simple loop round measurement and use the total loop impedance as the impedance of the rod.

BUT the PME earth is showing a small but significant and very variable voltage above true ground due to phase imbalance in the network. This is going to affect the voltage divided by current measurement of the rod's impedance and while a go / nogo ( less than 100 or greater than the upper limit ) result might be obtained it might be that the officer will want to see a recorded value and not something like " varied between 25 ohm and 75 ohm so assumed worse case is less than 100 ohm " I certainly would not like that sort of result handed to me in a safety critical situation.

As others have said, just do a loop reading of the rod using the line from the supply cable. This method is outlined in the on site guide where the TT system is protected by RCDs which I'm pretty sure you'll have.

BUT the PME earth is showing a small but significant and very variable voltage above true ground due to phase imbalance in the network. This is going to affect the voltage divided by current measurement of the rod's impedance and while a go / nogo ( less than 100 or greater than the upper limit ) result might be obtained it might be that the officer will want to see a recorded value and not something like " varied between 25 ohm and 75 ohm so assumed worse case is less than 100 ohm " I certainly would not like that sort of result handed to me in a safety critical situation.
Unless I'm misundesratnding, I think you concerns are probably unjustified. If you do a L-E loop impedance test (i.e. measure Ze) on your rod, the 'loop' in question will be from the transformer, along the L supply cable, through your earth electrode to the mass of the earth and thence back from the mass of the earth to the other side of the transformer. The PNE conductor of your supply does not come into that loop (although it provides some of the connection of transformer to earth via PME - and the voltage (above true earth) of that PNE when it enters your property is irrelevant ... or am I missing something?

Kind Regards, John.

I certainly would not like that sort of result handed to me in a safety critical situation.

Bernard, as others have indicated, you can use a two/three wire calibrated loop impedance tester with one lead connected to phase and the other connected just to the earth rod (ensuring no other parallel connections) this indicates the AC impedance in the loop formed from: earth rod>mass of earth> DNO earth rod (at sub-transformer) > star point on secondary > secondary winding > phase cable > phase termination within property.

The reading (Ra ohms) will be perfectly acceptable to building control.

Thanks for all the replies and advice there in. If measuring the current that the rod will sink from the live and calculating impedance from that is acceptable then my worries are gone.

Thanks

If measuring the current that the rod will sink from the live and calculating impedance from that is acceptable then my worries are gone.

Confirmed, building control will accept that. You will probably need to document the reading on a PIR certificate and simply write the limitations as "inspection of earthing arrangements only"

A PIR won't be enough as it is a completely new installation which will have to have all the test results for a new installation.

A PIR won't be enough as it is a completely new installation which will have to have all the test results for a new installation.

Aha right, in that case yes an EIC.

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

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.

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.

The first method is normally used for generator/transformer installs etc., or where a larger degree of accuracy is required - normally the Ze measurement will suffice - just enter it as normal on the cert.

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

Were you really that confused? It was a small error on my part for which I humbly apologise. I should have written "external Ze"
In everyday speak Ra often gets used (technically incorrectly) as Ze in a TT arrangement. Many interpretations of BS7671 certificates ask for the rod Ra and the method of measurement (EER or EFLI)

The first method is normally used for generator/transformer installs etc., or where a larger degree of accuracy is required - normally the Ze measurement will suffice - just enter it as normal on the cert.

Or when there is no power available to perform a Ze with an EFLI tester.

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