Earth Resistivity Testing 1

[MRM]

I have a few questions about the subject and I can't think of many better that I could ask...

I've become involved in resistivity testing to provide grounding information for use in cell phone tower grounding design (pre-installation resistivity testing). We're using an AEMC 4500 Digital Earth Resistance Meter. I've reviewed the IEEE standard related to resistivity testing, but there are still a few things I'm not sure of. While my knowledge of what soil factors affect resistivity readings is up there, my knowledge of the electrical aspect and the methodolgy may be lacking. I was wondering about the following:

1. Is the four point (Wenner arrangement) method the most suitable for pre-installation testing for grounding design? We are using spacings of 5, 10, 20, 30, and 40 feet and performing that arrangement along 5 different lines.

2. Are the terms impedance and resistance used interchangeably?

3. When I finally measure my resistance in the field and convert it to resistivity, is that considered the "apparent" resistivity or the "effective" resistivity, or are those terms also used interchangeably?

4. Is the four point method also considered a "fall-of-potential" test? I believe that's how the test works; by passing a current from the two outside probes and measuring the drop in the middle two, but again, I'm not sure about the terminology.

5. What are the signs that stray voltage may be present during the test? Will the meter give you different readings at different current levels or something?

6. Could you please explain step and touch voltages to me?

7. Would using different length probes be acceptable as long as the lengths are accounted for in the proper equation related to the four-point method and with the spacing in mind?

You can answer any or all of the above! Thanks in advance for your answers or comments.

 

[DanDel]

1. The four-terminal method is the only test to determine actual soil resistivity(as opposed to two- and three-point ground testing).

2. Impedance and resistance are not used interchangeably, but, for soil resistivity and ground testing, the value are equal to each other. Impedance refers to the total opposition to the flow of current in an AC circuit(the vectoral sum of resistance and reactance), and since the test uses AC voltage, it is actually measuring impedance. The total component of that impedance, however, is resistance.

3. I haven't heard of the use of the terms "apparent" resistivity or "effective" resistivity in ground testing, but this may refer to the difference between impedance and resistance as explained in #2.

4. The phrase 'Fall-of Potential' is typically used to describe the three-point test for a ground rod; though your understanding of the four-point test is accurate and both three-point and four-point tests measure voltage drop.

5. The AC voltage used by the ground test set is typically produced at a frequency which will be different from the system frequency and its harmonics just for the purpose of eliminating any such interference.

6. Touch potential, or touch voltage, typically refers to the voltage between the any two pieces of grounded equipment(including the ground grid) in close vicinity which can be 'touched' simultaneously by personnel, during a fault condition.
The system voltage during the fault is dissipated through the grounded equipment.
The lower the resistance of the connection between the equipment, the less chance of a significant touch potential between two points that a person might be touching.
If there is not a good grounding or bonding connection, the touch potential could be hundreds or thousands of volts.
The better the bonding and grounding between these components, the lower the potential between them, and the less risk to personnel near them.
This is also the reasoning behind 'step voltage', and why there is a ground grid to bring the potential of the ground to near-equal values everywhere in a switchyard.

7. I believe that consistent depths of test rods are necessary for consistent results.

 

[jghrist]

DanDel gives a very good explanation - I give him a star. A good explanation of resistivity and resistance testing is in

http://www.aemc.com/ground/document_layout01.html

which you probably already got with your AEMC instrument. For a more detailed explanation, see IEEE Std 81, "IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System (Part 1).

For cell phone tower grounding, you are as concerned about lightning protection as you are with electrical safety. The impedance of the grounding system at high frequencies becomes important. At high frequencies, the reactance becomes significant. Unfortunately, determining the impedance of a grounding system at high frequencies requires some pretty high-end analytical tools. See

http://www.sestech.com/software_packages/software.asp?from=EMF

for a description of software offered by SES and links to a couple papers. Getting a low impedance grounding system at lightning frequencies is an art as much as a science unless you do an extensive study with expensive software.

The term "apparent resistance" is the resistance that you measure at each probe spacing. It is converted to a resistivity value which represents the average resistivity of the ground at a depth equivalent to the distance between probes.

 

[ausphil]

Just a few more points that may be of interest

Q1. The pitfalls of earth resistivity measurements are if you have buried pipes, metalwork, fences or shallow water tables in the area you are measuring, as these will have an effect on the readings. Depending on the size of the grid you are looking at installing will determine the number of lines you need to measure, and the depth to which you will go (ie the maximum electrode spacing). We start at about 2 feet in your speak, and may go to about 100 foot spacings. Always (space permitting) perform measurements in both directions (perpendicular to each other) so get the best picture of the earth as possible.

Q7. Imagine the electrode as an equivalent hemisphere with a radius equal to the depth of the electrode. If you place your electrodes too close together, then the spheres will overlap, and you will get erroneous readings. If you play around with your electrode depths at the same spacing (say at your 5 foot spacings), start with the electrodes only just in the ground, and then hammer them in further and further, taking readings each time, you will find that the values change. If you use these same electrode depths at the larger spacings, you will find that there is no significant difference. I think the rule of thumb is that the electrode depth should not exceed 1/10 the electrode spacing to eliminate this issue. The depth only really becomes an issue at the close spacings.

Hope this rambling helps.