Soil Resistivity:
We mostly measure soil resistivity using the Wenner method. Other methods that we have used generally infer the soil resistivity from ground electrode measurements. For example, we have measured the ground resistance of steel pilings, for which the size and depth was known and have determined the average soil resistivity by modeling the configuration in the computer.
The Wenner method involves inserting four evenly-spaced test probes into the soil, in a straight line. A test current is applied to the outer two probes. This produces a potential difference between the inner two probes. The apparent resistance (obtained by dividing the potential difference by the test current) is used to determine the apparent soil resistivity for that probe spacing. We typically take readings for a series of probe spacings of between 1 and 40 metres. This enables us to determine an average layered soil resistivity structure for the area. Much wider probes spaced readings are also possible if required.
We mostly use a digital electronic four-terminal grounding tester for the measurements. The instrument provides a direct resistance reading in ohms. Sometimes, and particularly in mountainous areas, the surface soil resistivity is too high for reliable operation. It is then necessary to use a basic current and voltage method with a higher voltage source to circulate sufficient current. We can used a variable frequency ac source and tuned voltmeter to minimize the effects of stray currents and noise.We can also take readings using dc.
We have developed software to resolve the soil resistivity test data into the equivalent layered soil structure. We use two methods for this. Both of them develop a perfect layered soil structure that would produce as close as possible to the measurement results obtained in the field.
Steepest Decent Method This method is described in the IEEE Guide 81-1983 "IEEE Guide for Measuring Earth Resistivity, ground Impedance and Earth Surface Potentials of a Ground System".
The steepest decent method is used for the analysis of soil resistivity data that fits a two-layered model. It converges more slowly that the Filter Function Method, but does not modify the original input data.
Filter Function Method The Filter Function Method is based on a 1979 technical paper by P.A. Davis, Faculty of the Graduate School of the University of Minnesota, which is titled: "Development and Application of Resistivity Sounding Inversion for Several Field Arrays". The paper was submitted by Mr. Davis in partial fulfillment of the requirements for the degree of Master of Science, which was subsequently granted.
The method processes the field data through a filter function that rapidly converges on a multi-layered soil structure that best fits the measurement data.
The method relies on the input of logarithmically spaced data, which means that the readings must be taken with probe spacing increased logarithmically for each reading. Readings are usually not taken with this spacing pattern. The software incorporates a curve fitting algorithm to develop logarithmic spaced data from the non-logarithmic field data. In so doing, some information is lost and for this reason it is best to use this method only for resolving the soil into more than two layers of resistivity.
Ground Impedance:
We usually use the fall-of-potential (FOP) method for measuring ground impedance. The FOP method consists of creating an artificial current return electrode some distance from the object being measured and injecting a test current into the object, using this electrode for current return. The test current will cause a voltage rise on the object which is measured using another reference electrode also located some distance from the object being tested.
Many people use small portable, battery powered three and four terminal grounding testers to measure the ground resistance of objects. These instruments work well for small ground electrodes or ground grids that have a purely resistive impedance of 1 ohm or more. They will still give readings for larger ground systems with lower impedances, but the readings may contain considerable error.
Our experience enables us to determine when other methods should be used, in particular the variable frequency method:
Around 1980, David Bensted pioneered the use of the variable frequency method of measuring ground impedance. The method consists of using a power source with a frequency other than 60 Hz (in areas where the normal power frequency is 60 Hz) and a frequency selective voltmeter, to measure ground impedance. It enables measurements to be undertaken without shutting down equipment, even though there may be relatively high levels of 60 Hz "noise" present. We use a portable generator and sine-wave power source operated at50 or 70 Hz to provide the test current for these measurements. We use a spectrum analyzing instrument to filter out 60 Hz and other noise. Readings are possible with high levels of noise.
The conventional FOP test configuration is to measure the voltage rise of the ground system with reference to a test probe that is progressively moved closer to the current return electrode, along a straight line between the two. The test results are plotted as a curve of apparent measured resistance against the distance along the measurement traverse. If the current return electrode is sufficiently far away, there should be a flat portion of the curve where the test probe was in an area of zero soil voltage. The ground resistance is given by the resistance measured in that part of the curve. For an ideal measurement where the ground electrode being measured is small compared with the distance to the current return electrode and the soil resistivity is completely uniform, the true resistance can be shown to be indicated at 61.8% of the distance along the traverse towards the current return electrode. We have noticed that this is often incorrectly used as a criteria, when the plotted curve has no flat portion. The real reason for such a curve may be that the ground system is interconnected with other objects and is much larger than was thought.
The conventional method often fails to work properly, particularly in the case of large ground systems with a low ground resistance. If the measured resistance is in the order of 1 ohm or less, significant error will be introduced by inductive coupling between the test lead that carries current to the remote current return electrode and the test lead to the voltage probe. The coupling can be reduced by spacing the leads wider apart, but the reduction is not sufficient to eliminate the error. An alternative is to run the voltage traverse at 90 degrees to the direction of the current return electrode. The reading nearest to the correct reading will then be at the end of the potential traverse.
There are a number of other pitfalls in carrying out the fall-of-potential test. Attempts have been made to minimize these problems by developing "smart" testers and special test configurations. Closer examination shows that these devices and methods cannot produce as accurate a result as a well-configured fundamental fall-of-potential test.
Electrical Bonding:
These tests are used to evaluate existing bonding systems. We use a high current, four terminal resistance measurement method to test the bonding between different areas of a plant. Resistances as low as a few milli-Ω can be measured over several 100 metres in this way.
We have carried out 1000's of these tests at many different kinds of industrial plants. The tests provide confidence in the existing grounding system.