An earth resistance test set is commonly made up of a voltage source, an ohmmeter to measure resistance directly, and switches to change the instrument’s resistance range. Extension wires connect terminals on the instrument to the earth and reference electrodes or ground rods, as will be described. A battery-powered oscillator supplies the required current and you read resistance in ohms from a pointer on a scale or a digital readout.
There are three basic test methods, namely:
1. Fall-of-potential method also referred to as a three-terminal test.
2. Direct method, or two-terminal test.
3. Clamp-on/Stakeless method.
The Fall-of-potential or three terminal test is the basic method for measuring the resistance of grounding systems. However, it may only be practical on small, single ground rods because of limitation on the size of area available to perform the tests. Additional test methods have been derived, based on the principles of the Fall-of-potential method, to overcome difficulties encountered in real world field applications. We will explore these variations in future blogs.
This test is commonly performed with a three terminal earth tester, but can be performed with a four terminal tester. If a four terminal test set is used the C1 and P1 terminals on the instrument are jumpered and connected to the ground rod under test. With a three-terminal instrument, connect the X terminal to the ground rod. The driven reference rod C should be placed as far from the ground rod under test as practical; this distance may be limited by the length of extension wire available, or the geography of the surroundings.
Potential-reference rod P is then driven in at a number of points roughly on a straight line between the ground rod under test and C. Resistance readings are logged for each of the points. A curve of resistance versus distance is then drawn. Correct earth resistance is read from the curve for the distance that is about 62 percent of the total distance from the earth electrode toC. In other words, if the total distance is D, the 62 percent distance is 0.62D; for example, if D is 120 ft., the distance value for earth resistance is 0.62 x 120 or 74 ft.
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