Grounding Test Instruments

There are a number of different grounding test instruments available. Considerable strides have been made in recent years to improve them. They are mostly resistance measuring instruments that apply a test current to the resistance being measured and measure the resulting voltage difference. In most cases the current and voltage readings are converted to a resistance by the instrument.

Although the instruments are designed for measuring ground resistance, they can be used to measure other kinds of regular electrical resistance, for example a fixed resistor as may be used in electronic equipment. Such a resistor is often useful as a calibration tool for the instrument if a unit with 1% accuracy is used.

Ground resistance can also be measured using a fundamental test method in which a power source is used to apply the test current (I) which is measured with an ammeter. The potential difference (V) is measured separately, with a voltmeter and the resistance is, by Ohm's law V/I.

Click here (255k) to see a picture of soil resistivity measurement being done using a portable generator to apply the test current at 50 Hz (in an area with 60 Hz power systems) and a tuned voltmeter to measure the potential difference between the inner probes. The measured surface soil resistivity in this field was very high at about 12,000 ohm-metres. Even though more than 120 volts was used to drive the test current and salty water was applied to the test probes, the current was only between 3 and 12 mA. Notice how the current injection and voltage measuring equipment are separated to avoid errors from capacitive or leakage resistance coupling. They are also insulated from ground by being placed on insulating material. The actual test results are listed in this table. The project name has been blanked out. The curve fit to the measurement results done using our N-Layer program can be seen here. It can be seen that the calculated and measured curves fit very well and the equivalent layered resistivity structure is a top layer 1.03 metres thick with a resistivity of 16,486 ohm-metres over a deep layer with resistivity 186 ohm-metres.

Ground resistance cannot be readily measured using direct current because the current will usually cause electrolytic polarization in the soil that will affect the readings. Just the differences in acidity of the soil and oxidation of metal on the test probes will give erroneous potential readings that will appear before the test current is applied. Therefore some kind of alternating test current is usually used.


The rest of this page is currently under construction.

The figure below shows the ground resistance test configuration.

[Diagram of three and four terminal measurement configuration to be inserted]

Three Terminal Testers

These are mainly used for ground resistance measurement. They use a common terminal for connection to the ground grid and have separate terminals for current injection and potential reference.

Four Terminal Testers

These are the most widely used type of grounding tester. Older instruments date back to the 1950's. They used a battery operated mechanical vibrator or hand-cranked generator to develop the test current. The resistance was usually measured by balancing a bridge circuit. Newer instruments use some form of battery operated constant current generator and automatically calculate the resistance. Some incorporate pilot lights to warn of excessive noise levels or inadequate (too high) probe contact resistance. Others contain a microprocessor that automatically carries out tests to verify the adequacy of the test configuration and to switch current ranges and other parameters for the best measurement.

Clamp-on Testers

These are high frequency devices. Cannot be used on circuits with ac inductance. Require a low resistance ground reference.

Effect of Inductance of Ground Impedance Under Test

This is often not properly understood.

Effect of Test Lead Impedance

This is often not properly understood.