Ground Resistance Testing
The term "ground" is defined as a conducting connection by which a circuit or equipment is connected to the earth. The connection is used to establish and maintain, as closely as possible, the potential of the earth on the circuit or equipment connected to it. A "ground" consists of a grounding conductor, a bonding connector, its grounding electrodes, and the soil in contact with the electrode.
Grounds have several protection applications. For natural phenomena such as lighting, grounds are used to discharge the system of current before personnel can be injured or system components damaged. For foreign potentials due to faults in electric power systems with ground returns, grounds help ensure rapid protection by providing low resistance fault paths. This provides the removal of the foreign potential before personnel are injured and the power or communications system is damaged.
Ideally, to maintain a reference potential for instrument safety, protect against static electricity, and limit the system to frame voltage for operator safety, a ground resistance should be zero ohms. In reality this value cannot be obtained.
The ground resistance value for maintenance purposes should preferably not exceed 15 ohms when installed and should be inspected by qualified personnel once every two years in dry weather only. Results of inspections and tests shall be recorded and available upon an inspector's request.
All ground rods should be 5/8 of an inch (15.875 mm) or larger in diameter, circular, solid steel rods with a copper cladding, each 10 feet (3 metres) long. Rods should be plainly and permanently marked with manufacturer's name, rod length and nominal rod diameter. Length markings should be within the top 12 inches of the rod to permit determining length after installation. Connectors can be of the compression, impact or exothermic welded connection and made of a low resistance copper alloy containing not less than 80% copper. To prevent a tripping hazard, ground rods should be buried below ground level and covered with ground electrode boxes for easy access for inspection.
The most reliable post-installation testing procedure involves the fall-of-potential (three-point) method. With the help of a digital ground resistance meter, two auxiliary electrodes are driven into the soil at predetermined distances - as per testing specifications - in a straight line from the ground rod under test. The meter supplies a constant current between the ground rod being tested and the most remote electrode. The Three Point Vibroground, with a range of at least 0 to 300 ohms, shall be used to perform these tests. Manufacturer's instructions for using this instrument must be followed.
Clamp On Testers
The use of a ground resistance clamp-on meter provides an alternative method of verification testing and should not be used in our application. The meter uses the connection between site ground and the utility neutral - exactly the opposite of the fall-of-potential test.
Care must be exercised at any time when connecting or disconnecting a made ground at any part of a grounding system. Discharge or leakage through surge protectors, induced or lighting surges, or apparatus connected to the ac supply could cause the voltage on the disconnected part of the grounding system to reach dangerous levels.
There are various brand names of ground resistance testers which can be found on the web by searching "Ground Resistance Meters".