In power systems, grounding of electrical equipment is intended to protect personnel and equipment and to ensure correct operation. Ground resistance is measured using a ground resistance tester, also called an earth resistance test set. It is mainly used to measure the resistance of equipment grounding systems and lightning protection grounding. Because its shape resembles a megger, it is sometimes colloquially called a ground "shake" meter.
Minimum Acceptable Ground Resistance Values
In power systems, the resistance of functional grounding should not exceed 4 Ω. Protective grounding should not exceed 4 Ω. Repeated grounding should not exceed 10 Ω. For lightning protection, an independent lightning rod should not exceed 10 Ω. Station-type lightning arresters in substations should not exceed 5 Ω.
Structure of the Ground Resistance Tester
The ground resistance tester is a portable instrument specifically for measuring ground resistance; it can also measure small resistances and soil resistivity. The tester mainly consists of a hand-cranked AC generator, a current transformer, a potentiometer, and a galvanometer.
Principle: the hand-cranked AC generator is turned by the handle. The output current I flows through the primary of the current transformer TA → grounding electrode E → earth → current probe C' → back to the generator, forming a closed loop. When current I enters the earth at the grounding electrode E, it disperses outward through the soil.
The farther from the grounding electrode, the larger the cross-sectional area through which the current flows, and the lower the current density.
It is generally considered that at about 20 m the current density is effectively zero and the potential is zero, reaching the reference earth potential used in electrical engineering.
When current I flows through the tested ground resistance Rx, a voltage drop I·Rx is produced; a voltage drop I·Rc is also produced across Rc. The value of the tested ground resistance Rx can be determined by the current transformer ratio K and the potentiometer resistance RS, and is independent of RC.
Using the Ground Resistance Tester
- Disconnect the main grounding conductor from the grounding electrode under test.
- Make the test instrument connections.
- Place the instrument level and check that the galvanometer pointer rests on the center line.
- Verify all wiring is correct.
- Set the range switch to the maximum range. Slowly crank the generator handle while turning the measurement dial to bring the galvanometer pointer to the center line. When the galvanometer approaches balance, increase the cranking speed to the rated speed of 120 r/min and adjust the measurement dial so that the galvanometer pointer stabilizes at the center line. Read the value of RS at this point.
- After each measurement, withdraw the probes, wipe them clean, and store the leads neatly for next use. Store the instrument in a dry, shaded, vibration-free place. Handle the instrument carefully during transport and use to avoid damage to the jewel bearings and to prevent erroneous indication due to vibration.
Precautions
- Before measurement, insert the two test probes into the soil: the test grounding electrode E, the potential probe P, and the current probe C should be in one straight line and 20 m apart. Insert P between E and C. Connect E, P, and C to the corresponding terminals on the instrument using dedicated leads.
- Preparation before measurement: disconnect the equipment under test from the grounding system. Place the instrument level and perform mechanical zero adjustment.
- Typical probe placement for a hand-cranked ground tester: insert the current probe C into the soil about 40 m from the tested electrode E at a depth of about 40 cm. Insert the potential probe P midway between E and C. Ensure E, P, and C are collinear and spaced 20 m apart. Finally, connect E to the instrument E terminal, P to the P terminal, and C to the C terminal.
