When the human body comes into contact with the casing of electrical equipment that is accidentally charged, the current will flow in along the two parallel branches of the grounding device and the human body respectively. The resistance of human body is generally greater than 1000 ohm, and the resistance of grounding body cannot be greater than 4 ohm according to regulations, so the current flowing through human body is very small, and the current flowing into grounding device will be very large, thus avoiding the occurrence of electric equipment leakage accidents.
Grounding resistance refers to the resistance value that current flows into the earth through grounding device, and then flows to another grounding body through the earth or spreads to a distance.
The measurement methods of grounding resistance are: voltage ammeter method, ratio table method and bridge method.
Technical specifications for grounding resistance test are as follows:
When measuring, the grounding device should be disconnected from the grounding shell of the equipment.
The current electrode and voltage electrode are generally 30 to 50 cm long and should be driven into wet soil.
The arrangement of current electrode and voltage electrode should be far away from the grounding grid, and do not drive into the grounding grid.
Testing immediately after rain should be avoided.