Classification of Equipment Grounding
Earthing refers to the connection between neutral points of power system and electrical devices, exposed conductive part of electrical equipment and outer conductive part of equipment. Equipment grounding
can be divided into working grounding, lightning protection grounding and protective grounding.
Protective grounding is designed to prevent equipment from being charged due to insulation damage which endangers personal safety, such as metal enclosures, reinforced concrete poles and metal poles and towers of power equipment. Protective grounding can only have current when the insulation of the equipment is damaged, and its value can be changed in a large range.
Function of Equipment Grounding
1. Preventing Persons from Being Shocked by Electric Shocks
In order to protect the safety of human body, a good metal connection is made between the non-electrified metal part of electrical equipment and the grounding electrode under normal conditions. For electrical equipment with grounding devices, when insulation is damaged and the housing is charged, the grounding current will flow along both the grounding electrode and the human body.
The current flowing through each path will be inversely proportional to its resistance. The smaller the resistance of the grounding pole, the smaller the current flowing through the human body. When the grounding resistance is very small, the current flowing through the human body tends to be zero. The human body therefore avoids the danger of electric shock. Therefore, in any case, the grounding resistance should be guaranteed not to exceed the grounding resistance specified in the design or regulation.
2.Guarantee the Normal Operation of Electrical System
The grounding of power system is generally neutral point grounding, so the potential between neutral point and ground is close to zero. When the phase line touches the shell or grounding, the other two relative ground voltages will rise to twice the phase voltage in the neutral point insulation system. In neutral grounded systems, it is close to the phase voltage. Because of the neutral grounding wire, the reliability of relay protection can be guaranteed.
DC power supply in communication system usually adopts positive grounding, which can prevent noise from entering and ensure the normal operation of communication equipment.