Updated: Mar 1
What is earthing?
The symbol of electrical earthing is as given below.
It is not desired to transfer the electric current to the ground or earth in power system healthy condition. The equipment are designed to carry the rated current without failing. However, the insulation of the electrical equipment may fail after giving its useful life. When insulation of the equipment fails, the metallic enclosure attains the potential of the live phase.
If anyone touch the enclosure at the instant of fault, the person will get an electric shock. It is desired that metallic enclosure of the electrical equipment must attain the potential equal to the earth potential at the time of occurrence of the fault in order to protect the equipment and person. The fault current can be diverted to the earth by earthing the metallic enclosure or frame.The process of diverting fault current to ground or earth is called earthing. In other words, under earth fault condition, transfer of electric energy to earth through a low resistance path is called earthing.
The electrically conducting body or enclosure of the electrical equipment which does not have any electrical potential when there is no fault is connected to ground or earth through galvanized iron strip of suitable current carrying capacity. This process is known as electrical equipment earthing.
When the live part of the supply system termed as a neutral point which is at zero potential when there is no fault in the system or there is balanced current in all the three phases is connected to earth pit. This process is called the system grounding.
The earthing or grounding provides the least impedance path to fault current and the fault current quickly sinks in the ground and faulty phase potential immediately drops to zero value equal to earth potential . Thus, the earthing system protects the system, equipment and person working in the area.
Types of Electrical Earthing
The equipment and neutral point of supply system which does not carry the current during normal operation. However, at the time of the fault the equipment body or the neutral point of supply system attains the phase voltage. According to process of earthing of the frame or enclosure of the electrical equipment and neutral point of the supply system, the earthing is classified into two categories.
The neutral earthing is provided in the equipment which has star point. The neutral of transformer and generator which have neutral point is directly connected to the earth through GI strip or GI wire. In normal operation the neutral point is at zero potential and no current flows from neutral point to earth.
However, in the case of fault, the fault current flows from neutral point to earth.This type of earthing is called grounding or system grounding. The neutral earthing can be of solid earthing,resistance earthing and reactance earthing.
The electrical equipment metallic frame or metallic enclosure is connected to the earth through a conducting GI strip or GI wire.
In two points earthing, two distinct points of the frame is connected to two separate earth pit in order to ensure perfect earthing. In case of fault, the current flows from the frame of the equipment to earth and thus protect the equipment, power system and person.
Why Earthing Important?
The earthing is very important part of electrical installation. The earthing must be done as per the electricity rules. The earthing system must be checked regularly to ensure correctness and readiness of the earthing system. The earthing is of the paramount importance because of the following reasons.
The earthing protects the personnel from an electric shock.
The earthing protects the equipment from high voltage caused by fault, transients or lightning because the fault current passes through the least impedance path of earthing system and potential fall to zero.
The earthing minimize the risk of fire.
Thus as per the electricity law, every electrical equipment frame or enclosure must be earthed from two points and there should be two separate earth pits. Also, for system grounding there must be separate earthing circuit. The GI strip or wire connected to neutral point must run on the insulators and must be directly connected to earth pit.
How Earthing Protects person from an electric Shock?
If the metallic enclosure of the electrical equipment is not earthed, the enclosure attain the live potential if fault occurs in the equipment due to insulation failure. Now, if a person touch the enclosure his body will attain the live potential and the current completes the path through his body. The factors like amplitude,duration and frequency of current decides the severity of shock. Magnitude of current flowing through the body cause following effects on human body.
A 30mA current passing through the human body can cause electrocution.Therefore, Eearth Leakage circuit breaker is used to isolate the faulty circuit if the leakage current is 30mA.
Now if the equipment frame or enclosure is connected to earth, the live potential attained by metallic enclosure will immediately drops to zero because the metallic frame now is at zero potential. Thus, when equipment is earthed, the fault current does not flow through the human body and the fault current directly sinks in the earth.