• Raj

Why double earthing for 3-phase electrical machines and equipment?

Updated: Dec 21, 2021

There are two types of earthing used in electrical system. The equipment frame earthing and system earthing. In equipment earthing the non current carrying metallic part of the electrical enclosure is connected to earth.

The conducting part which remains at zero potential in normal condition attains phase voltage when the insulation of the electrical panel fails. In this condition, the panel body attains phase voltage. If earthing is not done, and if any body touches the frame at the time of fault one may get an electric shock. The purpose of earthing is to divert the current to earth so as to maintain the metallic frame potential to zero. The earth has very low resistance and fault current starts flowing towards ground. The protection circuit senses the earth fault current and trips the supply system.

Thus, the earthing provides protection against an electric shock fire and machine protection.

The earthing resistance must be as minimum as possible. The earthing pit resistance must be minimum and the earthing strip or conducting wire used between equipment frame to earth pit should have very low resistance.

In similar way, the neutral point of supply system is at zero potential under normal operating condition. However, in faulty condition neutral point voltages rises and the other healthy phases voltage also rise if system in ungrounded. This condition may lead to insulation failure due to high voltage. Therefore, in order to maintain the system stability, the neutral point is earthed. This is called the grounding or system grounding.

we are connecting the frame of electrical appliance to earth and also connecting neutral point to earth, then why double earthing is recommended? why not ,single point earthing is OK.

Why Double Earthing?

Two separate earthing circuit is made and connected to frame of the equipment and neutral of the supply system. Thus total four numbers earth pit is required, two numbers for frame earthing and two numbers for neutral earthing.The double earthing provides the following advantages.

2 points or double earthing of electrical equipment
  1. If there is one earth circuit, the fault current flows through the circuit. However, if the earth circuit is not closed the frame or enclosure body may attain live potential. However, if two circuit is there, one circuit will take care of diverting the current to earth if one of the circuits gets defective. Thus, the earthing system reliability gets enhanced with double earthing.

  2. In event of the fault, the current will flow in two earth circuits for double earthing scheme. The effective resistance of the circuit is always less than the resistance of one circuit because both the circuits are in parallel. If we connect 2 ohms resistance in parallel the equivalent resistance is 1 Ohm. The same property applies here. The low resistance of earth circuit cause easy flow of earth fault current to earth.

Read More : Why double earthing for electrical machines?

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