Earthing and its types

What is earthing?

As per IEEE 80-2000, the process of transferring the immediate discharge of electrical energy (fault current) directly to the earth with the help of the low resistance path/wire is known as earthing.

Types of electrical earthing

Neutral Earthing

• With the help of GI wire, the neutral is directly connected to the earth in  neutral earthing. It is also referred to as “system earthing.” Most systems with star windings have this type of earthing. An example of the same could be motors, transformers, and generators that have neutral earthing.

Equipment Earthing

• This type of earthing is done to the electrical equipment The non-current-carrying part of the equipment, like the metallic   frame/body of electrical equipment, is connected to the earth by the help of the conducting wire. If any fault occurs in the apparatus, the short circuit current is passed to the earth.

What is Earth Resistance?

The resistance offered by the earth to the flow of fault current is called earth resistance. It should ideally be ZERO, but the NFPA and IEEE bodies have recommended a value of less than 5 Ohms

The Goal of Earthing

• To ensure the safety of living beings around the vicinity of the substation

• To ensure there is a low resistance path for fault current,

• To ensure there is a proper discharge path for lightning  (e.g., a lightning arrester connected to the substation).
• Proper functioning of the system under fault conditions can improve system performance.
• Equipment failures are mitigated by increased equipment life.

Substation earthing system

• An earthing (or) grid, earthing electrodes, earthing conductors, and earth connections comprise the earthing system.

Earth Mat or Grid

The primary requirement of earthing is to possess a really low earth resistance. Individual electrodes have a fair resistance, but if individual electrodes are connected together and linked then resistance parallel path is created so overall value of earth resistance decreases.

Maximum permissible resistance of earthing system for substation

Large power sub-station : 0.5 ohms

Major sub-station : 1.0 ohms

Small sub-station : 2.0 ohms

In all other cases : 5.0 ohms

Ideally a ground should be of Zero ohms’ resistance. However, the National Fire Protection Association (NFPA) and Institute of Electrical and Electronics Engineers (IEEE) have recommended a ground resistance value of 5.0 ohms or less. The earthing resistance of a power station should not be greater than 0.5-Ohm because a power station is required to carry high earth currents.

Step and Touch Potential

Step Potential

Step potential is the voltage between the two feet of a person standing near an energised grounded object. To escape step potential hazards; it is recommended using very short steps, or move away from the electrical fault keeping both feet close together. Employees in danger should avoid taking large steps.

Touch Potential

It is the potential within a charged object and the feet of the person in connection with object. To avoid touch potential, operator will need to use high insulated gloves and shoes.

Expertise shared by:

Vishwadeep Nanda,

Product Manager & Profit Centre Head – Test and Measuring Instruments,

Rishabh Instruments Pvt. Ltd.