Earthing or Grounding
Grounding is the process of creating an alternative(different) path for the flow of fault(excessive) currents safely into the ground in the presence of minimal resistance or impedance.
It connects non-current carrying parts of an equipment to the general mass of earth in such a manner as to ensure, at all times, an immediate discharge of energy without danger.
Classification of Earthing
- Equipment earthing
- Neutral earthing
1. Equipment Earthing
It means connecting the non-current carrying metallic parts in the neighbourhood of electrical circuits to earth. The non-current carrying parts include the following
motor body, tower, poles, transformer tank, sheet, and cables, etc.
2. Neutral Earthing
The neutral point of generator, transformer, transmission, and distribution system or circuit, rotating machine, etc. is connected to earth either directly or through a resistance or a reactance.
Objects of Earthing
- It provides protection to personnel and equipment by ensuring operation of the protective control gear and isolation of the faulty circuit.
- to ensure that all exposed conductive parts do not reach a dangerous potential.
- Maintain the voltage at any part of an electrical system at a known value so as to prevent over current or excessive voltage on the appliances or equipment.
Step Voltage
A person walking on the surface of the ground will experience a voltage between his/her feet. This voltage will generate electric body currents. In this case, the voltage to which the person is subjected is called step voltage.
Touch Voltage
A person touching a structure which has a potential that is different from that of the point of ground at which the person is standing. In this case, the person is subjected to a voltage that will generate an electric current through his or her body. The voltage to which the human body is subjected is called the touch voltage.
The magnitude of these voltages depends on the following factors:
- The value of fault current
- Upper and lower resistivity of the soil
- Area of the grid
Exercises
Q. What is meant by earth continuity conductor?
Ans. The conductor, by means of which the metal body of an or an appliance is connected to the earth, is know as earth continuity conductor (ECC).
Q. What are maximum permissible values of resistance of system for (i) large power stations (ii) major power station (iii) small substations (iv) in all other cases?
Ans. Maximum permissible values bf resistance of earth system as follows :
Large power stations – 0.5 ohm
Major power stations – 1.0 ohm
Small substations – 2.0 ohm
In all other cases – 5 ohm maximum.
Q. What should be the minimum size of earth continuity conductor?
Ans. The cross-section of earth continuity conductor should not be either less than 2.9 mm2 (14 SWG) or half of the installation conductor size.
Q. What is harm if earth resistance exceeds the permissible value of earth resistance?
Ans. The sensitivity of the protective equipment, system voltage, and the maximum fault current directly relate to permissible valued earth resistance. In case the earth resistance exceeds the permissible value, then in the event of an earth fault, the fault current may not reach a sufficient value to operate the protective equipment (such as fuses or relays) and dangerous condition may arise.
Q. Why charcoal and salt is used in earthing?
Ans. Generally, alternate layers of charcoal and salt are used to increase the effective area of the earth and to reduce the earth resistance respectively.
Q. Do you know how to measure the earth resistance?
Ans. Actually, the Earth resistance is measured using “earth resistance meter”. You might have also heard that using megger, but megger is used only for high resistance measurement i.e., in Megaohms but when considering upon earth resistance it should not exceed 1ohm so earth resistance meter is required.
In this meter, there will be four terminals(E, P1, P2, P3) in which E and P1 will be shorted and connected to the grid or electrode and P2 and P3 are reference electrodes which will be kept on the earth surface at two different locations for taking the reference resistance of the earth.
The meter shows the resistance of the earth. But it is necessary that it should be below 1ohm.
Q. How to maintain the earthing resistance?
Ans. Earthing Resistance can be maintained by adding water, charcoal & salt per month, Not only by adding those three, Its rusted bolt & Nuts must be renewed by a new one for every three months, this maintenance should be varied depending upon the climatic conditions, such as in rainy season.Earth pit must be fenced with cement.
Q. What is the difference between grounding and earthing?
Ans. In America, Canada etc. word Grounding is used ( GND ) and in the rest part of the world including India, word earthing is preferred.
Generally, both are same but in detail when we connect any equipment (i.e. motor, mixture, washing machine or any household equipment) with the earth through earth wire its called earthing and when we earth any system like star point of generator or transformer is called Grounding.
