In this topic, you study Equipment Earthing – Purpose, Theory & Advantages.
The earthing of equipment consists of connecting to ground or earth the metallic frames of electrical apparatus, generator frames, motor bodies, transformer bodies, metal conduits, switch boxes etc. By earthing we reduce the resistance of the path which the fault current has to travel. Earth resistance varies from time to time depending upon the moisture content of soil. Lower is the value of earth resistance, the better protection it gives. Earth resistance should be measured with a megger at least once in a year. If the earth resistance is found to be high, the soil around the electrode is dug, rust is removed from the surface of earth plate and earth pipe is cleaned, coke or charcoal is added and common salt dissolved in water is sprinkled upon it. In case earth resistance is found to be still high, more electrodes will have to be provided in parallel to reduce the earth resistance. Generally, the values of earth resistance given in Table 1 must be achieved while earthing.
Advantages of Equipment Earthing
- Safety of operating personnel.
- Safety of equipment from damage.
- Proper operation of protective equipment which will avoid system breakdown.
- Reduction in repair and maintenance cost.
Note: In the case of household appliances we use a three pin plug. The third additional pin, which is bigger in diameter, is provided for connection to earth so that if the enclosure becomes alive it will still be at earth or zero potential only and the fault current will flow to earth directly rather than through the body to the operator.