Earthing - Basics, Methods, Requirements as per I.S.I
What is Earthing?
Earthing is the process of making metallic connection of the body to the earth (which is at zero potential) for the safety of the human body from shocks.
Generally all metallic covers of machines, starter or sheathing of wiring are electrically dead.
Due to failure of insulation or bad workmanship, they can become alive.
If we touch such faulty parts of machine or installation, we will receive a serious shock.
To avoid this, all metallic covers and frames of machines has to be earthed.
An earthing system should have very low resistance and easily allow the leakage current through it.
Objectives of Earthing:
Earthing is should be done to achieve the following objectives:
- To save human life from danger or shock or death by blowing fuse of any apparatus which becomes leaky.
- To protect large building from atmospheric lightning.
- To protect all machines fed from overhead lines from lightning arrestors.
- To maintain the line voltage constant ( since neutral of every alternator, transformer is earthed).
The general rules for earthing are given below:
- The earthing must be continuous by a single solid conductor in a house wiring system.
- All metallic covering of the main switch, distribution box, electrical equipment, ceiling fans etc. along with the earth of the walls socket should be earthed.
- The resistance of the earth continuity conductor should not be more than 1 Ω though out the system.
- All medium voltage machine metallic covers should be earthed by two separate earth connections.
- The earth resistance of the electrode should not exceed 3 ohm for ordinary soil. The earth resistance of the electrode should not exceed 8 ohm for rocky soil.
Methods of Earthing:
The various methods of earthing are:
- Strip or wire earthing
- Earthing through water mains
- Rod earthing
- Pipe earthing
- Plate earthing
Required Specifications for earthing as per I.S.I
For providing good earthing the following are recommended specifications as per ISI
- The earthing electrode should be situated at a place atleast 1.5 meters away from the building (outside) whose installation system is being earthed.
- The earth wire should be of same material as that of earth electrode used.
- The minimum sectional area of earth lead wired should not be less than 0.02 sq. inch and not more than 0.1 sq.inch.
- As a general rule, the size of earth conductor should not be less than half of the section of the line conductor.
- The earth wire should be taken through G.I pipe of 1mm diameter for at least 32cm length above and below ground surface to the earth electrode to safeguard against mechanical wear and tear.
- For effective earthing, loose earth and coal salt mixture should be filled around the earth electrode .
- All the joints in the earth wire should be firmly done with nuts and bolts of the same material as of earth wire.
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