Earthing is a system through which part of the electrical system is connected to ground.
Earthing system tells the electrical potential of the conductors in comparison to the conductive surface of earth.
For the safety of electrical equipment and for human earthing system plays an important rule.
Good Earthing must have low impedance enough to ensure that sufficient current can flow through the safety device so that it disconnects the supply as early as possible Fault current is much more than the full load current of the circuit which melts the fuse.
Hence, the appliance is disconnected automatically from the supply mains. Earthing is a technique of making a low resistance connection to the earth.
TYPES OF EARTHING
There are two types of earthing.
1-wiring system earthing.
DESIGN OF EARTHING SYSTEM
The earth electrode voltages are the very important factor for the design of earthing system. The voltage between earth electrodes or earthing system to which current is flowing and the reference earth (zero potential) is called the earth electrode voltages.
The resistance between consumer earthing terminals should not exceed to 1 ohm.
Earth electrode voltage= earth electrode current * earth electrode resistance
SIZE OF EARTH CONTINUITY CONDUCTORS
The size of earth continuity conductor depends on the size of cable that is to be protected.
The earth continuity conductor area should not be less than half of largest current carrying conductor supplying on the system
TYPES OF SOIL
Another important factor is the resistivity of the soil. So, it needs to be measured before the earthing system in installed. Smaller the resistivity lowers the earth electrode resistance.
The earth electrodes should be in touch with the soil.
INSTALLATION OF EARTHING PLATES
The purpose of electrode is to provide good continuity with the earth at all time. The best method to maintain the continuity is the installation of plates at well below the level of water. This plate should be installed in the upright position in the ground and it should be surrounded by at least 1 foot of the mixture of charcoal and lime this leads to increment of the continuity.
In some situation coal can also be used but it is not a convenient method so, it should be used where charcoal is not present. We cannot use coke because it has a corrosive action.
Sometimes salt is also mixed with the mixture of charcoal and lime, but again it is not a good method because it also have a corrosive property. Although, salt increase the continuity, but for a particular time.
Normally, the center of the plate should be 10ft below the surface of the earth and at least 1ft below the water level. The depth of the plate should also depend upon the local conditions.
PRACTICE OF EARTHING
For the practice of the earth following points need to be considered.
The area of plate should be less than 2ft*2ft and, so the diameter should be ¼ inches for GI pipe and 1/8 inches for the copper plate.
For lower resistance, buried 2 plates inside the earth keeping distance 10ft between them.
If water level too far below the surface of the earth then plates are placed at 10ft down inside the earth. For this 30mm diameter pipe is provided so as to inject the water as per requirement for the improvement of earth resistance.
Earthing lead should be connected to the earthing electrodes by means of nuts and bolts.
STANDARD EARTHING RULES
Earthing rules are follows.
All exposed noncurrent carrying metallic parts of the electric equipment, switch gear, transformer neutral should be efficiently earthed.
The earthing of individual points shall be done as specified exclusively and independently of the substation earthing.
For earthing of substation, two separate and distinct shall be provided with the copper plate earthing electrode for earthing of HT and LT equipment as neutral point of the transformer.
There shall be no joint in the earthing lead between the earthing lead and the earthing block.
The earth other than the substation earth shall be consist of 2ft2ft1/8 inch copper plate as specified and buried in the ground at a depth of 15 feet or more according to the moisture.
The shortest route to the earth electrode shall be adopted sharp bends and joint shall in all cases be avoided