The earthing is of great importance because it protects both the equipment as wells as the persons using them. As the name implies the earthing means that the equipments or installations connected to the earth. The earthing is also called grounding. The resistance of the earth is very low.
So when the equipment or installation develops some fault then the current will take the path through the earth and the equipment will remain safe. Therefore all power supply systems, consumer installations and electrical appliances are grounded.
Purpose of earthing
- To save human life from danger of electrical shock or death by blowing a fuse i.e. To provide an alternative path for the fault current to flow so that it will not endanger the user
- To protect buildings, machinery & appliances under fault conditions ie. To ensure that all exposed conductive parts do not reach a dangerous potential.
- To provide safe path to dissipate lightning and short circuit currents.
- To provide stable platform for operation of sensitive electronic equipments i.e. To 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 .
- To provide protection against static electricity from friction
The features of good earthing system
- Must be of low electrical resistance
- Must be of good corrosion resistance
- Must be able to dissipate high fault current repeatedly
Types of Earthing Systems
- Conventional Earthing
- Maintenance Free Earthing
- The Conventional system of Earthing calls for digging of a large pit into which a GI pipe or a copper plate is positioned in the middle layers of charcoal and salt.
- It requires maintenance and pouring of water at regular interval.
Maintenance Free Earthing
- It is a new type of earthing system which is Readymade, standardized and scientifically developed.
- No need to pour water at regular interval- except in sandy soil.
- Maintain stable and consistent earth resistance around the year.
- The conductive compound creates a conductive zone, which provides the increased surface area for peak current dissipation. And also get stable reference point.
- Highly conductive. Carries high peak current repeatedly.
Types of Maintenance Free Earthing
1. Plate Earthing
2. Pipe Earthing
3. Rod Earthing
4. Strip Earthing
In this type of earthing plate either of copper or of G.I. is buried into the ground at a depth of not less than 3 meter from the ground level. The earth plate is embedded in alternative layer of coke and salts for a minimum thickness of about 15cm. The earth wire(copper wire for copper plate earthing and G.I. wire for G.I. plate earthing) is securely bolted to an earth plate with the help of bolt nut and washer made of copper, in case of copper plate earthing and of G.I. in case of G.I. plate earthing.
Pipe earthing is best form of earthing and it is cheap also in this system of earthing a GI pipe of 38 mm dia and 2meters length is embedded vertically in ground to work as earth electrod but the depth depend upon the soil conditions, there is no hard and fast rule for this. But the wire is embedded upto the wet soil. The earth wire are fastened to the top section of the pipe with nut and bolts. The pit area arround the GI pipe filled with salt and coal mixture for improving the soil conditions and efficiency of the earthing system. It can take heavy leakage current for the same electrode size in comparison to plate earthing. The earth wire connection with GI pipes being above the ground level can be checked for carrying out continuity test as and when desired, while in plate earthing it is difficult. In summmer season to have an effective earthing three or four bucket of water is put through the funnel for better continuity of earthing.
In this system of earthing 12.5mm diameter solid rods of copper 16mm diameter solid rod of GI or steel or hollow section of 25mm GI pipe of length not less than 3 meters are driven vertically into the earth. In order to increase the embeded length of electrod under the ground, which is some time necessary to reduce the earth resistance to desired value more than one rod section are hammered one above the other. This system of earthing is suitable for area which are sandy in character . This system of earthing is very cheap.
Strip or Wire Earthing
In this system of earthing 12.5mm diameter solid rods of copper 16mm diameter solid rod of GI or steel or hollow section of 25mm GI pipe of length not less than 3 meters are driven vertically into the earth. In order to increase the embeded length of electrod under the ground, which is some time necessary to reduce the earth resistance to desired value more than one rod section are hammered one above the other. This system of earthing is suitable for area which are sandy in character . This system of earthing is also very cheap.
Testing of Earthing System by 3 point test
The 3-point or fall-of-potential method is used to measure the resistance-to-ground of existing grounding systems. The two primary requirements to successfully complete this test are the ability to isolate the grounding system from the utility neutral and knowledge of the diagonal length of the grounding system (i.e. a 10’ x 10’ grounding ring would have a 14’ diagonal length). In this test, a short probe, referred to as probe Z, is driven into the earth at a distance of ten times (10X) the diagonal length of the grounding system (rod X). A second probe (Y) is placed in-line at a distance from rod X equal to the diagonal length of the grounding system.