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Wednesday, December 30, 2015

Earthing - Basics

Earthing is a connection which connects parts of the electric circuit with the ground or earth. Regulations for earthing system vary considerably among countries and among different parts of electric systems. Most low voltage systems connect one supply conductor to the earth (ground). Normally, the earth connection should be without the intervention of a fuse, switch, circuit breaker, resistor etc.

Need for earthing:

  • Stable operation of the system. 
  • Safety of the men and material. 
  • Protection against lightning. 

 Some of the important terms related to grounding:
  • Ground
A ground is a conducting connection, whether intentional or accidental, between an electrical circuit or equipment and earth or to some conducting body that serves in place of earth.
  • Grounded
Grounded means connected to earth or to some conducting body that serves in place of earth.
  • Grounded conductor
A system or circuit conductor that is intentionally grounded, such as grounded neutral conductor.
  • Grounding conductor
A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.
  • Grounded rod
A conductive metal rod driven in the ground to serve as a grounding electrode, usually 10 to 15 ft long and 1 in. In diameter.

  • Ground loop
A continuous ground electrode of wire encircling the area and connecting three or more ground rods.
  • Ground clip
A metal clip connected to the ground loop to enable one or more extensions of the ground loop to be extended.
  • Ground well
A concrete encasement around the ground rod to allow access to the rod to measure ground resistance.
  • Ground grid
A steel mesh or wire grid buried beneath the surface of an area to serve as a ground electrode.
  • Ground bus
A metal plate to which multiple ground leads are attached.

Methods of earthing:

There are two methods of earthing, namely
  1. Pipe earthing 
  2. Plate earthing 

1. Pipe earthing

Pipe earthing is done by permanently placing a pipe in wet ground. The pipe can be made of steel, galvanized iron or cast iron. Usually GI pipes having a length of 2.5m and an internal diameter of 38mm are used. The pipe should not be painted or coated with any-non-conducting material.

The pipe should be placed at least l.25m below the ground level and it should be surrounded by alternate layers of charcoal and salt for a distance of around 15 cm. This is to maintain the moisture level and to obtain lower earth resistance. The earth lead of sufficient gauge should be firmly connected to the electrode and it should be carried in a GI pipe at a depth of 60cm below the ground level.

2. Plate earthing

The plate electrode should have a minimum dimension of 600x600x3.15mm for copper plate or 600x600x6.3mm for GI plates. The plate electrode should be placed at least 1.5m below the ground level. The earth conductor is to be securely connected to the plate by means of bolts and nuts. The bolts and nuts should be of the same material as that of the plate. The earth conductor should be carried in a GI pipe buried 60 cm below the ground level. The prate electrode should be surrounded by a layer of charcoal to .reduce the earth resistance.

For more details about power system earthing refer to Earthing in electrical network – purpose, methods and measurement
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