Vertically driven conductors called Earthing Rods are commonly used in earthing existing buildings and for improving or reducing electrical resistance of existing earth electrodes. See common terms used in earthing to gain understanding of earthing rod and earth electrode.
Characteristics of Earthing Rods
The rods used for earthing buildings may be:
(a) Copper or more commonly Copper clad steel. Copper clad steel are generally 1 or 2 meters long and provided with screwed ends and sockets in order to reach
considerable depths, if
necessary (for instance, the water-table level in areas of high soil resistivity).
(b) Galvanized steel pipe greater than or equal to 25mm in diameter or rod greater than or equal to 15mm in diameter with lengths greater than or equal to 2 meters long. Note however that where galvanized conducting materials are used for earth electrodes, sacrificial cathodic protection anodes may be necessary to avoid rapid corrosion of the electrodes where the soil is aggressive.
When earthing is done by rods, It is often necessary to use more than one rod as shown above, in which case the spacing between them should exceed the depth to which they are driven, by a factor of 2 to 3.
The total resistance (in homogeneous soil) is then equal to the resistance of one rod, divided by the number of rods in question. The approximate resistance R obtained is given by:
Use of Vertical Plates
Rectangular plates, each side of which must be greater or equal to 50cm, are commonly used as earth electrodes, being buried in a vertical plane such that the center of the plate is at least 1 meter below the surface of the soil.
The plates may be:
(a) Copper of 2mm thickness or
(b) Galvanized steel of 3mm thickness
The approximate resistance, R, in ohms is given by:
Where:
L = the perimeter of the plate in meters
? = resistivity of the soil in ohm-meters.
Characteristics of Earthing Rods
The rods used for earthing buildings may be:
(a) Copper or more commonly Copper clad steel. Copper clad steel are generally 1 or 2 meters long and provided with screwed ends and sockets in order to reach
considerable depths, if
necessary (for instance, the water-table level in areas of high soil resistivity).
(b) Galvanized steel pipe greater than or equal to 25mm in diameter or rod greater than or equal to 15mm in diameter with lengths greater than or equal to 2 meters long. Note however that where galvanized conducting materials are used for earth electrodes, sacrificial cathodic protection anodes may be necessary to avoid rapid corrosion of the electrodes where the soil is aggressive.
When earthing is done by rods, It is often necessary to use more than one rod as shown above, in which case the spacing between them should exceed the depth to which they are driven, by a factor of 2 to 3.
The total resistance (in homogeneous soil) is then equal to the resistance of one rod, divided by the number of rods in question. The approximate resistance R obtained is given by:
Where:
L = the length of the rod in meters
? = resistivity of the soil in ohm-meters.
n = the number of rods
L = the length of the rod in meters
? = resistivity of the soil in ohm-meters.
n = the number of rods
Use of Vertical Plates
Rectangular plates, each side of which must be greater or equal to 50cm, are commonly used as earth electrodes, being buried in a vertical plane such that the center of the plate is at least 1 meter below the surface of the soil.
The plates may be:
(a) Copper of 2mm thickness or
(b) Galvanized steel of 3mm thickness
The approximate resistance, R, in ohms is given by:
Where:
L = the perimeter of the plate in meters
? = resistivity of the soil in ohm-meters.
Mr. Muhammad Saqib, M.Sc. (PAK), Research Fellow (PAK), a member of IEEE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from University Of Engineering And Technology Taxila, PAKISTAN.
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