EARTHING

Objective of Earthing Connecting neutral point of the supply system or non current carrying metal parts used in electrical distribution system to the general mass of earth by wire of negligible resistance . To ensure of immediate discharge of energy without danger. Prime objective of earthing is to provide a zero potential surface in and around and under the area where the electrical equipment is installed or erected. The current always passes through the path having low resistance. If earth wire is not provided fault current passes through human body.

Necessity of Earthing 3

Necessity Of Good 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      

Qualities of good earthing The quality of earthing you need depends upon your application The qualities of a good earthing system are : Must be of low electrical resistance Must be of good corrosion resistance Must be able to dissipate high fault current repeatedly  .

IS specifications regarding earthing An earthing electrode shall not be situated within a distance of 1.5mts from the building whose installation is being earthed. The crossection of earth continuity conductor should not be less than 14 SWG. Resistance to earth: It should be low enough to cause of flow of leakage current to make the protective devices to operate.

Contd… The value of resistance to earth changes with weather. As per IS standards the max permissible values of earth resistances for various places are, For large power station- 0.5 Ω Major power station-1.0 Ω Small substation- 2.0 Ω In all other cases- 5.0 Ω

Contd… The value of resistance to earth changes with weather. As per IS standards the max permissible values of earth resistances for various places are, For large power station- 0.5 Ω Major power station-1.0 Ω Small substation- 2.0 Ω In all other cases- 5.0 Ω

Factors influencing earth resistance The resistance of earth system depends on Condition of soil Temperature of soil Moisture content of soil Size and spacing of earth electrodes Depth at which the electrode is embedded Material of conductor Quality of coal, dust and charcoal in the earth electrode pit.

PIPE EARTHING A galvanised steel and perforated pipe of 2.5mts(ordinary soil) and 40mm dia is placed upright in a permanently wet soil. The pipe is tapered with the tapered casting at the bottom. Charcoal and salt are filled in the pit alternately of 15cm layers for a distance of about 2mts. The pipe has 12mm dia holes drilled in it to facilitate spreading of dampness and thereby decreasing soil resistivity.

Contd.. At the top cement concrete work is done to protect the pipe from mechanical damage. A funnel with wire mesh is provided with the concrete work. The funnel is connected to another pipe which is connected to main earth GI pipe. Another GI pipe is taken from funnel to outside to connect to the earth wire of not less than 8 SWG( small installations) should be used for earthing. It should be carried in a pipe of 12.7mm dia at a distance of 60 cm below the ground

PLATE EARTHING An earthing plate of Cu of dimensions 60 cm x 60 cm x 3 mm and GI of dimensions 60 cm x 60 cm x 6 mm is buried in the ground with its face vertical at a depth of not less than 3 mts from the ground level The earth plate is embedded in alternate layers of coke and salt for a thickness of 15 cm. The electrode or plate is connected to GI pipe of 12.7mm dia for connecting the earth wire to the earthplate. The earthwire is bolted on the Earth plate with bolt and nut of the same material of that of earth plate. The pit filled with charcoal and salt is connected with a pipe for carrying water fro the concrete work to the pit where earth plate is kept to increase the dampness