05-10-2010, 10:34 AM
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Introduction
What is Earthing
-The process of earthing is to connect all the objects which could become charged to the general mass of the earth, to provide a path for electrical current and to hold the parts as close as possible to earth potential.
-For tying the electrical supply system to earth, supply transformer neutral conductor (Y point of 3 phase Supply) is connected to earth using an earthing electrode or the metal sheath and armouring of a buried conductor.
Why Earthing required
-The purpose of grounding system is to provide a low impedance electrical contact between the neutral of an electrical system and earth
-1. To provide safety during normal or fault condition.
-2. To stabilize the voltage during transient conditions and therefore to minimize the probability of a flashover during transients.
-3. To dissipate lightening strokes
-Due to impedance of the system, the potential of the grounding system may become different than the potential at various points on earth during abnormal operating condition or fault condition.
-Due to this potential difference a touch and step voltages are subjected by the persons which generate an electrical current through his body. This flow of current is a source of danger.
-Electrical body current below 116/(sqrt(t) mili amps, where t is shock duration time
-Vtouch, allowable = 0.116(1.5 + 1000)/ (sqrt(t)
-Vstep, allowable = 0.116(6.0 + 1000)/ (sqrt(t)
