Reasons For Earthing:There are four basic
reasons for earthing an electrical system:REDUCTION OF SHOCK HAZARDAn
earthed chassis will provide a low impedance path to ground for any
active voltage that appears on the case or metalwork.SAFE DISCONNECTION
OF SUPPLYIf the exposed metalwork of an appliance were connected by a
fault condition to an active conductor, the active current path needs to
be able to flow to ground via a low impedance path so an excessive
current will flow that will cause a protective device such as a fuse or
circuit breaker to open circuit, thus prenting an electric shock
situation.MINIMISE THE DANGER OF FIRE HAZARDThe earth wire should always
be in good contact with all metal parts of any appliance the user may
touch. Should the equipment become faulty by the active wire touching
the case, then current will rush to earth, smoke will rise from the
equipment, and if the current is very high the fuse in the active line
will blow, disconnecting mains power from the faulty appliance. Without
the earth wire, this fault would go undetected, allowing high current to
flow through the patient whenever the patient formed a circuit to
ground. Standards limiting current in any circuit are vital to prevent
excessive heating of the power supply wires leading to fires.VOLTAGE
STABILITYNeutral earthing : Star point of Wye connected transformers and
alternator windings are earthed primarily to provide voltage stability
to system on account of imbalance in loading resulting in neutral
current flow. Neutral point allows vector sum of three phase currents
displaced at 120 degrees nullifying it to zero. Secondary purpose served
is detection of earth faults or earth leakages and allow the protective
gear to operate in desired time.In the view of high harmonic contents
found in modern facilities, the zero sequence or triplen harmonic
currents overload the neutrals if not adequately sized. This is because
these are in phase and hence do not cancel out at neutral point. So
neutrals are required to be sized equal to or sometimes double the size
of line conductors to carry “algebraic sum” of triplen components in the
lines. Neutral earthing for applicable to low voltage distribution
systems upto 1000 V ac comprise of solid earthing with no intentional
resistance connected in the path. Visit following
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