Answer / abhinav yadava

Earthed System
 Earlier the generators and transformers were of
small capacities and hence the fault current was less. The
star point was solidly grounded. This is called earthed
system.

Unearthed System
 Today generators of 500MVA capacities are used
and therefore the fault level has increased. In case of an
earth fault, heavy current flows into the fault and this
lead to damage of generators and transformers. To reduce
the fault current, the star point is connected to earth
through a resistance. If an earth fault occurs on one
phase, the voltage of the faulty phase with respect to
earth appears across the resistance. Therefore, the voltage
of the other two healthy phases with respect to earth rises
by 1.7 times. If the insulation of these phases is not
designed for these increased voltages, they may develop
earth fault. This is called unearthed system.

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