Answer / binod

EARTH/EARTH fault occurs when electricity travels outside
an intended path and tries to get to the ground by the
shortest route. If you touch electricity while you are
grounded (touching the ground or something resting on the
ground, like a ladder), your body becomes electricity’s
shortest route to ground. You could also become part of
electricity’s path to ground if you touch water that
touches electricity. In either case, you could be hurt or
even killed.

In three-phase systems, a fault may involve one or more
phases and ground, or may occur only between phases. In
a "ground fault" or "earth fault", charges may flows to
earth.This isan asymmetric fault, where the three phases
are not affected equally. In practice, most faults in power
systems are unbalanced. With this in mind, symmetric faults
(only 3 phases) can be viewed as somewhat of an
abstraction; however, as asymmetric faults are difficult to
analyze, analysis of asymmetric faults is built up from a
thorough understanding of symmetric faults.
An asymmetric or unbalanced fault does not affect each of
the three phases equally. Common types of asymmetric
faults, and their causes:
•line-to-line - a short circuit between lines, caused by
ionization of air, or when lines come into physical
contact, for example due to a broken insulator.
•line-to-ground - a short circuit between one line and
ground, very often caused by physical contact, for example
due to lightning or other storm damage
•double line-to-ground - two lines come into contact with
the ground (and each other), also commonly due to storm
damage.


An asymmetric fault breaks the underlying assumptions used
in three-phase power, namely that the load is balanced on
all three phases. Consequently, it is impossible to
directly use tools such as the one-line diagram, where only
one phase is considered. However, due to the linearity of
power systems, it is usual to consider the resulting
voltages and currents as a superposition of symmetrical
components, to which three-phase analysis can be applied.

BACK UP EARTH FAULT-BACK UP WORD IS USED IN PROTECTION

Backup protection of transformer has four elements, three
over current relays connected each in each phase and one
earth fault relay connected to the common point of three
over current relays as shown in the figure. The normal
range of current settings available on IDMT over current
relays is 50% to 200% and on earth fault relay 20 to 80%.

Another range of setting on earth fault relay is also
available and may be selected where the earth fault current
is restricted due to insertion of impedance in the neutral
grounding. In the case of transformer winding with neutral
earthed, unrestricted earth fault protection is obtained by
connecting an ordinary earth fault relay across a neutral
current transformer.

The unrestricted over current and earth fault relays should
have proper time lag to co - ordinate with the protective
relays of other circuit to avoid indiscriminate tripping

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