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Depending upon the time of operation the
overcurrent relays are categorized as :-

1.
Instantaneous over-current relays

2.
Inverse time-current relays

3.
Inverse definite minimum time (IDMT)
Over-current relay

4.
Very inverse relay and

5.
Extremely inverse relay

Now let us study the same one by one:-

1.

__Instantaneous Over-current relay:-__
This relay as clear from it’s name is that
it has instantaneous tripping without any delay. Time of operation of such
relay is about 0.1 sec. This type of characteristic can be achieved by using
hinged armature relays.

2.

__Inverse time-current relay:-__
In that type of relays operating time
reduces as the actuating quantity ,i.e. current In case of over-current ,increases
in magnitude. More pronounced the effect more inverse will be the
characteristics. They are normally more inverse near the pick up value of the
actuating quantity and become less inverse as it is increased. This type of
characteristic can be achieved with induction type relays by using suitable core
which doesn’t saturate for large value of fault current. If saturation occurs
at very early stage, the time of operation remains the same over the working
range. Characteristic for the same in diagram as (a )

Characteristics of Over current Relay |

3.

__Inverse definite minimum time (IDMT) Over-current relay:-__
In that type of relay operating time is
approximately inversely proportional to the fault current near pick up value
and becomes substantially constant slightly above the pick up value of the
relay as shown (b). This is achieved by using a core of the electromagnet which
get saturated for current slightly greater than the pickup current.

4.

__Very inverse relay:-__
In this relay saturation of the core occurs
at later stage, characteristic is shown in (c ). The time current characteristic
is inverse over greater range and after saturation rends to definite time.

5.

__Extremely inverse relay:-__
In this relay saturation occur even at later
stage. Same is shown in (d) curve. The equation describing the curve is K= I

^{2}t , where I is the operating current and t is operating time.