How does definite time Over current relay work?

Answer 1

These! relays operate at a constant time predetermined by adjustment

and are independent of current magnitude as long as it is sufficient

to operate the relay. The time-current characteristic is shown by curve

D in Fig. 2.

These days are generally us&l for radial circuits or for those loop

circuits which have a few sections in series and there is no difference between

the fault current at the end of one s~&on and the beginning of the next

one. This relay is also used for back-up protection to differential and

distaze relays.

If there are several sections in se&s, say more than three or four,

the time of operation increases as the fault location moves closer towards

the source of power and thus the heavier faults get cleared in longer times.

Nom - The resetting current or time or both arc important where hi

is uSed or if the breaker is not supposed to trip. There is a poleibility 8!2zz3

condition may cause the overcurren t relay to operate but a return to normal condition

for a relay wth a low resetting current or a high resetting time is not possible and an

undesired operation may result.

This relay 'may also be advantageously used qn systems where the

Eault current at a particular point varies widely due to the changes in source

impedance, as there is relatively small change in time with the variation of

Gult current.

For earth-fault protection, if the fault current is severely limited

by neutral impedances or by arc or earth resistances, which is often the

case in industrial plant systems, the definite time overcurrent relay may be

used to facilitate co-ordination between two successive sections. In such

cases it is recommended to use relays with similar charact&istics.