This document discusses over-current protection of transmission lines and describes various types of relays used for over-current protection including fuse, thermal relays, bimetallic relay, over-current relay. It defines important terms related to over-current relay operation such as pick up current, current setting, plug setting multiplier, and time setting multiplier. The document also discusses different types of over-current relays based on operation including instantaneous, definite time, inverse time, and various inverse relays. It describes electromagnetic attraction relays and induction type relays. Finally, it covers applications of definite time and inverse definite minimum time relays for distribution feeder protection and over-current protection of a three phase feeder.

1. Over-current protection of
transmission lines

2. Fuse

3. Thermal Relays
• Bimetallic Relay

4. Over-current Relay

5. Important terms
• Pick up current: It is the
minimum current in the
relay coil at which the
relay starts to operate.
• Current setting:
Pick-up current = Rated
secondary current of
C.T. × Current setting

6. Plug-setting multiplier (P.S.M.):
• P.S.M. = Fault current in relay coil / Pick - up
current
= Fault current in relay coil/
(Rated secondary current of CT × Current
setting)

7. Time setting multiplier:

8. Time/ P.S.M curve:

9. Relay Timing
• Instantaneous over-current relay

10. Definite time over-current relay

11. • Inverse time over-current relay
• Inverse definite minimum time (IDMT) over-current
relay

12. Very Inverse type over-current relay

13. Extremely inverse type over-current
relay

14. Inverse definite minimum type relay Characteristics
(TMS=1.0)

15. Electromagnetic Attraction Relays
i. Attracted armature type relay

16. ii. Solenoid type relay.

17. iii. Balanced beam type relay.

18. Induction type Relay

20. Shaded pole structure

22. Watt-hour meter structure

23. Induction cup structure

24. Application of DTOC Relays for
protection of a distribution feeder

26. Plug setting of DTOC relay

28. Time setting of DTOC relay

30. Purpose and setting of relays

32. Setting of DTOC relays for OC
protection of a feeder

35. Application of IDMT relay on a
distribution feeder

37. Variation of fault current against the
fault location for IDMT relays under
maximum fault condition

41. Choice between IDMT and DTOC relays

42. Earth-Fault Protection
Residually connected earth-fault relay

43. Connection of CTs for earth fault
protection

45. Earth fault relay connected in neutral
to earth circuit

47. Combined earth-fault and phase fault
protection

49. Protection of a Three Phase Feeder

51. Over-current protection of a three
phase feeder

53. Two phase fault relays and one ground
fault relay for over-current protection of a
three phase feeder

55. Directional Over-current relay

61. Characteristics of directional over-
current relay