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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