1) Inverse definite minimum type relays (IDMT):-
a) Normal Inverse
i) 3.0 sec relays - i.e. 3.0 sec. at ten times pickup with T.L of 1.0
ii) 1.3 sec relays - i.e. 1.3 sec. at 10 times pickup
b) Very Inverse relays
c) Extremely Inverse relays
2) Definite Time Relays
Instantaneous highest O/L relay supplementing the above O/C relays. By providing a timer,the required time delay can be obtained.
The O/L, E/L relays are used for line protection (for 11KV to 132KV) and for Transformer
Protection.
O/C relaying is very well suited to distribution system protection for the following reasons:-
1. It is basically simple and inexpensive
2. Very often the relays do not need to be directional and hence no PT supply is required.
3. It is possible to use a set of two O/C relays for protection against inter-phase faults and a
separate O/C relay for ground faults.
Pick-up Setting
For coordination of the inverse time O/C relays, the pickup current and time dial setting are to be choosen. The pickup of the relays must be choosen such that it will operate for all short circuits in its own line and provide backup for adjoining lines, keeping in view of maximum full load current.
O/C relay
Pickup setting = I max.load
E/F Relay
Pickup setting = 20% of rated current.
For the E/F relay, the load current is not a factor in the selection of pickup settings and is normally set at 20% of rated current.
Time Settings
The actual operating time of the O/C & E/F relays can be varied by proper selection of the
‘Time Dial Setting’ which is selectable from 0.1 to 1.0.
Time dial settings are to be chosen by having proper coordination and gradation in the
system. Gradations between successive relays are obtained by ‘Selective time interval’ which is usually set between 0.3 to 0.4 Sec.
