a) Very inverse b) extremely inverse relays c) definite time O/C Relay d) instantaneous O/C Relay.
a) Very Inverse – Used where inverse protection reduces substantially as distance from source increases, operating time doubles for a fault current reduction from 7 in to 4 in, used where the short ckt current is independent of generating conditions.
b) Extremely inverse – Used for feeders subjected to peak making currents. Grade with HRC fuses, e.g. Refrigerator, pumps.
c) Definite time O/C Relay – Where neutral is resistance earthed- fixed ground current.
d) Instantaneous O/C Relay – Used along with inverse O/C relay – to get higher grading
margin. Disadvantage – Under minimum generation it may not operate.