Over Current Protection of 11KV MetalClad Switchgear according to WAPDA Specification P-44:96

• All panels shall have overcurrent & earth fault protection which shall be provided by means of inverse induction type/numeric type relays with two over current elements and one earth fault element. The relays shall have three instantaneous elements.
• For 12.5KA switchgear, the relays shall be arranged to operate on AC and without any external source of supply using saturable auxiliary current transformer.

• For 25KA switchgear, the over current relays shall operate on DC.
• For 25KA Incoming Panels, in addition to the DC protection scheme, a back up AC series protection scheme shall also be provide. The back up AC series protection shall be operated by a separate earth fault relay. The characteristics of earth fault relay for back up protection shall be same as for earth fault unit in the main over current  and earth fault relay. For AC back up protection the trip coils shall be energized directly from the CTs without  requiring any external source