Directional over current relays

The fault current flows into the non-polarity mark of the CT connected to Circuit Breaker 4, so the Directional Overcurrent (67) relay sees the fault in the reverse direction.  The orange/red shaded region indicates the typical region for a fault behind a relay.

Overcurrent directional relays can be set to trip for faults in the forward direction, which will protect the equipment in front of the relay. Or they can also be set to trip for faults behind the relay in the reverse direction. Forward and reverse are typically determined by the normal flow of current into the relay, so be sure to confirm the CT connections before you make any assumptions.

Incorrectly determining forward and reverse is an easy mistake to make.  If I ever have doubts about some relay settings or directional overcurrent tests, I usually ask the design engineer, “Did you mean to trip if the fault is on the transmission line, or on the buss?” (You can use whatever easy-to-define characteristic for your situation.) Once they answer that question, I will review the CT connections and build a test on the transmission line and see if it trips.  I then apply the fault

in the reverse direction to make sure it doesn’t trip. Always ask the engineer what they intended if there is any doubt.