Faults on transmission lines are associated with increasing of current and decreasing of voltage which yields decreasing the value of Z. The idea of distance relays are based on the fact that :
- On normal condition, The value of Z is very high so the distance relay does not operate .
- On fault condition, The value of Z is decreased and distance relay will operate.
figure (22) shown the change from health condition ( Z1 ) to fault condition (Z2) in the impedance relay characteristics.
