PURPOSE: To prevent erroneous function due to circulation current by providing first and second current direction relays for detecting the direction of current flowing through each line of parallel two-line transmission line and tripping a circuit breaker when AND condition is satisfied for the outputs from an overcurrent relay, a short circuit selection relay, and the first and second current direction relays.

CONSTITUTION: When a circulation current flows through a parallel two-line transmission line upon occurrence of bus short circuit fault, any one of first and second current direction relays 167, 267 functions but the other does not function because current flow in different directions through respective lines. Consequently, AND condition is not satisfied for the outputs from an overcurrent relay OC, a short circuit selection relay 50S, and the first and second current direction relays 167, 267, and no circuit breaker trip signal is delivered from an AND circuit 23. On the other hand, both of the first and second current direction relays 167, 267 function for forward fault to satisfy the AND condition and a circuit breaker trip signal is delivered from the AND circuit 23.