Over current relay can be used as main or backup protection relays in the radial system. Protection and the grading of the over current relay are needed in order to isolate the faulty component from the health system in a short time. This paper proposed a radial power system and offered the study on the coordination of the definite time over current relay of the proposed system. Design and simulation of this work have been performed using MATLAB / Simulink. The proposed model was tested with various faults such as three phases to the ground, single line to ground and double line to ground faults, with various fault locations. The results show that the over current protection relay responds and trip the circuit breaker within its coordinated time delay and isolated only the fault zone. The time delay of zone-1, zone-2 and zone-3 are 0.06s, 0.12s and 0.18s respectively. The results also show the effectiveness of backup protection, the dependability and reliability of the over current relay during the testing.
