The effective time-current curve for ground-fault protection consists of the curve for the ground-fault relay and the phase overcurrent protective device.

The effective time-current curve for ground-fault protection consists of the curve for the ground-fault relay and the phase overcurrent protective device.

• A. True
• B. False
• C. Only the relay curve is used
• D. Only the phase overcurrent curve is used

Answer: (A)

The key idea is that protection timing is defined by how quickly devices trip for a given fault current, and ground-fault protection relies on two devices working together: the ground-fault relay and the phase overcurrent protective device. For a ground fault, you have the relay’s curve that responds to the ground-current imbalance, and you also have the phase overcurrent device’s curve that responds to high line current. The effective time-current behavior is the envelope created by these two curves: at every fault current level, the protection will trip by whichever device reaches its pickup and operates first. This coordination ensures both rapid clearance for faults and proper selectivity so the correct point in the system clears first. So stating that the effective curve consists of both the ground-fault relay curve and the phase overcurrent device curve is correct.

The key idea is that protection timing is defined by how quickly devices trip for a given fault current, and ground-fault protection relies on two devices working together: the ground-fault relay and the phase overcurrent protective device. For a ground fault, you have the relay’s curve that responds to the ground-current imbalance, and you also have the phase overcurrent device’s curve that responds to high line current. The effective time-current behavior is the envelope created by these two curves: at every fault current level, the protection will trip by whichever device reaches its pickup and operates first. This coordination ensures both rapid clearance for faults and proper selectivity so the correct point in the system clears first. So stating that the effective curve consists of both the ground-fault relay curve and the phase overcurrent device curve is correct.