What does good discrimination refer to in overcurrent protection?

• A. Discrimination between two or more overcurrent protective devices in series is achieved when in overload or short circuit conditions only the device required to operate does so and the others remain intact.
• B. Discrimination requires all devices trip.
• C. Discrimination refers to voltage rating of devices.
• D. Discrimination is not important.

Answer: (A)

Discrimination in overcurrent protection is about selectivity between protective devices so that only the device closest to the fault operates. When a fault or overload occurs, the downstream device should trip first to clear the problem, while the upstream device remains intact, keeping power on to the rest of the system.

This is why the best description says that, in a series of protective devices, only the device required to operate trips and the others stay closed. It preserves service continuity and limits damage by isolating the fault locally rather than causing a wide outage.

In practice, you achieve this by coordinating the time-current characteristics of the devices: the downstream device is set (through pickup current and curve type) so that it will trip for a given fault before the upstream device does. If all devices tripped for every fault, you’d lose power to more of the system than necessary, which is undesirable. Discrimination is about current levels and timing, not about voltage ratings, and it remains an important reliability consideration.