In a 3-wire, 3-phase system with one grounded conductor, which arrangement correctly describes motor overload protection fusing?

• A. a fuse must be inserted in each ungrounded conductor
• B. a fuse must be inserted only in the grounded conductor
• C. a fuse must be inserted in each ungrounded conductor and also in the grounded conductor
• D. no fuses required

Answer: (C)

The main idea here is that motor overload protection must interrupt every conductor that can carry current to the motor. In a 3-wire, 3-phase system with a grounded conductor, the motor circuit includes the three ungrounded (hot) conductors and the grounded conductor can also carry current under certain conditions or fault scenarios. To guarantee that a fault or overload clears completely, the protective devices must be placed in each current-carrying path, which means one fuse in each ungrounded conductor and one in the grounded conductor as well. This ensures that when the protection trips, all conductors that could energize the motor are simultaneously opened, preventing any continued current through part of the circuit. If you only fuse the ungrounded conductors, the grounded path could still carry current during a fault and keep the motor energized; and without any fuses, there would be no automatic means to interrupt the fault.