In a 3 wire, 3 phase AC system feeding a motor, with one conductor grounded, where must fuses for motor overload protection be inserted in the circuit?

• A. a fuse must be inserted in each ungrounded conductor and also in the grounded conductor
• B. no fuses are required for motor overload protection
• C. a fuse must be inserted only in each ungrounded conductor
• D. none of the above answers is correct

Answer: (A)

The main idea is that motor overload protection must interrupt current in every conductor that can carry motor current. In a three-phase system fed to a motor with a grounded conductor, there are three ungrounded conductors (the hot phase wires) and a grounded conductor (the neutral). Even though the motor usually runs on the three hot lines, protection is required on each current-carrying path, including the grounded conductor, because under certain faults or multi-wire conditions that neutral can become part of the current path. Placing fuses in all the ungrounded conductors plus the grounded conductor ensures that any overload or short in any path to the motor is promptly opened, protecting the wiring and the motor windings. This aligns with the rule that overload protection must be provided for each motor conductor that carries current.