What is the minimum number of conductors a motor controller must open?

• A. only the grounded conductor
• B. enough ungrounded conductors to stop the motor
• C. all ungrounded conductors
• D. no conductors need to be opened

Answer: (B)

The essential idea is that to de-energize a motor, you must interrupt the power supply that actually drives it. The ungrounded conductors are the hot lines that deliver voltage to the motor windings, while the grounded conductor (neutral) and the equipment grounding conductor are safety paths and not the active power path for running the motor. By opening enough ungrounded conductors to remove all energizing paths, the motor cannot be supplied with voltage and will stop. In practice, that means opening all the hot legs feeding the motor (for a three-phase motor, all three hot conductors). Leaving the grounded or grounding conductors connected does not energize the motor, but those hot lines must be opened to ensure it cannot run.