A three-phase, induction-type motor experiences an open in one phase. Which of the listed automatic protective devices will prevent the motor from being damaged?

Correct Answer: Option C — Thermal overload relay

When one phase of a three-phase motor opens, the motor experiences 'single-phasing,' leading to increased current in the remaining phases. A thermal overload relay is designed to detect this elevated current and trip the control circuit, protecting the motor windings from overheating and subsequent insulation failure as required by 46 CFR.