Why is the motor "locked rotor" condition problematic for overcurrent protection?

The "locked rotor" condition occurs when an electric motor's rotor is unable to turn, causing it to draw a significantly higher current than its normal operating state. This excessive current can be problematic for overcurrent protection systems, particularly because it can lead to a situation where the current decreases and potentially falls below the trip levels of circuit protection devices such as circuit breakers or fuses.

When a motor is locked, the current may initially be very high, triggering the overcurrent protection; however, depending on the characteristics of the protection device, as time progresses, the current might decline as components within the system begin to react to the heat or damage being produced. If the current drops enough due to heating effects or other limitations, it may not stay above the protection device's threshold long enough to trip and interrupt the circuit, allowing the motor to continue operating in a damaging condition. This poses a serious risk of overheating and may lead to eventual failure.

This situation highlights the importance of selecting appropriate protection devices that can accurately respond to transient conditions in motors, especially under locked rotor scenarios, to avoid undetected overheating or equipment damage.