Oklahoma Electrical Practice Exam

A 3 phase motor controller that also serves as a disconnect is required to open which of the following?

1. One phase

2. Two phases

3. All three phases

4. Only the neutral

The correct answer is: All three phases

In a three-phase motor controller that also serves as a disconnect, it is essential to open all three phases. This requirement ensures that the motor is completely de-energized, providing safety for maintenance and servicing. The disconnection of all three phases prevents any potential for unintentional start-up or electrical shock risk, as remaining live phases could still energize the motor unintentionally. In addition to safety considerations, opening all phases helps to reduce the risk of phase imbalances, which can lead to motor damage and decreased efficiency. It is also important to adhere to electrical code requirements, which typically specify that all phases of a three-phase circuit must be disconnected when the motor is taken offline. Although disconnecting only one or two phases might seem sufficient, doing so does not guarantee complete isolation of the motor, posing a safety hazard. The neutral, while relevant in single-phase systems, does not typically serve the same role in three-phase systems when disconnecting. Thus, the practice of opening all three phases is crucial for ensuring both safety and compliance with industry standards.