New York State Master Electrician Practice Exam

Which type of switch must not be opened under load when rated for more than 1200A at 250V or less?

1. Isolating Switch

2. Load Break Switch

3. Transfer Switch

4. Disconnect Switch

The correct answer is: Isolating Switch

An isolating switch is designed specifically to interrupt the electrical circuit for maintenance or to ensure safety without breaking the load. When dealing with high current ratings, such as those exceeding 1200A at 250V or lower, it is crucial that these switches are not opened under load. This is because opening an isolating switch while under load could lead to arcing, which poses significant safety risks, including potential equipment damage and fire hazards. A load break switch, on the other hand, is designed to safely interrupt the circuit while it is under load, making it suitable for operational situations where you might need to disconnect the load. The transfer switch is utilized to facilitate the changeover between different power sources, and a disconnect switch is generally designed to provide a means of isolating sections of the circuit but may also have limitations on load interruption depending on the design. Thus, the key factor that makes the isolating switch the correct choice is its intended function and construction, which is not meant for operation under load conditions, especially at higher ratings. Understanding these devices' operational limitations is critical for maintaining safety and system integrity in electrical installations.