NEC Commercial Electrical Practice Test

What device is defined as one designed to open and close a circuit manually and automatically interrupt the circuit on an over-current?

1. Fuse

2. Circuit breaker

3. Relay

4. Contactor

The correct answer is: Circuit breaker

The device designed to open and close a circuit manually and automatically interrupt the circuit during an over-current condition is a circuit breaker. Circuit breakers serve a crucial safety function by breaking the circuit in response to excessive current, which helps prevent overheating, electrical fires, and damage to electrical components. In addition to their automatic features, many circuit breakers also allow for manual operation, enabling users to open or close the circuit as needed. Fuses, while also serving a protective role, function differently. They provide over-current protection by melting and breaking the circuit but cannot be reset and reused like circuit breakers can. Relays and contactors, on the other hand, are primarily used for controlling circuits, usually employing an electromagnetic mechanism to switch elements on and off, but they do not inherently provide over-current protection like circuit breakers do.