What device automatically opens a circuit when current exceeds a specified level?

A circuit breaker is a safety device designed to automatically open or disconnect an electrical circuit when it detects that the current flowing through it exceeds a predetermined limit. This action helps prevent damage to the wiring or equipment that could occur due to overheating or electrical overload. The circuit breaker can be reset once the issue has been resolved, allowing the circuit to function normally again.

In contrast, a fuse serves a similar purpose but works by melting and breaking the circuit when the current gets too high, requiring replacement rather than reset. An unload switch, typically used in different contexts, is designed for safely disconnecting devices but doesn't specifically monitor current levels in the same way. A transformer, on the other hand, does not interrupt a circuit based on current levels; rather, it is used to change the voltage of electricity for distribution or use.

Thus, the circuit breaker's ability to automatically cut off power when excess current is detected makes it the suitable choice for this question.