Which of the following devices is primarily used to interrupt excessive current flow?

A circuit breaker is primarily used to interrupt excessive current flow to protect electrical circuits from damage caused by overloads or short circuits. When the current exceeds the rated capacity of the breaker, it automatically trips, breaking the circuit and stopping the flow of current. This prevents overheating, potential fires, and equipment damage, maintaining safety in electrical installations.

In contrast, a relay is typically used for switching operations or controlling circuits rather than protecting against excessive current. A transformer is designed to change the voltage levels in an electrical circuit and is not involved in interrupting current flow. A capacitor is used for storing electrical energy and cannot interrupt current flow; rather, it functions in power factor correction and energy storage applications. Therefore, the circuit breaker is the correct choice, as its main function is to provide overcurrent protection.