Relays and circuit breakers are not interchangeable. Together, they form the backbone of reliable power protection systems. There are some important differences between these components, which are highlighted in the sections below. But while both devices respond to abnormal electrical conditions, their functions are entirely different—and understanding that difference is critical for engineers in testing and. Relays are generally cheaper, but the total cost depends on system complexity and rating. Apply technology to. Relay and circuit breaker coordination determines whether faults are cleared selectively, arc flash energy is limited, and protection behaves as intended under real fault conditions by aligning relay operation, breaker response, and short-circuit behavior before failure.