In electrical engineering, a protective relay is a relay device designed to trip a circuit breaker when a fault is detected. The relays are in round glass cases. The selection and applications of. A protection relay is a crucial component of electrical systems that safeguard infrastructure, employees, and equipment from electric problems and malfunctions. Types of Protective Relays: Protective relays are categorized by their mechanism (electromagnetic, static, mechanical) and function. Relion protection and control relays for several application reduce complexity.