A general-purpose relay is an electromechanical device that is used to control a circuit by opening and closing electrical contacts. General-purpose relays are typically housed in an enclosure, and they are typically designed with two sets of mechanical contacts, one to open and one to close a circuit. The contacts are operated by one or more coils (electromagnets) which, when energized, cause the contacts to open or close. The act of opening and closing a circuit This can be used to control various functions such as turning a motor on and off, operating a valve, or turning on a circuit breaker. Common sizes range from miniature up to 30A or higher.