A relay is an electronic switch. It inputs external signals (such as electrical quantities like voltage and current or non-electrical quantities like temperature, pressure, and speed) through the measurement link and transmits them to the intermediate mechanism. This mechanism compares the signals with the set value (i.e., the setting value). When the setting value is reached (either over or under), the intermediate mechanism causes the actuator to produce an output action, thereby closing or disconnecting the circuit to control the circuit. When selecting a relay, the following points can be analyzed and studied item by item: Appearance and installation method, including installation pins; Input parameters; Output parameters; Environmental conditions; Safety requirements; Installation and use requirements.