The residual current device (RCD) continuously monitors the difference between the outgoing and return current in a circuit. If part of the current flows via an unintended path, such as through the human body to earth, the switch detects this fault current and disconnects within 30 to 40 milliseconds. The standard tripping value for personal protection is 30 mA. In addition to Type AC (alternating fault currents), there is Type A for pulsating DC fault currents, relevant for modern electronic appliances, and Type B for pure DC fault currents in PV systems and frequency converters. Installation is mandatory in wet rooms, children's rooms and outdoor installations.