Residual current device, has three types if protection:
1.Magnetic help to detect short circuits, with the use of an internal coil and a magnet, open close when a fault occurs.
2.Termical protection, detects overloads with the help of a bi-metallic plate that gets hot and bend (open the circuit) under overload conditions.
3. Life saver protection, the RCD current is balanced with the flowing of current from phase conductor to neutral. But if for example a person touches the cable, it becomes a discharger (leakage). This will make the RCD detect it and open at the speed of 30 milliseconds.