RCD / Residual current device

RCD means residual current device, a protective device that monitors a circuit and disconnects the electricity supply when it detects residual current. In plain terms, residual current is an imbalance that suggests some current may be leaving the intended path, for example through a person, damaged cable or faulty appliance. RCDs provide residual-current protection against some electric-shock risks and some fire risks from earth faults, but they do not provide ordinary overload or short-circuit protection in the way an MCB does. In a home consumer unit or distribution board, one RCD may protect several circuits, while an RCBO combines this residual-current function with MCB-style overcurrent protection for an individual circuit. If an RCD trips repeatedly, the circuit and connected equipment need checking before it is reset again.