Residual Current Devices
These are intended to safeguard a person rather than equipment and to minimise the risk of injury due to electric shock. An RCD is one which detects when electricity is leaking from a circuit, potentially causing electrocution. When this is detected it acts as a circuit breaker and breaks the circuit. Many RCDs are combined with a circuit breaker and so will protect in the overload case as well. RCDs are also known as earth leakage circuit breakers (ELCB), safety switches and ground fault circuit interrupters (GFCI).
An RCD monitors the flow of current to a device along the live conductor and back via the neutral conductor. It detects when there is less current returning due to leakage (potentially through a person) and breaks the circuit.
RCDs are often incorporated in the distribution panels in a building to provide safety for all sockets. They are also available as portable units that plug into a standard socket, allowing the device to be plugged into the RCD.
Note that RCDs should be tested frequently to ensure their continued safety coverage. RCDs come with instructions for testing and most have an obvious test button, which when pressed should trip the circuit.