An RCD is a Residual Current Device. It’s designed to protect against electrocution and electrical fires by cutting off the supply of power when it senses a leakage of electrical current from a circuit. An RCD monitors electric currents in circuits or individual items and will switch off the electricity automatically if there is a problem. For example, if you cut through a cable with a power tool, the power switches off, eliminating the chance of injury from electrocution. RCDs also offer protection against electrical fires.