A Residual Current Device (RCD) is a safety device used in electrical installations to prevent electric shock and reduce the risk of electrical fires. It works by monitoring the flow of electricity through a circuit and quickly disconnecting the power if it detects an imbalance, which may indicate a fault or leakage of current to the ground. RCDs are commonly installed in homes and commercial buildings as part of the electrical system. They can be found in various forms, including RCCBs (Residual Current Circuit Breakers) and RCBOs (Residual Current Circuit Breaker with Overcurrent protection). Regular testing of RCDs is essential to ensure they function correctly and provide the necessary protection.