RCD stands for Residual Current Device. It is a safety device that switches off electricity automatically if there is a fault. RCDs are far more sensitive than normal fuses and circuit-breakers and offer a level of personal protection that ordinary fuses and circuit-breakers simply cannot provide.
A Surge Protective Device. It is used to protect the electrical installation, which consists of the consumer unit, wiring and accessories. The protection is against electrical power surges due to the sudden release of energy previously-stored or induced by other means. These can be either naturally occurring such as indirect lightning strikes or man-made.
Indirect lightning strikes most likely happen from a lightning strike on an adjacent overhead power or telephone line. It causes a temporary overvoltage to travel along the lines, which can cause significant damage to the electrical installation and connected equipment.
SPDs are also used to protect your sensitive electronic equipment connected to the installation. Computers, televisions, washing machines and safety circuits, such as fire detection systems and emergency lighting are all protected. Equipment like these with sensitive electronic circuitry can be vulnerable to damage by such events.