What is a residual current device? Residual current devices (RCD) and Residual current circuit breakers (RCCB) provide earth leakage protection that can cause electric shock or electrocution. RCDs are used with miniature circuit breakers (MCB) to provide protection for overloaded circuits (over heating of your wires) and earth leakage protection. Residual current breakers with overcurrent protection (RCBO) combine the benefits of a MCB and a RCD in one compact device saving installation time and space.