This guide covers split load vs dual RCD vs RCBO board configurations, circuit arrangement and allocation, BS 7671 labelling requirements, type testing under BS EN 61439, SPD installation, wiring best practice, and the common mistakes found during EICR inspections. A distribution box is the heart of any electrical system. It takes the incoming power and safely distributes it to different circuits throughout your building. It includes isolator, RCCB (Residual current circuit breaker) or RCD (Residual-current device) devices, protective fuses or MCB's (Miniature Circuit Breaker).