Connects to end-use equipment via switch boxes, forming a three-tier power distribution system. Residual current devices (RCDs) at both the tertiary (equipment-level) and secondary (zone-level) stages. Ensures safe disconnection in case of faults or leakage currents. A residual-current device (RCD), residual-current circuit breaker (RCCB) or ground fault circuit interrupter (GFCI) is an electrical safety device, more specifically a form of earth-leakage circuit breaker, that interrupts an electrical circuit when the current passing through line and neutral. The equipment within these boxes varies: primary distribution cabinets usually contain isolating switches, circuit breakers, and residual current devices (RCDs); secondary cabinets contain large three-phase circuit breakers; tertiary cabinets contain single-phase circuit breakers. Equipped with larger three-phase circuit. Selectivity between RCDs is achieved either by time-delay or by subdivision of circuits, which are then protected individually or by groups, or by a combination of both methods. Analogously, three-phase users as, for instance, UPS, computed tomography (CT) equipment, nuclear mag-netic resonance (NMR) equipment, variable speed drives and AC/DC converters in case of earth faults, generate such direct currents that the. people have been injured in electrical accidents at home and in industry. In Australia and New Zealand as well as in global markets, electrical safety authorities have advocated the wide use of residual current devices (RCDs), also known as safety switches or earth leakage devices, as an could.