For three-pole or four-pole residual current devices, all the conductors (phases and neutral) go into the core. But you should take it with caution: The neutral conductor must always go through the residual current device and the PE conductor must never go through the residual current. 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. Such selectivity avoids the tripping of any RCD, other than that immediately upstream of a fault position. Selectivity. 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. RCDs work together with Miniature Circuit Breakers (MCB) or fuses, covering the whole system against potentially damaging thermal and dynamic stresses of any overcurrent.
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