The Schneider Electric A9N18542 is an add-on residual current device (RCD) from the Acti9 Vigi C120 range, designed to clip onto the outgoing side of a compatible Acti9 MCB or switch-disconnector. It provides earth-leakage protection for a 4-pole circuit rated at 125 A, with a 30 mA sensitivity and instantaneous trip characteristic.
Rated at 125 A continuous and 400-415 V AC, this unit handles the full load current of a large submain or a heavy commercial outgoing group. The instantaneous trip means no intentional delay; it clears a fault within tens of milliseconds, which is appropriate for final circuits but not for selectivity against downstream RCDs. The residual current trip mechanism is voltage-independent — it does not rely on the mains voltage to operate. That means it still trips even if the neutral is lost upstream, a genuine safety advantage over electronic RCDs in TN or TT systems where a broken neutral can otherwise leave the circuit unprotected.
The unit is 198 mm wide, which is 10 modules of 9 mm pitch. It connects to the upstream MCB or switch via screws and accepts tunnel terminals up to 50 mm² rigid or 35 mm² flexible. Factor the 198 mm width into your DIN-rail layout — it occupies ten 9 mm slots, so plan the enclosure fill accordingly.