The Schneider Electric A9N18543 is an Acti9 Vigi C120 add-on residual current device (RCD) — a 4-pole, 300 mA sensitivity, Type AC earth-leakage block that clips onto the outgoing side of a compatible Acti9 C120 MCB. It's voltage-independent, so it trips on residual current alone without needing a separate power supply to the electronics. Rated 125 A continuous at 230...415 V AC, 50/60 Hz, it's sized for the main incomer or a large subfeed in a commercial or light-industrial distribution board.
The 300 mA sensitivity is the standard for fire-protection RCDs — it won't nuisance-trip on normal leakage from long cable runs or equipment filters, but it still clears a ground fault before the fault energy reaches ignition levels. The 125 A rating matches the C120 MCB frame, so the RCD and MCB share the same continuous-current capacity — no derating mismatch on the outgoing feeder. Pollution degree 3 per IEC 60947-2 means it's rated for the conductive-dust and condensation environment inside a typical distribution panel — no need for a sealed enclosure unless the panel itself is in a washdown zone. The IP20 finger-safe terminals are fine behind a locked cabinet door. The 6 kV impulse withstand voltage (Uimp) tells you it survives the switching surges and indirect lightning strikes common on 230/400 V networks without flashover across the internal gap.
The 4-pole block occupies 10 x 9 mm pitches (90 mm of rail width). It connects to the MCB via screws through the tunnel terminals — 1 to 50 mm² rigid or 1 to 35 mm² flexible, stripped 15 mm, torqued to 3.5 N·m. That's a heavy cable range for a 125 A feeder; you can land a 50 mm² tail without a pin ferrule.