The A9N18555 is an add-on residual current device (RCD) from the Acti9 Vigi C120 series, designed to bolt onto the line side of a compatible Acti9 MCB via screw terminals. It is rated for 125 A continuous current on a 4-pole (4P) AC network at 230...415 V, 50 Hz. The 300 mA earth-leakage sensitivity with Type A-SI (Super Immunised) classification means it handles pulsed DC fault currents and resists nuisance tripping from high-frequency harmonics — common on VFD-fed or UPS-backed circuits. The instantaneous time delay clears faults without intentional lag, so coordination with downstream RCDs needs a check on selectivity. The voltage-independent tripping technology means the device does not need an external power supply to open on a residual fault — it uses the fault current itself to energise the trip solenoid. That makes it inherently fail-safe on a lost neutral or phase dropout, which matters for circuits where continuity of protection is critical.
The 198 mm width across 10 x 9 mm pitches means it occupies ten modular spaces in the enclosure — plan the DIN rail fill factor accordingly. Tunnel-type terminals accept 1...50 mm² rigid or 1...35 mm² flexible cable; strip length is 15 mm, tighten to 3.5 N·m. The screw connection to the adjacent MCB is a mechanical and electrical link — no extra busbar needed, but the MCB must be from the same Acti9 C120 family. IP20 finger protection on the terminals; the device is intended for enclosed panel installation, not open washdown areas.