The Schneider Electric A9N18557 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 the downstream circuit without requiring a separate enclosure or additional DIN-rail space — the unit mounts directly to the load-side terminals of the upstream device via the screw connection. Rated at 125 A on a 230…415 V AC 50 Hz network, this 2-pole device is intended for outgoer group incomer positions. The residual current protection class is Type A-SI (Super Immunised). Type A detects AC sinusoidal and pulsating DC fault currents; the SI suffix adds immunity to high-frequency leakage from variable-speed drives, UPS systems, or LED drivers that would otherwise cause nuisance tripping on a standard Type A device. This makes the A9N18557 a practical choice for commercial or light-industrial panels where non-linear loads are present.
The unit occupies 7 modules of 9 mm pitch, so the total width on the rail is 63 mm — the 117 mm overall width includes the overhang of the terminal housing. Depth is 73 mm, height 95 mm. Tunnel-type terminals accept 1…35 mm² flexible or 1…50 mm² rigid conductors. The rated impulse withstand voltage is 6 kV, and the rated insulation voltage is 500 V AC 50/60 Hz per IEC 60947-1.
Standards and lifecycle
These figures cover most indoor panel environments without derating, though the upper operating limit of 60 °C should be checked against the internal panel temperature rise if the device is mounted near high-current busbars or transformers.