The Schneider Electric A9N18589 is an add-on residual current device from the Acti9 Vigi C120 range, designed to clip onto the outgoing side of a compatible MCB or MCCB. It provides earth-leakage protection for a 4-pole circuit rated at 125 A, with a 1000 mA sensitivity and Type A waveform detection — meaning it catches sinusoidal AC faults plus pulsating DC residual currents, which covers most modern electronic loads like VFDs, UPS systems, and switching power supplies. The unit is voltage-independent for its tripping mechanism, so it will still disconnect on a residual fault even if the line voltage collapses — a critical safety feature for circuits where loss of neutral or phase could otherwise leave a ground fault undetected. It carries a selective (time-delayed) characteristic, meaning it will ride through short-duration earth-leakage pulses (like those from capacitive inrush on filter circuits) and only trip on sustained faults. This lets it coordinate downstream with faster RCDs, so a nuisance trip on a branch doesn't take out the whole submain.
It connects to the upstream MCB via screw terminals, with tunnel-type terminals accepting 1…50 mm² rigid or 1…35 mm² flexible cable.