The Schneider Electric A9N18564 is an add-on residual current device (RCD) from the Acti9 Vigi C120 range, designed to clip onto the outgoing side of a compatible MCB or MCCB. It is a 2-pole (2P) unit rated for 125 A continuous current with 300 mA AC-type earth-leakage sensitivity, instantaneous trip.
The 125 A rated current (In) means this RCD can sit on the feed to a sub-distribution board or a large load group, not just a single branch circuit. The 300 mA sensitivity is a common choice for earth-leakage protection on outgoing groups where nuisance tripping from normal leakage currents (cables, filters, motor drives) would be a problem if you used a 30 mA device. The instantaneous time delay means no intentional delay; it trips as soon as the leakage exceeds 300 mA. Rated operational voltage is 230…415 V AC 50/60 Hz, so it covers both single-phase (L-N at 230 V) and two-phase or three-phase line-to-line configurations up to 415 V. Insulation voltage is rated 500 V AC, and impulse withstand is 6 kV per IEC 60947-2 — solid for standard industrial and commercial distribution panels. Pollution degree 3 per IEC 60947-2 means it's rated for the conductive-dust and humidity conditions typical of factory-floor enclosures. For a distribution RCD that sees maybe a few operations a year (testing, maintenance, fault reset), that's more than enough. The tunnel-type terminals accept up to 50 mm² rigid or 35 mm² flexible cable, so wiring in a 125 A feeder with a 35 mm² conductor is straightforward.