The Schneider Electric A9Z21491 is a 4-pole residual current circuit breaker (RCCB) from the Acti9 iID series, rated at 100 A with 30 mA Type A earth-leakage sensitivity. The 30 mA trip threshold means it catches ground faults that could be a shock hazard or start tracking on wet cable runs, which matters out here in the grease where moisture and vibration are part of the daily route.
The 100 A rated current (In) is the continuous load it's built to carry — sized for a main incomer or a large subfeed where the downstream load sums close to that figure. Type A means it detects both sinusoidal AC residual currents and pulsed DC ones, which covers most modern electronics and variable-speed drives that can generate pulsating faults. Rated insulation voltage (Ui) is 500 V AC, and the rated impulse withstand (Uimp) is 6 kV — that's the surge it can take without flashover, so it's fine in a standard industrial distribution board where you're not expecting direct lightning hits.
Clip-on DIN rail mounting — 8 modules wide at 9 mm pitch, so it takes 72 mm of rail space. Neutral is on the left, which matters if you're replacing an existing unit and the busbar arrangement is fixed. The toggle handle has a padlocking option for lockout/tagout, and there's a contact position indicator so you can see at a glance whether it's tripped.