The C254MG250D is a thermal-magnetic trip unit from the ComPacT NSX100...250 DC range, designed for DC generator protection circuits. The instantaneous pick-up is fixed at 625 A, meaning it trips on short-circuit faults above that threshold without adjustment.
If your generator's full-load current is 250 A or less, this unit matches. The long-time pick-up (Ir) is adjustable from 0.7 to 1 x In, so you can dial it down to 175 A if the load is lighter. Rated for 750 V DC — that's the maximum system voltage it can interrupt. In a 500 V DC or 600 V DC bus, it has headroom. The 4-pole configuration with 4D protection means all four poles are switched and protected, not just three with a neutral. IP40 protection means it's suited for indoor panel mounting where tools and fingers are the main concern, not hose-down. No earth-leakage protection built in — if you need that, you'll add an external Vigi module or separate RCD.