The C253MG250D is a TM-G thermal-magnetic trip unit for the ComPacT NSX250 DC range, built for DC generator protection. It's the brain that sits inside the breaker frame — you don't wire it separately, it's the replaceable protection element. The LI protection profile covers overload (thermal, adjustable long-time pick-up from 0.7 to 1 x In) and short-circuit (magnetic, fixed instantaneous pick-up at 625 A). That's the standard two-step curve for generator feeders — thermal handles the gradual overcurrent, magnetic snaps on a hard fault. The long-time delay is fixed, so coordination studies need that constraint baked in.
This is a DC-only trip unit — the network type is DC, and the rated operational voltage is 750 V DC. That's not a typo or a dual-rated AC/DC part; it's purpose-built for DC circuits. The 3D pole configuration means all three poles switch the DC current, which matters for arc extinction in DC — you need the series poles to stretch and cool the arc. Don't drop this into an AC breaker frame expecting it to work. Ambient operation from -25 to 70 °C covers most plant-floor conditions; storage from -50 to 85 °C is generous for spares on the shelf. Class II electrical shock protection means no earth connection is required for the user — the double-insulated design handles it. That simplifies wiring in ungrounded DC systems common on generator skids.