The Schneider Electric A9N3P25DGN is a 3-pole miniature circuit breaker from the Acti9 xC60 range, rated 25 A with a D tripping curve. The D curve's magnetic trip threshold of 10 to 14 times In means it holds through the starting surge of a motor or the magnetising current of a control transformer, provided the fault current stays below that multiple.
The 3-pole body occupies 54 mm of rail width (6 x 9 mm pitches). IP20 finger-safe protection applies to the front face; inside the enclosure, the pollution degree 3 rating means it tolerates conductive dust and occasional condensation, which is typical for switchgear in a non-conditioned electrical room.
The thermal-magnetic trip unit combines a bimetal element for overload protection (the 25 A continuous rating) with a solenoid for short-circuit response. The magnetic trip fires between 10 and 14 times the rated current — so for a 25 A breaker, instantaneous trip occurs between 250 A and 350 A. That window is deliberately wide to accommodate the manufacturing tolerance of the solenoid and the temperature dependence of the coil resistance; in practice, a hard short at 1 kA will clear it, but the coordination study should use the lower end of the band for the worst-case let-through energy.