What it is and what it does
The Siemens SIRIUS 3RT1025-1AR64 is a size S0 power contactor built for switching motor loads in industrial control panels. It's rated for 15.5 A in AC-4 duty at 400 V — that's the category for reversing, plugging, and inching motors where the contacts make and break under full locked-rotor current. The 24 VDC coil holds in at 18 V and drops out below that, so it plays nice with standard 24 VDC control supplies. Out here in the grease, that AC-4 number is the one that matters: it tells you the contactor can handle the arc a motor throws when you reverse it hot, not just the running current.
Mounting and panel integration
This contactor snaps onto a 35 mm DIN rail per EN 50022 and takes up 45 mm of width. Side-by-side mounting is allowed with a 6 mm gap at the side for heat dissipation. The screw terminals accept up to 2x 4 mm² solid or 2x 10 mm² stranded on the main circuit — that's enough for the 7.5 kW motor load at AC-2 or the 11 kW at 690 V. The front is IP20, the terminals themselves are IP00, so it's a panel-mount part, not a standalone enclosure device. It'll live inside a cabinet with the rest of the SIRIUS gear.
Coordination and protection
For type 2 coordination (no damage to the contactor on a short circuit), the upstream fuse needs to be a gL/gG 25 A. For type 1 coordination (contactor can be damaged but no fire or shock hazard), you can go up to a 63 A gL/gG. That's the factory recommendation — don't guess it; the fuse size is part of the contactor's short-circuit rating.