The Siemens SIRIUS 3RV1611-1DG14 is a size S00 circuit breaker designed for distance protection, packing a 50 kA breaking capacity at 400 V AC into a 45 mm wide housing that snaps onto standard 35 mm DIN rail per DIN EN 60715.
No ground fault detection here, so pair it with a separate RCD or GFCI if that's required on the circuit. At 240 V it jumps to 100 kA, so on lower-voltage distribution you've got headroom for high-fault installations. Upstream fuse coordination is specified: gL/gG 40 A at 400 V, stepping down to 35 A at 500 V and 690 V. Mounting position is unrestricted, so it fits horizontal or vertical buswork without derating. The screw terminals accept 2x 0.5 to 2.5 mm² solid or stranded; a Pozidriv size 2 bit is what you'll want on the van for termination.
The S00 frame is the smallest SIRIUS breaker footprint — 45 mm wide, 90 mm tall, 75 mm deep. Clearance requirements: 20 mm upward and downward, 9 mm at the sides, zero forward and backward. That's tight enough to pack into a crowded enclosure without leaving extra breathing space.