What it is and what it does
The Siemens 3RV2031-4SB15 is a SIRIUS motor protection circuit breaker, designed specifically for protecting three-phase motors against overload and short-circuit faults. It combines a CLASS 20 overload relay with a magnetic short-circuit release in one compact package that screws or snaps onto a 35 mm DIN rail per DIN EN 60715. Rated for supply voltages from 20 to 690 V AC, it covers the common low-voltage motor circuits found across European and North American panels. The CLASS 20 trip curve means it allows a 20-second overload at 600% of the set current before tripping — a good match for standard induction motor starting profiles on pumps, fans, and compressors where the start isn't prolonged. Breaking capacity is 30 kA at 400 V, so it safely interrupts a bolted fault up to that level without needing an upstream current-limiting fuse in most industrial distribution panels. At 690 V it still clears 3 kA, adequate for most 690 V motor circuits.
Key ratings and what they mean for fit
The auxiliary contact ratings tell you what control voltage you can switch through the breaker's status contact: 1 A at 24 V DC, 0.15 A at 60 V DC, and 0.5 A at 230 V AC. That's enough to signal a PLC input or a contactor coil, but not to drive a large load directly — keep it as a status signal, not a power switch. Motor rating is 3 hp at 230 V, which gives a quick cross-check for North American motor sizing. For 400 V circuits, the breaker's own current setting range (not listed here but implied by the CLASS 20 and breaking capacity) will cover the typical 3 to 14 A motor full-load current band. Phase failure detection is built in — if one phase drops out, the breaker trips on the resulting current imbalance, preventing single-phasing damage to the motor. No ground-fault detection on this variant, so if you need that, look at the 3RV20 series with the GF option. Maximum operating rate is 15 switching cycles per hour under AC-3 duty (motor starting). That's fine for a conveyor or pump that cycles a few times per hour, but not for a rapid-reversing application like a stamping press or a crane hoist.
Panel integration and mounting
Width is 55 mm. Depth is 149 mm, so check your enclosure depth before committing. Mounting position is any orientation, which helps in tight panels where you have to rotate the breaker to fit. Spacing requirements: 50 mm above and below for cooling, 10 mm to the side. Ignore the side clearance and you'll derate the trip curve — the breaker will nuisance-trip on a warm day. Main contact terminals accept M6 bolts or two conductors from 0.5 to 2.5 mm² solid or stranded. Strip length is 10-12 mm for M6 ring terminals.