The 3RT5045-1BF40 is a SIRIUS size S3 power contactor with a 110 V AC coil. The 110 V control voltage is common in legacy European panels and some North American installations where a control transformer steps down from 480 V. The S3 frame size tells you the physical footprint: 70 mm wide, 146 mm high, 152 mm deep — so it occupies a standard four-module width on a 35 mm DIN rail and leaves room for side-by-side mounting without derating. Rated at 21.1 kW at 690 V AC-3e, this contactor handles motor loads up to that power in standard three-phase induction motor duty. If you are driving a pump, fan, or compressor at 690 V, this contactor carries the full motor current through the rated mechanical life of 10 million operations. The switching frequency limits tell you how often it can cycle without overheating: 1 000 operations per hour at AC-3e, 300 per hour at AC-4 (plugging/inching). That is a comfortable margin for a conveyor line cycling every few seconds; for a high-speed pick-and-place running 3 000 cycles per hour, you would need a larger frame or a contactor rated for higher frequency.
Main circuit terminals are screw-type, accepting 2x (2.5 to 16 mm²) solid or stranded copper, or 2x (10 to 1/0 AWG). At 40 °C ambient, the minimum permissible conductor is 50 mm²; at 60 °C, it drops to 35 mm² — the higher temperature derates the current-carrying capacity, so you need a larger wire to keep the temperature rise within limits. Auxiliary and control circuits also use screw terminals, and the contactor accepts an auxiliary switch block (product extension available).
For BOM freeze or PCN watch, this is a low-risk line item. The SIRIUS platform is Siemens' mainstream contactor family, widely stocked and supported. If you need to second-source, the closest functional peer is the 3RT5045-1AF00 (same S3 frame, same ratings, 110 V 50 Hz / 120 V 60 Hz coil) — it drops into the same panel footprint without rewiring, though the coil voltage differs slightly at 60 Hz. Verify the control transformer taps before swapping.