The Siemens SIRIUS 3RT5975-5AD31 is a replacement magnet coil for size S12 contactors. It's the part that pulls the contactor in and holds it closed — when the coil fails, the contactor drops out regardless of the main contacts' condition. Rated for 48 V AC/DC control supply voltage, so it works on either a 48 VAC control transformer or a 48 VDC power supply without rewiring the coil circuit. The 50 Hz and 60 Hz rated values both land at 48 V — no derating needed when switching between line frequencies. Size S12 is a large-frame contactor class in the SIRIUS family — typically used for motor loads in the 100+ A range. This coil is the service part you order when the original coil on an S12 contactor opens, shorts, or draws excessive holding current. It's not a standalone component; it mounts into the contactor's magnet system.
Because it's a current-production service part, availability is generally steady.
What the 48 V AC/DC rating means on the panel
The AC/DC control supply voltage type means this coil accepts both AC and DC without a separate rectifier module. On a 48 VDC control bus — common in battery-backed or UPS-fed panels — the coil holds in cleanly. On a 48 VAC transformer, same part, same wiring. Just verify the control supply is within the coil's operating range (typically 0.8–1.1 × rated voltage for SIRIUS contactor coils). For the panel builder: the coil's inrush and holding power draw determine the control transformer sizing. On a 48 VAC system, the apparent pick-up power of an S12 coil runs several hundred VA momentarily; the holding power drops to a fraction of that. Make sure the control transformer can supply the inrush without dipping below the dropout threshold.