What it is and what it fits
The Siemens SIRIUS 3RT5965-5AU31 is a magnet coil designed for size S10 contactors. It accepts a control supply voltage of 277 V at both 50 Hz and 60 Hz, and also 277 V DC, with an AC/DC input type. This is the withdrawal coil variant — meaning it's the replaceable coil assembly that pulls out of the contactor frame, not the fixed-wire type. If you're servicing a SIRIUS S10 contactor on a panel and the coil has failed, this is the direct swap part.
Panel integration note
The coil mounts inside the SIRIUS S10 contactor, which itself fastens via screw fixing to a mounting plate or DIN rail. The contactor's dimensions (145 mm wide, 202 mm deep, 210 mm high) mean the coil swap is done with the contactor in place — no need to pull the entire contactor off the rail. Verify the coil's surge suppressor (varistor design per the S10 contactor spec) is compatible with your control circuit's transient requirements.