The Siemens SENTRON 3VA9978-0BB10 is a 12V DC undervoltage release designed as an auxiliary release for the 3VA4, 3VA5, and 3VA6 molded-case circuit breaker families.
Rated for 12V DC at 2.5 W maximum power consumption, so it's a straight DC coil — no AC rectifier inside. That 2.5 W is the holding power once the coil is energized; inrush is higher for the pickup transient, so size your control transformer or DC supply accordingly.
Panel integration
This release snaps into the 3VA breaker's accessory slot — three slots are available on the breaker, and this occupies one. The body measures 28.8 mm deep by 25.7 mm wide by 65.1 mm tall, so it stays within the breaker's own envelope; no extra DIN-rail space needed. Wiring is via screw terminals accepting AWG 18 or up to 750 mm² (likely a typo for 0.75 mm² or 1.5 mm² — strip 10 mm / 0.4 in).