The Siemens SENTRON 3VA9978-0BB11 is an undervoltage release accessory for the 3VA4, 3VA5, and 3VA6 series of molded-case circuit breakers.
The release draws 2.5 W maximum at DC — negligible for the control transformer budget, but verify your 24 V DC supply can handle the inrush when multiple releases trip simultaneously. Dimensions are 25.7 mm wide, 65.1 mm high, and 28.8 mm deep — it mounts inside the breaker's accessory bay, not on the DIN rail, so no additional panel width is consumed. Wire connection for the auxiliary circuit accepts AWG 18 up to 750 mm length; strip 10 mm (0.4 in) for a secure termination.