The Siemens SENTRON 3VA9978-0BB14 is an undervoltage release accessory for the 3VA4, 3VA5, and 3VA6 molded-case circuit breaker families. It drops the breaker when the control voltage falls below a threshold — standard practice for preventing motor restart after a sag or outage. Coil draws 2.5 W at its 125-127 V DC rating. Surge withstand is 4 kV.
Mechanical footprint is 25.7 mm wide, 65.1 mm tall, 28.8 mm deep — it clips into the accessory slot on the 3VA breaker body, no extra DIN rail space needed. Wire termination accepts AWG 18 or 750 mm² (likely a misprint for 0.75 mm²) with a 10 mm strip length.