The Siemens SENTRON 3VA9978-0BB24 is an undervoltage release accessory designed for 3VA4, 3VA5, and 3VA6 molded-case circuit breakers. It trips the breaker when supply voltage drops below the release threshold, protecting motors and sensitive loads from undervoltage conditions.
The 4 kV surge voltage resistance ensures it withstands switching transients common in motor circuits without failing. The 3-slot design and compact dimensions — 28.8 mm deep, 25.7 mm wide, 65.1 mm high — allow it to snap into the accessory slot of the host breaker without increasing the panel's DIN-rail footprint. Stripped length of 10 mm (0.4 in) on the auxiliary circuit connection (AWG18 / 750 mm) is the standard for panel wiring.