The Siemens SENTRON 3VA9978-0BB25 is an undervoltage release accessory that trips the parent breaker when the control voltage drops. It operates on a 208-230 V AC supply at 50/60 Hz. The 2 VA maximum apparent power draw is low enough that it won't load the control transformer on a multi-device string.
The accessory occupies 3 slots in the 3VA breaker's accessory bay — a standard fit for this family. Dimensions are 25.7 mm wide, 65.1 mm high, and 28.8 mm deep, so it nests entirely within the breaker envelope without increasing the panel footprint. The auxiliary circuit connection accepts AWG 18 to 750 mm² wire; strip length is 10 mm (0.4 in). This is a snap-in accessory for the 3VA4/5/6 line — no DIN-rail space consumed beyond the breaker itself. The 4 kV surge rating and overvoltage category III mean it's rated for the distribution side of a building's main panel, not just downstream sub-panels.