The Siemens SENTRON 3VA9978-0BF23 is a fixed-delay time-delay device, a 24 V DC accessory designed to pair with the 3VA UL undervoltage release. It provides a minimum OFF-delay time of 100 ms, which means the undervoltage release holds the breaker closed for that window after control power drops — enough to ride through a brief sag without nuisance tripping the line. That's the whole point of this accessory: it keeps the SENTRON 3VA molded-case breaker from dropping out on a momentary dip that the process can survive.
Mechanically, it clips onto the 3VA breaker's accessory slot — no extra DIN-rail real estate needed. The body measures 72.3 mm deep by 44.8 mm wide by 57.5 mm high, so it stays within the breaker's own envelope. Wiring lands with a 10 mm strip length (0.4 in), which is the standard termination for the spring-cage terminals on these SENTRON accessories.
That means this order code is the current production part, not a superseded variant.