The Siemens 3VM9908-0BB26 is a SENTRON undervoltage release — an auxiliary accessory that trips a 3VM molded-case circuit breaker when supply voltage drops below its dropout threshold. It's wired to the line side of the breaker and holds the mechanism closed only while voltage is present; on loss or dip, it releases the trip bar. Rated 380-400 V AC, 50/60 Hz, with an apparent power draw of 2.5 VA. This release fits the 3VM breaker family up to 630 A. Three accessory slots in the breaker accept one or more releases; this unit occupies one slot.
Fit and panel integration
Physical envelope: 65.1 mm tall, 25.7 mm wide, 28.8 mm deep. Stripped length for the connecting conductors is 10 mm (0.4 in). The release snaps into the designated slot on the breaker — no DIN-rail footprint of its own, it's a captive accessory.
Status is Active per the manufacturer. This is a current-production accessory for the 3VM line.