What it is and what it does
The Siemens SENTRON 3VM9908-0BB27 is an undervoltage release accessory for 3VM moulded-case circuit breakers rated up to 630 A. It trips the breaker when supply voltage drops below its operating threshold, protecting downstream loads from undervoltage conditions — a common requirement on motor feeders and process lines where a brownout could stall contactors or damage drives. Coil is rated 440-480 V AC at 50/60 Hz and draws 2.5 VA maximum. That draw is small enough that the control transformer sizing is rarely an issue, but verify the control circuit fuse is rated for the inrush — typical undervoltage releases pull a brief surge on dropout recovery. Rated surge voltage resistance is 4 kV, with overvoltage category III and pollution degree 3. That means it's designed for fixed-installation panels where the supply is permanently connected and the environment can have conductive dust or condensation — standard for industrial switchboards, not clean-room gear.
Mounting and panel fit
Dimensions are 28.8 mm deep, 25.7 mm wide, 65.1 mm high. It clips into the accessory slot on the 3VM breaker body — no DIN-rail space consumed, no extra wiring channel needed. The stripped length spec of 10 mm (0.4 in) tells your panel wireman the exact strip length for the coil terminals; no guessing, no whiskers. The three accessory slots on the 3VM frame mean you can stack this undervoltage release alongside a shunt trip or auxiliary switch without crowding. Plan the slot assignment before wiring — once the breaker is in the panel, swapping slot order means pulling the whole accessory stack.