The Siemens 3VA9978-0AA23 is a leading auxiliary switch from the SENTRON family, designed as a snap-on accessory for 3VA4, 3VA5, and 3VA6 molded-case circuit breakers. It provides a changeover contact set (type HQ, 7 mm width) that signals the breaker's ON/OFF state to a PLC, indicator lamp, or safety circuit. The 0.3 A thermal current rating (Ith) governs the continuous current the contacts can carry — sized for signal-level loads, not power switching.
The contact ratings break down by utilization category. At AC-15 (solenoid/contactor control) and AC-12 (resistive) the switch handles 0.3 A at both 12 V and 24 V. On DC circuits the ratings are lower: DC-12 (resistive) at 0.1 A, DC-13 (solenoid/inductive) at 0.07 A — both at 12 V and 24 V. The 24 V minimum operating voltage means the switch needs at least that to reliably change state; don't expect it to work on a 12 V DC control bus.
Environmental and installation specs
The 4 kV rated surge voltage matches the insulation coordination of standard 400 V distribution panels. Strip length is 15 mm (0.6 in) for the wiring terminals. Contact operating time is 20 ms — fast enough for most signaling but not for high-speed counting.