What this auxiliary switch does in a 3VA panel
The Siemens 3VA9978-0AA21 is a leading auxiliary switch from the SENTRON family, designed as an accessory for 3VA molded case circuit breakers. Rated thermal current (Ith) is 10 A, which sets the continuous carry capacity through the switch contacts. The actual switching capability depends on the load type and voltage. For AC-12 (resistive loads and solid-state outputs with optocoupler isolation) it handles 10 A up to 240 V, derating to 6 A at 400 V and 2 A at 600 V. For AC-15 (solenoid valves, electromagnetic loads > 72 VA) it switches 6 A up to 240 V, 2 A at 400 V, and 0.6 A at 600 V. For DC-13 (solenoids with series diode suppression) it handles 3 A at 12/24 V, dropping to 0.8 A at 48 V, 0.2 A at 110 V, and 0.1 A at 250 V. Surge voltage withstand is 4 kV. This puts it in the right class for industrial control panels where the breaker sees line-side surges and the environment isn't climate-controlled.