What this accessory does in the panel
The Siemens SENTRON 3VA9978-0AB22 is an electrical alarm switch with changeover contacts (type HQ, 7 mm width) designed as an accessory for 3VA6 molded-case circuit breakers. It mounts inside the breaker accessory slot and tracks the breaker's tripped or switched state, giving the control system a hardwired signal — no separate monitoring relay needed. Rated for a thermal current of 6 A, it switches AC control loads up to 6 A in resistive duty (AC-12) and 3 A in inductive solenoid duty (AC-15) across 12–240 V. On DC control circuits, it handles 6 A at 12 V (DC-12) and derates to 0.8 A at 24 V (DC-13) for solenoid valves or contactor coils. The 4 kV surge voltage resistance and overvoltage category III rating mean it's built for the transient environment inside a main distribution panel, not just a clean control cabinet.
Switching capacity by duty — the real fit check
The 3VA9978-0AB22's switching ratings are given per IEC duty class, and the right one decides whether the contact holds up. For AC resistive loads (AC-12) — think panel indicators, relay coils, or PLC digital inputs — it's good for 6 A at any voltage from 12 V to 240 V. For AC solenoid or contactor coils (AC-15), the make/break current drops to 3 A across the same voltage range; that's the inductive inrush that welds undersized contacts. On DC, the derating is steeper: at 24 V it switches 4 A in DC-12 (resistive) but only 0.8 A in DC-13 (solenoid/valve). At 110 V DC, DC-13 drops to 0.2 A — enough for a small contactor hold coil, not a power circuit.
Wiring and integration notes
Strip length is 15 mm (0.6 in) for the screw terminals — use a ferrule on stranded wire to avoid whiskers under the clamp. The changeover contacts (form C) give one normally-open and one normally-closed signal from the same trip event, so you can wire a fault lamp to the NO and a PLC alarm input to the NC without extra relays. The 7 mm width fits the standard 3VA6 accessory slot; no DIN-rail space consumed beyond the breaker itself. Verify free-wheel diode polarity on any DC solenoid load switched through the DC-13 contact — the 0.8 A at 24 V rating assumes the coil's inductive kick is suppressed.