The Siemens 3VA9978-0AB12 is a tripped signaling switch — a changeover contact module that mounts inside the SENTRON 3VA molded-case circuit breaker family (3VA4, 3VA5, 3VA6 frames) to report a trip event back to a PLC, annunciator, or safety relay. It occupies the 7 mm auxiliary-contact slot on the left side of the breaker and terminates with screw terminals; the stripped length is 15 mm (0.6 in) for the wire termination. Rated for a thermal current (Ith) of 6 A, it carries a full set of utilization-category ratings that define what it can switch in real control circuits: 6 A in AC-12 (resistive loads like pilot lights and relays) at 24 V or 240 V, 3 A in AC-15 (solenoid and contactor coils) at the same voltages, and 0.8 A in DC-13 (DC solenoids) at 24 V. The DC-13 rating at 48 V drops to 0.4 A, and at 110 V to 0.2 A — so if you're switching a DC contactor coil at line voltage, stay within those limits.
Switching capability and isolation
The minimum operating voltage at DC is 250 V, so it can handle the full DC bus voltage of common 24 VDC or 48 VDC control systems without issue. Compared to a standalone miniature circuit breaker like the SENTRON 5SY4140-7 (a 1P, C-curve MCB), the 3VA9978-0AB12 is not a protective device — it's a signaling accessory. You don't use it to switch power; you use it to tell the system the breaker tripped. The 5SY4140-7 lives on a DIN rail in the distribution board; this switch lives inside the breaker itself and sends a dry-contact signal back to the controller.