What this part is and where it fits
The Siemens 3RV1956-0BA0 is a lever-type rotary operating mechanism with a lock and door interlock, designed specifically for size 3RV13 5 circuit breakers in the SIRIUS family. This is the mechanical interface that lets you open or close the breaker from the panel door while keeping the door interlocked — the breaker must be in the OFF position before the door can open, which is standard for safety isolation in industrial control panels. It mounts on the front of the breaker and couples to the rotary handle on the enclosure door.
What it does — the mechanical interlock function
The lever-type rotary mechanism translates a 90-degree rotation of the door-mounted handle into the breaker's toggle action. The integral door interlock prevents the enclosure door from opening when the breaker is in the ON position — a key safety feature for lockout/tagout procedures on motor branch circuits and distribution panels. This is the same class of accessory used across the SIRIUS 3RV13 breaker range, so if your panel already uses that breaker family, the 3RV1956-0BA0 is the correct handle mechanism for size 5.