What is the difference between a "handle tie" and "common trip" in circuit breakers?
It ain't about the handle-tie
Common trip does NOT work by one handle tripping, and that action dragging the other handle over via the handle-tie.  That is not how that works.
That wouldn't work because the handles &quot;trip free&quot; - that is, if the breaker needs to trip and a padlock or naughty human is impeding the handle from moving, the breaker just trips anyway.  So if the other half of the breaker impeded the handle from moving, the side that tripped would just trip anyway, and common trip would not occur.
This means you cannot 'bolt on' common trip by adding a handle-tie.
Common trip works by an internal mechanism within the breaker. Either the breaker has it, or does not.  In the case of this Siemens breaker, it does not.   Eaton also makes a line of non-common-trip quadplex breakers just like these, but they also make nearly identical breakers with common trip inner and outer.  Other makers build them with inner common-trip and outer independent trip.
So from a common-trip perspective, a handle-tie is nothing but &quot;trip theater&quot;.
The real purpose of a handle-tie, as manassehkatz discusses, is to assure common shutoff when the handle is used as an &quot;off&quot; switch.  And provide a visual cue that the circuits are related.