Did you check the breaker to panel buss connection? meter? any obvious signs of overheated wires?
Yes, this is the first place I would look. The thermal element is located close to the load end connections so a loose connection which can cause the thermal element to create causing the breaker to trip.
On small breakers a poor connection at the line side stabs can also cause the thermal element to create.
Should there be heating at the line side end the breaker is apt to be damaged. What can happen is the heating can be conducted from the stab or terminal through to the stationary contact, to the moving contact which weakens the spring that holds the moving contact against the stationary contact. The spring can weaken and fail no longer capable of holding the moving contact against the stationary contact causing the contacts to fail.
Also, anytime other is heating at the stabs the spring tension will deteriorate which increases heating. Any time a terminal is not torqued sufficiently the termination will heat with loading and the cool when unloaded. This cycling causes the terminal to loosen further causing heating to a point were I have seen the results of arcing, enough arcing that the arc my involve a grounded surface.
I had one one 3p breaker where the arc then involved a second phase which is not a good thing. You would have expected the breaker to trip thermally except in this case it was a MCP mag only breaker for a motor starter.
Anyway, I've done many autopsies on breakers to determine a cause of failure.
