The SP circuit breaker is a good design and uses a pneumatic operator. I've worked on numerous units and have found that the horizontal operating rod in the unit can develop problems. Below are some pictures of the unit with problem linkage. Typically the linkage problems develop with units that have higher operating cycles. A visual inspection can determine if this condition exists.

Horizontal operating rod—Note the bow in the operating spring and the angle of the tensioning plate on the left side of the spring. The linkage in this unit is starting to deflect which will lead to a failure. It was changed prior to failure.

SP breaker in transit without legs. 72kv—1200 amp.

Sister breaker with broken horizontal operating rod. When this happens—the contacts no longer work together which may leave 1 open and 2 closed or the opposite. The operating rod is not solid and the manufacturer strongly advises that only OEM parts be used due to timing issues.