Which type of circuit breaker does not utilize a pneumatic operating mechanism?

The correct choice is a bulk-oil circuit breaker, which does not depend on a pneumatic operating mechanism. Instead, bulk-oil circuit breakers use oil as both an insulating medium and as a cooling agent, allowing for the arc-extinguishing process to occur within the oil. The mechanism for operation in these breakers relies more on the oil itself and the switchgear's mechanical design rather than pneumatic pressure.

In contrast, other types of circuit breakers, such as vacuum and miniature circuit breakers, generally utilize different operating principles. Vacuum circuit breakers operate by creating a vacuum that allows for effective interruption of the current, while miniature circuit breakers rely on a bimetallic strip and electromagnet for tripping and are compact in size with a design suited for lower voltages. Airtight circuit breakers also typically use air pressure in a sealed environment to operate, which emphasizes the difference in the operating mechanisms compared to the bulk-oil type.