synchronism. the three mechanisms were mechanically trip free. Figure 11 shows the trip-free operation in which the mechanical linkage disconnects from the closing piston for an air cylinder diameter of 10 in. Mechanisms that used air pressure for both closing and operating had valves that provided fast release of the closing air pressure to allow immediate opening air pressure to be applied. these were identified as pneumatically trip free. Later Model Design Features the design of early circuit breakers consisted of two interrupters bridged by a movable metal member (cross arm) for conducting current when the breaker is closed or breaking the circuit and providing a disconnect gap for isolation when it is open. this was an important feature maintained in the later-model oil breakers. additionally, there had been significant changes in figure 11. The Westinghouse pneumatic mechanism type AA-10 in a trip-free position. (From Westinghouse Descriptive Bulletin 33-350, June 1959.) figure 12. A cross-section view of a 138-kV OCB. (From Westinghouse Descriptive Bulletin 33-253, May 1960.) structure, mechanical operation, and bushings for insulating the conductors from the grounded tank structure and current transformers for intelligence. Figure 12 is the cross section of a pole unit of a modern OCB. the bushing insulates the conductor from the grounded structure consisting of the tank andhttp://www.nrec.com