figure 9. A trip-free mechanism schematic. (From Westinghouse Descriptive Data 33-123, Sept. 1938.) figure 10. The Westinghouse pneumatic mechanism types AA-7 and AA-14. (From Westinghouse Descriptive Bulletin 33-350, June 1959.) 92 ieee power & energy magazine mechanisms were capable of providing five-cyle interrupting times. this is one of the first mechanisms providing the trip-free feature. trip free is now a standard requirement that permits a breaker to be opened immediately, even if the closing operation is in progress, thus resulting in a close-open operation. Figure 9 is a schematic of a trip-free operation of the solenoid mechanism. the shorter interrupting times and higher currents required increased energy, resulting in mechanisms using high pressure air, and hydraulic mechanisms were subsequently developed. the trigger systems for faster release required complete redesigns. Figure 10 shows a family of pneumatic mechanisms with air cylinder diameters of 7 and 14 in with operation at air pressure up to 270 psig. the largest single mechanism with a 14-in diameter operated all three phases of a 345-kV OCB in may/june 2018