to the three interrupting breaks connected in series. The first break was adjacent to the blast valve, and breaks 2 and 3 were fed high-pressure gas from the valve manifold by tubes. The 138-kV design was similar, except it had only two breaks in series. The Evolution of the Interrupter Design figure 1. A two-pressure prototype interrupter in the Westinghouse High Power Laboratory. (Photo courtesy of the Westinghouse Electric Corporation with contrast enhanced by the authors using Photoshop Elements 13.) direct tests of 40 kA at 44 kV, or onethird of the 230-kV breaker line-toground voltage. 86 ieee power & energy magazine The high-pressure reservoir for the 230-kV design was on one end of the interrupter and supplied the flow of gas The power systems in the United States are based on a 60-Hz (60 cycles/s) alternating current (ac) design. The ac current goes through zero twice every cycle. For symmetrical currents, zeros occur every 8.33 ms. Fault currents are limited by the inherent inductance of the connected system and are many times larger in magnitude than normal load currents. Because of the inherent inductance in the circuit, conservation of flux linkages requires that the current not change magnitude may/june 2016