Historically, electrical engineers have had only two choices when it came to specifying power circuit breakers: oil-filled or air. Today, with established technologies for insulating and interrupting methods, the application of circuit breakers is sometimes an afterthought. Although 2kV-38kV class of AC air circuit breakers has virtually disappeared from the scene, they still have a prominent place in the 250V-2000V class of equipment, particularly in AC and DC traction power systems.