Control limit track type switches operate at 600 V maximum, ac or dc. Types of operators for these switches are roller lever (460 FPM), standard fork lever (700 FPM), heavy-duty fork lever (340 FPM) and counter, and counter weight with reset for over-hoist applications. Standard spring return switches operate in one direction only (contacts change state when lever is moved in one direction from center). Crane power circuit limit switches are mounted on a crane trolley to prevent over travel of the crane hook when hoisting. These enclosed switches are available as single and duplex types from 50 500 hp. Power circuit limit switches utilize double-pole double-throw power circuit contacts to disconnect the main power to the hoist motor. On dc applications, a resistor is required with the limit switch to provide dynamic braking.