Instrument and control

Dec. 8, 2005
Instrument and control switches are used to manage relatively large currents and voltages as found in operator panels at electrical utility substations.


switches are used to manage relatively large currents and voltages as found in operator panels at electrical utility substations. Typical ratings are 600 V at 20 A. The switch design was originally conceived by Westinghouse for its utility controls. Modern versions frequently serve as replacements for these devices.

The moving contact is a roller. The metal roller is in the shape of a dumbbell with the contacts at the two ends. The roller seats in a glass polyester rotor which turns when the operator twists the handle. The metal rollers are typically springloaded and push up against stationary terminal studs in a frame. This provides a double, series-break contact. The silver-plated bronze-alloy roller contacts move in a rolling, wiping action and are self-aligning on assembly. Thus as the front knob turns the roller contacts move radially, held in and guided by an insulated slot in the rotor arm.

These switches come as both maintained and spring-return types. Maintained switches employ a starwheel mechanism to hold the switch contacts in place. The mechanism consists of a pointed, notched, molded wheel rotating between the two metal rollers mounted on spring-loaded arms. This arrangement provides a positive snap action as the handle turns. The momentary type uses a rotor that returns to its original position by a double coil-wound spring.

Contact features include options such as shorting, NC, slip, and lateral contacts. Shorting contacts use a roller having a diameter large enough to span the space between stationary contacts. NC types use a shorting bar and cam actuated by the switch handle and operating in parallel with the regular rollers. Slip contacts are for three-position switches and operate when the handle turns right or left. They open when the handle turns to the opposite position and stay open as the handle returns to the center. Finally, lateral contacts give the switch a push-pull actuation mode. Here roller contacts span two adjacent terminals on a band and move back and forth between two bands in a stage of the switch.

About the Author

Leland Teschler

Lee Teschler served as Editor-in-Chief of Machine Design until 2014. He holds a B.S. Engineering from the University of Michigan; a B.S. Electrical Engineering from the University of Michigan; and an MBA from Cleveland State University. Prior to joining Penton, Lee worked as a Communications design engineer for the U.S. Government.

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