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A Mechanical limit switch is a mechanical
device which can be used to determine the physical position
of equipment. For example, an extension on a valve shaft
mechanically trips a limit switch as it moves from open to
shut or shut to open. The limit switch gives ON/OFF output
that corresponds to valve position. Normally, limit switches
are used to provide full open or full shut indications as
illustrated below.
Many Mechanical limit switches are the
push-button variety. When the valve extension comes in
contact with the limit switch, the switch depresses to
complete, or turn on, the electrical circuit. As the valve
extension moves away from the limit switches, spring pressure
opens the switch, turning off the circuit.
Mechanical Limit switch failures are normally
mechanical in nature. If the proper indication or control
function is not achieved, the limit switch is probably
faulty. In this case, local position indication should be
used to verify equipment position.
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