Mechanical limit switches and reed switches provide valve open and shut
indications. They also are used to determine the physical position of equipment.
DESCRIBE the following switch position indicators to include
basic construction and theory of operation.
A 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 in Figure 3.
Figure 3 Limit Switches
Many 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.