Limit Switches:

A limit switch is a mechanical device that could be used to determine the physical position of equipment. For instance, 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 provides ON/OFF output which corresponds to valve position. Generally, limit switches are used to gives full open or full shut indications as described in Figure.

Figure: Limit Switches

Several limit switches are the push-button variety. While the valve extension comes in contact along with the limit switch, a switch depresses to finish, or turn on, the electrical circuit. As the valve extension moves away from the limit switches, spring pressure opens the switch and turning off the circuit.

Limit switch failures are generally mechanical in nature. The limit switch is probably faulty if the proper indication or control function is not achieved. In that case, local position indication should be used for verify equipment position.