Mechanical limit switches and reed switches provide valve open and shut indications. They also are used to determine the physical position of equipment.

EO 1.2DESCRIBE the following switch position indicators to include basic construction and theory of operation.

a. Limit switches

b. Reed switches

Limit Switches

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.

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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.

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.