Position IndicatorsSWITCHESSWITCHESMechanical limit switches and reed switches provide valve open and shutindications. They also are used to determine the physical position of equipment.EO 1.2 DESCRIBE the following switch position indicators to includebasic construction and theory of operation.a. Limit switchesb. Reed switchesLimitSwitchesA limit switch is a mechanical device which can be used to determine the physical position ofequipment. For example, an extension on a valve shaft mechanically trips a limit switch as itmoves from open to shut or shut to open. The limit switch gives ON/OFF output thatcorresponds to valve position. Normally, limit switches are used to provide full open or full shutindications as illustrated in Figure 3.Figure 3 Limit SwitchesMany limit switches are the push-button variety. When the valve extension comes in contactwith the limit switch, the switch depresses to complete, or turn on, the electrical circuit. As thevalve extension moves away from the limit switches, spring pressure opens the switch, turningoff the circuit.Rev. 0 Page 5 IC-05
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