Position IndicatorsSWITCHESFailures are normally limited to a reed switch which is stuck open or stuck shut. If a reed switchis stuck shut, the open (closed) indication will be continuously illuminated. If a reed switch isstuck open, the position indication for that switch remains extinguished regardless of valveposition.SummarySwitch position indicators are summarized below.Switch Position Indicators SummaryA limit switch is a mechanical device used to determine the physical position ofvalves. An extension on a valve shaft mechanically trips the switch as it movesfrom open to shut or shut to open. The limit switch gives ON/OFF output whichcorresponds to the valve position.Reed switches are constructed of flexible ferrous strips placed near the intendedtravel of the valve stem or control rod extension. The extension used is apermanent magnet. As the magnet approaches the reed switch, the switch shuts.When the magnet moves away, the reed switch opens.Rev. 0 Page 7 IC-05
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