Failures are normally limited to a reed switch which is stuck open or stuck shut. If a reed switch
is stuck shut, the open (closed) indication will be continuously illuminated. If a reed switch is
stuck open, the position indication for that switch remains extinguished regardless of valve
Switch position indicators are summarized below.
Switch Position Indicators Summary
A limit switch is a mechanical device used to determine the physical position of
valves. An extension on a valve shaft mechanically trips the switch as it moves
from open to shut or shut to open. The limit switch gives ON/OFF output which
corresponds to the valve position.
Reed switches are constructed of flexible ferrous strips placed near the intended
travel of the valve stem or control rod extension. The extension used is a
permanent magnet. As the magnet approaches the reed switch, the switch shuts.
When the magnet moves away, the reed switch opens.