Reed switches, illustrated in Figure 4, are more reliable than limit switches, due to their simplified construction. The switches are constructed of flexible ferrous strips (reeds) and are placed near the intended travel of the valve stem or control rod extension.

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Figure 4 Reed Switches

When using reed switches, 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. This ON/OFF indicator is similar to mechanical limit switches. By using a large number of magnetic reed switches, incremental position can be measured. This technique is sometimes used in monitoring a reactor's control rod position.

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 position.

Summary

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.