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.
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.
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 reactors control rod position.