Vol. EE Reference
Chapter 9 Circuit Schematic Symbols

Switches, Process Actuated

It is very important to keep in mind that the “normal” contact status of a process-actuated switch refers to its status when the process is absent and/or inactive, not “normal” in the sense of process conditions as expected during routine operation.

For instance, a normally-closed low-flow detection switch installed on a coolant pipe will be maintained in the actuated state (open) when there is regular coolant flow through the pipe. If the coolant flow stops, the flow switch will go to its “normal” (un-actuated) status of closed.

A limit switch is one actuated by contact with a moving machine part. An electronic limit switch senses mechanical motion but does so using light, magnetic fields, or other non-contact means.