Flow Switch

A flow switch is built to detect fluid flow through a pipe. In a schematic diagram, the switch
symbol appears to be a toggle switch with a “flag” hanging below. The schematic diagram, of course,
only shows the circuitry and not the pipe where the switch is physically mounted:

This particular flow switch is used to trigger an alarm light if coolant flow through the pipe ever falls to a dangerously low level, and the contacts are normally-closed as evidenced by the closed status in the diagram. Here is where things get confusing: even though this switch is designated as “normally-closed,” it will spend most of its lifetime being held in the open status by the presence of adequate coolant flow through the pipe. Only when the flow through the pipe slows down enough will this switch return to its “normal” status (remember, the condition of minimum stimulus?) and conduct electrical power to the lamp. In other words, the “normal” status of this switch (closed) is actually an abnormal status for the process it is sensing (low flow)!