Perhaps the most confusing aspect of discrete sensors is the definition of a sensor’s normal status. Electrical switch contacts are typically classified as either normallyopen or normallyclosed, referring to the open or closed status of the contacts under “normal” conditions. But what exactly defines “normal” for a switch? The answer is not complex, but is often misunderstood due to the ambiguous nature of the word normal. The “normal” status for a switch is the status its electrical contacts are in during a condition of no physical stimulation. Another way to think of the “normal” status is to think in terms of the switch being at rest. For a momentarycontact pushbutton switch, this would be the status of the switch contact when it is not being pressed. Electrical switches are always drawn in schematic diagrams in their “normal” statuses, regardless of their application. For instance, the following diagram shows a normallyopen pushbutton switch controlling a lamp on a 120 volt AC circuit (the “hot” and “neutral” poles of the AC power source labeled L1 and L2, respectively):
We can tell this switch is a normallyopen (NO) switch because it is drawn in an open position. The lamp will energize only if someone presses the switch, holding its normallyopen contacts in the “closed” position. Normallyopen switch contacts are sometimes referred to in the electrical industry as formA contacts.
If we had used a normallyclosed pushbutton switch instead, the behavior would be exactly opposite. The lamp would energize if the switch was left alone, but it would turn off if anyone pressed the switch. Normallyclosed switch contacts are sometimes referred to in the electrical industry as formB contacts:
This seems rather simple, don’t you think? What could possibly be confusing about the “normal” status of a switch? The confusion becomes evident, though, when you begin to consider process switches (i.e. switches actuated by process measurements such as pressure, flow, level, etc.). In order to better understand this concept, we will consider a simple application of a flow switch: a switch built to actuate when a sufficient rate of fluid flows through a pipe.
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 normallyclosed as evidenced by the closed status in the diagram. Here is where things get confusing: even though this switch is designated as “normallyclosed,” 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 and conduct electrical power to the lamp. In other words, the “normal” status for this switch (closed) is actually an abnormal status for the process it operates within (low flow), for the simple reason that the switch should be stimulated and not at rest while the process is operating as it should. We often wonder why process switch contacts are labeled according to this convention of “no stimulation” instead of according to the typical status of the process in which the switch is used. The answer to this question is that the manufacturer of the switch has no idea whatsoever as to your intended use. A flow switch manufacturer does not know or care whether their product gets used as a lowflow detector or as a highflow detector. In other words, the manufacturer cannot predict what the typical status of your process will be, and so the definition of “normal” status for the switch must be founded on some common criterion unrelated to your particular application. That common criterion is the resting status: when the sensor is exposed to the least (or no) amount of stimulation from the process it sense
These are the conditions represented by the switch statuses shown in a schematic diagram. These may very well not be the statuses of the switches when they are exposed to typical operating conditions in the process. A helpful tip to remember about process switches and their respective schematic diagram symbols is that the symbols are conventionally drawn in such a way that an upward motion of the movable switch element represents increasing stimulus. Here are some examples of this, showing various process switch types and NO/NC contact configurations, comparing their states with no stimulus versus when the stimulus exceeds the each switch’s threshold or “trip” setting. The normal status of each switch as defined by the manufacturer is labeled in green text