Normally-open and Normally-closed Switch Contacts
The definition of a sensor’s normal status is perhaps the most perplexing part of discrete sensors.
Electrical switch contacts are usually classed as either normally-open or normally-closed, depending on whether they are open or closed under “normal” conditions. But what does it mean to be “normal” for a switch?
The answer is not difficult, yet it is frequently misconstrued due to the aforementioned aforementioned aforementioned aforementioned aforementioned aforementioned aforementioned aforementioned aforementioned.
The “normal” status of a switch is the state of its electrical contacts when no physical stimuli is applied. Another way to think about the “normal” status is to imagine the switch at rest.
This is the state of the switch contact when it is not pressed on a momentary-contact pushbutton switch. Schematic diagrams always include electrical switches.
Normally-open and Normally-closed
A normally-open pushbutton switch, for example, controls a lamp on a 120 volt AC circuit (the “hot” and “neutral” poles of the AC power source are labelled L1 and L2, respectively):
Because it’s drawn in the open position, we know it’s a normally-open (NO) switch.
The lamp will only turn on if the switch is pressed and the normally-open contacts are held in the “closed” state. Normally-open switch contacts are also known as form-A contacts in the electrical industry.
The behaviour would be exactly the reverse if we had used a normally-closed pushbutton switch instead. If the switch was left alone, the lamp would turn on, but if it was pressed, it would turn off.
Don’t you think this is rather straightforward? What could be more perplexing than a switch’s “natural” state?
When you start thinking about process changes, though, the confusion becomes clear (i.e. switches actuated by process measurements such as pressure, flow, level, etc.).
To better grasp this notion, consider a basic flow switch application: a switch designed to activate when a significant amount of fluid flows through a pipe.
A flow switch is a device that detects the movement of fluid via a conduit. The switch sign seems to be a toggle switch with a “flag” hanging down in a schematic diagram.
Of course, the schematic diagram just depicts the circuitry and not the pipe in which the switch is physically installed:
The connections are generally closed, as demonstrated by the closed position in the diagram, and this flow switch is employed to trigger an alarm light if coolant flow through the pipe ever dips to a dangerously low level.
Even though this switch is labelled as “normally closed,” it will spend the majority of its life in the open position due to the presence.
This switch will only return to its “normal” condition and conduct electrical power to the lamp once the flow through the pipe has slowed enough.
In other words, the switch’s “regular” state (closed) is actually an abnormal one for the process it controls (low flow), for the simple reason that the switch should be stimulated rather than idle while the process is running normally.
We frequently ask why process switch contacts are labelled with the “no stimulation” norm rather than the typical status of the process in which the switch is used.
The answer to this query is that the switch’s manufacturer has no idea what you’re going to do with it.
A flow switch manufacturer is unconcerned with whether or not their device is utilised as a low-flow or high-flow detector.
In other words, because the manufacturer cannot forecast what your process’s average condition will be, the definition of “normal” status for the switch must be based on some generic criterion unrelated to your specific application.
Here is a listing of “normal” definitions for various process switch types:
- Limit switch: target not contacting the switch
- Proximity switch: target far away
- Pressure switch: low pressure (or even a vacuum)
- Level switch: low level (empty)
- Temperature switch: low temperature (cold)
- Flow switch: low flow rate (fluid stopped)
These are the states depicted in a schematic diagram by the switch statuses. When the switches are exposed to typical operating conditions in the process, these may or may not represent the states of the switches.
One thing to keep in mind about process switches and their associated schematic diagram symbols is that the symbols are typically drawn in such a way that an upward motion of the symbol is indicated.
Here are some examples of this, showing various.
Process switch types and NO/NC contact configurations, contrasting their states when no stimulus is present versus when the stimulus exceeds the threshold or “trip” setting of each switch.
It’s important to keep in mind that the way a switch is shown in a schematic design only depicts its “typical” condition as described by the manufacturer.
This might or might not be the switch’s status during “normal” process operation, and it might or might not be the switch’s status when you’re looking at the schematic!
The “normal” position of a switch simply refers to what the switch will do when subjected to a minimum stimulus – that is, when the stimulus is less than the switch’s actuation threshold.
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