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In
industrial valve terms, a
limit switch is a device containing one or more magnetic or electrical switches, operated by the rotational or linear movement of the valve actuator. They are used to show the open or closed position of an industrial valve, or as safety interlocks to protect man or machine. The enclosure is usually classified in
NEMA (National Electrical Manufacturers Association) Standards, such as NEMA 4, NEMA 4X or NEMA 7 (explosion resistant).
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| Internal Mechanical Limit Switch |
The internal switches can be electromechanical or magnetic. Electromechanical switches respond to a mechanical cam on a shaft to change state (make or break electrical contacts). Magnetic switches use a magnetic field to change state in a reed switch, or to sense a disruption in the magnetic field, or through capacitance.
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| Internal Magnetic Limit Switch |
Along with an electrical signal, visual indication is an important part of a limit switch. Most switch come standard with a "dome" or "beacon" displaying "open" or "closed" or positions anywhere in between.
When applying limit switches on industrial valve application one should consider the following:
- Environment - is the limit switch in an area where there may be dust, spray-down, rain, corrosive gasses, combustible gases, or require sanitary conditions. Choosing the correct NEMA classification is critical.
- The electrical rating of the signal being transmitted. AC or DC? High voltage or low?
- Operation Frequency and Cycle life - will the switch only open/close occasionally, or will it cycle every few seconds? Mechanical switches have inherently shorter cycle lives than do proximity or magnetic switches. Choose the best switch considering load and cycle life.
- Visual indication - do you need to see the valve status from a distance?
- Auxiliary outputs - besides opening of closing the primary signal, are their other signals you must send by using additional switches in the housing.
