When selecting operator controls and pushbuttons, you must choose those that are rated for the environment in which they will be used. Will they be exposed to circulating dust, dripping liquids, freezing temperatures? Are they in a washdown area or in an environment that is highly corrosive? Standards developed by the International Electrotechnical Commission (IEC) and by the National Electrical Manufacturers Association (NEMA) assign ratings to pushbuttons and controllers according to how impervious they are to personnel contact and to the ingress of dust and liquids. This article discusses the various ratings and highlights some of the features that protect a controller from water, oil, and dust.
NEMA ratings for 30.5 mm pushbuttons and indicator lights.
30.5 mm heavy duty, watertight and oiltight pushbuttons and indicator lights are available in momentary, push-pull, and switch selector models. There are also joystick and roto-push models. These controllers meet NEMA ratings 1, 2, 3, 3R, 3S, 4, 4X, 12, and 13. Contact a group like Flodyne Incorporated to get an idea of what is available.
Levels 1 and 2 are for indoor equipment, and as in all of the NEMA ratings, indicate that the controller will protect personnel from incidental contact with enclosed equipment. 3, 3R, and 3S are for indoor or outdoor operation, with 3 also providing windblown dust protection. NEMA 3 and 3R rated controllers are not damaged by external ice formation, while NEMA 3S means external components are still operable when there is external ice formation.
NEMA levels 4 and 4X are indoor/outdoor rated. NEMA 4 controllers are protected from wind blown dust and rain, splashing water and hose directed water. NEMA 4X also protects against corrosion. Both 4 and 4X ratings mean there will be no damage from external ice formation. (This rating does not mean the internal components are protected from internal condensation or icing, however).
NEMA 12 is an indoor rated controller protected from falling dirt, dust, lint, and fibers as well as dripping and light splashing of liquids.
NEMA 13 is an indoor rating the same as 12 except with the added protection from spray, splash, or seepage of water, oil, or non-corrosive coolants.
International Protective Marking, or IPM ratings.
The IP, or Ingress Protection ratings, made standard by the IEC are similar to the NEMA ratings but are written in code. There are basically two numbers in an IP code as it relates to pushbuttons and other operator controls.
The first number rates the protection from personnel access to enclosed parts and the protection from infiltration of solid foreign objects. In this scheme, a first digit of 0 means there is no protection while a 6 indicates the controller is dust tight with no access to internal parts.
The second number indicates protection from water. 0 would mean no protection while a rating of 8 would mean the part could be immersed in water.
Comparing the NEMA and IP ratings.
NEMA 4 and 4X are comparable to IP-56, NEMA 12 is comparable to IP-52, and NEMA 13 is comparable to IP-54.
It’s important to note that while the two ratings can be compared, the tests for the two ratings are different, and the comparison does not mean similar tests have been run.
European plants tend to use the IP ratings, while in the US, the NEMA ratings are more popular. But 22–22.5 mm modular pushbuttons are rated with the IP system, with most offering IP66-67 ratings and even IP-69K ratings for washdown areas.
This article gives a comparison of the IP and NEMA systems.
Features that rate controllers and pushbuttons.
Some of the features that prevent ingress of—and controller failure from—liquids and solids are foam Neoprene sealing gaskets, stainless steel operating springs, and reliability nibs that allow switching even where there is corrosion or oxidation. There are also front-of-panel drainage holes in operator bushing to prevent buildup of liquid, to avoid freezing of trapped liquids, and to relieve pressure on diaphragms during pressure washdowns.
Understanding the environment in which your pushbutton controls will be operating is the key to knowing what protection your controllers must have. With this knowledge, you can select the NEMA or IP rating that ensures your operator controls will give you many years of service no matter where they are placed. Your pushbutton and operator controls specialist can help you choose the correctly rated controller for your specific process and plant environment.