by Ravi Jain, Bud Industries Inc.
Some 30 years ago, NEMA-rated enclosures were used only in electrical applications. Today, as electronics have invaded areas such as factory automation, indoor and outdoor controls, and telecommunications, NEMA is the fastest growing segment of the electronics enclosure industry.
NEMA standards define the various levels of environmental protection. When an application is wet, dirty, or washdown, NEMA enclosures are the obvious solution.
What’s not so obvious is which type of material to select for the NEMA enclosure. Most U.S. engineers automatically choose metal enclosures – wasting money and flexibility. In fact, economical NEMA enclosures are available in a wide variety of materials including plastic, die-cast aluminum, steel and aluminum sheet metal, and fiberglass. Each type of material has its pros and cons, and it’s an important decision, because it determines how and where the device can be used. The choice of material depends on the application, budget, and degree of customization required.
Despite the climbing price of oil, plastic is still the lowest cost option for NEMA enclosures. It isn’t as rugged as metal, but it has some unique properties that make it a useful choice. For starters, plastic enclosures are lightweight, which makes them easy to carry. Many small hand-held and portable devices are packaged in plastic enclosures.
Also unique is the option of transparency. NEMA enclosures can have a see-through cover, allowing LEDs and other indicators to be visible without the need to open the cover or provide gasketed or sealed openings.
Plastic enclosures can house instruments that may be affected by metal. If this is a requirement, then don’t forget to specify non-metallic fasteners and latches, which enclosure suppliers make available as an option.
Because plastic is naturally corrosion resistant, it can be used outdoors and in washdown environments. Especially in Europe, plastic enclosures are often used for junction boxes and to house industrial controls. Contrary to popular belief, plastic boxes are easily wall- and machine-mounted.
Stock plastic enclosures come in a limited number of colors, but for large quantities, custom colors are available. Finding custom size plastic enclosures is a bit more challenging. The tooling required for modifications can cost any where from $5,000 to $40,000 depending on the size and complexity. Fortunately, a large variety of dimensions are available from stock.
Die-cast aluminum housings are also lightweight, but they offer more compression strength and rigidity than plastic. Typically, die-cast aluminum enclosures are used to house electronic, pneumatic, hydraulic and electrical devices. They are especially good for housing sensitive electronics assemblies because of the EMI shielding and heat dissipating quality of aluminum.
Die-cast aluminum housings are commonly available with optional mounting flanges, which are either welded or formed as part of the die. Look for the mounting flange to be on the box body, rather than the lid – otherwise, the box cannot be opened without first removing it from the wall or machine.
Unlike plastic, die-cast aluminum enclosures can be ordered with a mar-resistant powder coat finish in a choice of colors. This not only enhances the relatively corrosion resistant property of aluminum, it is also a way to differentiate a product in the marketplace.
Steel sheet metal enclosures provide a nicer appearance than die-cast aluminum, while providing the same benefits of heat dissipation, EMI shielding, and compression strength. Sheet metal enclosures are more durable and also heavier than those made of other materials. In low quantities, sheet metal enclosures can be expensive, but in large quantities they are economical. However, custom enclosures in low quantities and large enclosures are generally most economical in sheet metal because of comparatively low tooling costs.
Steel is not as corrosion resistant as aluminum, but with a powder coat finish, it can meet the same NEMA standards as enclosures made of other materials. Stainless steel enclosures are also an option.
Typical applications include instrument cases, control housings, terminal wiring enclosures, meter cases, junction boxes, and marine applications.
Because sheet metal NEMA boxes are formed by bending metal, it is relatively easy to produce custom dimensions.
Fiberglass enclosures offer many of the same advantages of plastic enclosures. They are lightweight and corrosion resistant. In addition, fiberglass is stronger than ABS or polycarbonate plastic – in fact it is stronger than steel of the same weight. Fiberglass also offers higher operating temperature. The disadvantage is that fiberglass enclosures cost up to twice as much as ABS or polycarbonate plastic enclosures. They are also expensive to tool.
Increasingly, company managers are requesting customization from their enclosure suppliers. Customization lets companies focus on core competencies and lower manufacturing costs. It can add value by providing customers with tailored solutions, such as mounting holes in a custom configuration, or by enhancing the design, such as with CNC machining of custom shaped openings.
Unfortunately, NEMA box enclosures can be tricky to customize. Any openings cut in into the box could compromise the NEMA rating, and ventilation may be limited or impossible, depending on the desired NEMA rating.
Cable glands are an inexpensive and effective way to seal the holes where cables exit the enclosure. Other openings may require your supplier to add custom gaskets. Also, consider heat build-up in the design, and make use of passive heat dispersing tactics.
Often, plastic and die-cast NEMA boxes are molded with internal bosses and card guides. Upon request, the supplier will usually mill them off. Sometimes customization can be added to the outside without affecting the NEMA rating, such as adding latches, lock hasps, mounting brackets, and carrying handles. Of course, suppliers are often willing to provide custom finishes, silk screening, and labels.
More and more devices are being packaged in NEMA-rated enclosures, a trend fueled by increasing customer demand and falling costs. By choosing the ideal material for a NEMA enclosure, engineers can protect their designs for the lowest possible cost.
The National Electrical Manufacturer’s Association has standardized the various levels of environmental protection for enclosures. In a recent survey, more than 70 percent of engineers were confused by the different NEMA ratings. In addition, the survey suggests that 40 percent of all enclosures that require protection are specified to the wrong level. Most engineers use a level that does not provide adequate protection for their design’s intended application; the balance is overkill, which adds unnecessary cost.
At the low end of the range, NEMA 1 provides a basic level of protection – just enough that you cannot put your finger inside the unit and the internal components are protected from incidental contact. This is a basic level that provides the product with safety but not with environmental security.
At the high end of the range, a NEMA 4 enclosure provides all the protection of NEMA 1 plus protection against sprayed liquids and the ability to withstand the formation of ice. This is a high degree of security that is not required in many situations, except in outdoor applications.
The most misunderstood rating is NEMA 12. Although it has a higher number, it covers enclosures for indoor use only. It provides protection against falling dirt, circulating dust and fibers, and dripping water. Cabinet racks for indoor use are often specified to NEMA 12.
|NEMA enclosure definition||IEC
|1||Constructed for indoor use to provide a degree of protection to personnel against access to hazardous parts and to provide a degree of protection of the equipment inside against ingress of solid foreign objects (falling dirt).||IP10|
|4||Constructed for indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside against ingress of solid foreign objects (falling dirt and wind blown dust); to provide a degree of protection with respect to harmful effects on the equipment due to ingress of water (rain, sleet, snow, splashing water, and hose directed water); and that will be undamaged by the external formation of ice on the enclosure.||IP56|
|4x||Same as Type 4 except provides additional protection against corrosion.||IP56|
|12||Constructed (without knockouts) for indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside against ingress of solid foreign objects (falling dirt and circulating dust, lint, fibers, and flyings); and to provide a degree of protection with respect to harmful effects on the equipment due to ingress of water (dripping and light splashing).||IP52|
* NEMA type typically equals or exceeds the IEC IP rating shown.