Nov. 15, 2002
Planning for packaging and interconnections in the early phases of a design project eliminates the chances of poor design degrading long-term reliability.

Planning for packaging and interconnections in the early phases of a design project eliminates the chances of poor design degrading long-term reliability.

The integrity of system design depends on the reliability of conductors (wires, cables, and fiber optics), terminals, connectors, sockets, printed-circuit boards, backplanes, and the overall enclosure.

Enclosures typically come in two grades: commercial and industrial. Commercial cabinets suit office and light industrial environments. Industrial equipment is normally more robust. Operating environment determines the type of cooling, shielding, and NEMA protection needed. These considerations determine the type of materials and plating or finishes needed to withstand the environment and contain or shield radiation.

Another consideration may be shock and vibration, especially in factories and regions prone to earthquakes. Industrial cabinets with self-supporting welded frames, rubber mounts, and bracing pieces for additional support may be needed.

Overall enclosure dimensions dictate the closest standard catalog size available, and free space surrounding the cabinet works into calculations for selecting a cooling unit. Heat exchanger and air-conditioner equations include the exposed surface area of the cabinet -- enclosure bottoms and sides resting against walls and floors are ignored.

Intended use is important because some industries such as petroleum and food processing enforce rules and regulations concerning materials, finishes, paint, seals (for fluids and radiation), and explosion-hazard factors. For example, steel cabinets are a must for shielding against EMI/RFI, and many food and beverage-processing plants mandate stainless-steel enclosures. Some installations require fiberglass composite or polycarbonate enclosures.

Another issue crops up especially around local-area networks, telecommunications equipment, medical instruments, switchgear, machine tools, telephone links, and optical links. Many of these products are regulated by local, national, or universal standards, and special features may already be incorporated in the cabinet. Custom panels, interfaces, backplanes, and software that meet the requirements may be available.

Stick to standard racks and enclosures as much as possible to keep costs down. What might be a custom job for one enclosure manufacturer, may be a standard catalog item for another. Standard enclosures, climate controllers, and accessories typically are available off the shelf or within two to three days. Modifications, however, may take four to six weeks.

Three organizations have published electrical enclosure standards for U.S. industry.

Underwriters Laboratories Inc. (UL) standards for electrical enclosures formerly included requirements for just one enclosure type for indoor applications, and two enclosure classifications for outdoor applications. Other UL standards are for various hazardous locations defined by the National Electric Code.

The National Electrical Manufacturers Association (NEMA) enclosure standards include a series of classifications designated by arbitrarily assigned numbers. The enclosures are intended for use in specific industrial environments to provide both mechanical and environmental protection. The types are defined by a statement of the intended application, performance tests, and specific design features. Over the years, the number of classifications has expanded to include 18 different types.

The now defunct Joint Industrial Council (JIC) left its mark by establishing many of the specific design features that still exist in many Type 12 enclosures, pushbutton enclosures, "JIC" boxes, etc.

As a result of efforts to reach concurrence, UL and NEMA enclosure standards now include many of the same types. The exception is Type 5, which at one time was superseded by Type 12, but has since been reinstated. Although the requirements for NEMA and UL are nearly the same, at least two significant differences still exist.

Both concern the degree of corrosion protection required. For most enclosure types rated for outdoor applications, UL requires one of several alternate methods of corrosion protection. NEMA, on the other hand, requires the same protection as specified for indoor enclosures. (NEMA's logic is that additional finish requirements may be imposed by others as appropriate for each intended environment.)

Another difference is that UL does not differentiate the corrosion protection requirements for Types 4X and 6P from those for other outdoor enclosures. However, NEMA requires a greater degree of corrosion resistance equivalent to Type 304 stainless steel when an enclosure is subjected to a 200-hr salt spray test.

Enclosure standards originating in other countries are important to U.S. firms involved in foreign trade. Examples are the standards published by the Canadian Standards Association (CSA) and the International Electrotechnical Commission (IEC).

CSA functions similarly to UL. It is a nonprofit, independent organization comprising representatives of a wide range of occupations formed to develop industrial standards. CSA offers product testing and certification along with follow-up checking. Certification by CSA is increasingly necessary to market products in Canada. IEC is an international agency that publishes industry standards widely accepted in Europe and other parts of the world.

The IEC has no enforcement powers; certification is performed by each nation's testing agency.

Modular enclosures: Many electronic enclosures now come in what is called a modular format. Large cabinets are made by simply joining smaller units together. Shipping costs are reduced because the enclosure units are handled easily, which also reduces damage claims and labor costs.

Electrical equipment is usually mounted to a back panel while electronic controls and instruments can be mounted in several ways. Electronic components can be mounted to racks, placed on shelves, or on roll-out surfaces. Other accessories include writing surfaces, print pockets, drawers, casters, levelers, and eye bolts for lifting.

Some enclosures also are sealed well enough to qualify for NEMA-4, 12, or 13 ratings, providing indoor and outdoor protection from falling, dripping, or spraying noncorrosive liquids. Most enclosures have a NEMA-1 rating, providing protection only from contact with enclosed equipment and, to some degree, against falling dirt. Some modular cabinets use welded frames made with a 14-gauge, fourfold beam. Enclosures can be bolted together side-by-side for larger cabinets.

Cabinets for electronic components are 19 in. wide, recognized as an international standard. Frames are generally 11-gauge steel and limited to side-by-side mounting with few internal options.

One UL-approved line of welded steel multidoor cabinets now provides removable side panels to allow union with other cabinets. Designers can build large enclosures many meters long that meet UL and NEMA specifications by bolting enclosures together, and sandwiching a neoprene seal between them. Consoles are also available in different widths and basic unit shapes that can be mixed and stacked in many combinations. Typical are a base unit, consolet (writing surface), and top unit (similar to base unit, but with an inclined front door). Base and top units include provisions for optional accessory mounting. Panels, shelves, chassis guides and drawers as well as 19 and 24-in. rack-mounting angles from a standard line of options allow specialization of the cabinets.

Rather than welding the corners of the cabinet frame, the uprights in other types of cabinets are tabbed to fit into slots in the cabinet top and base. Corners are then bolted for a positive connection. The bolt-together approach also means that if a side brace is damaged, it can be removed and replaced.

Some cabinets offer small and medium heights, from 6U to 34U. The U is a height unit of 1.75 in. and reflects international standards. These smaller cabinets are available in three depths, and a 440-lb load capacity.

Cabinets are also available in heights from 25U to 43U, two widths, three depths, and a 1,300-lb load capacity.

Other enclosures are made of fiberglass and come in three sizes, each 5.31 in. high. Nonmetallic construction affords greater resistance in corrosive atmospheres than ferrous metal enclosures. Units are intended for industrial on-the-wall electrical installations that must be weatherproof and expandable. Enclosures have NEMA-3R, 4, 12, and 13 ratings.

Units can be joined together on all sides quickly with fiberglass wedges inserted into corner brackets, or flanges. Slides are removable, and available in a blank style or with a variety of knockout patterns for optional fiberglass cable glands. Also available are watertight cable entry grips with strain reliefs for all sizes of knockouts.

With smaller boxes ganged together, electrical systems can be designed into compartments. The watertight seal is achieved with a polyurethane foam gasket on all components.

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