Enclosures—Think Outside the Box

Considering the globalization trend of industry, a completed control system may be shipped anywhere in the world. To discover what users want and need in enclosures, Control Engineering surveyed 1,500 readers. Of the 465 responses, 50% work in process industries while 39% identified their primary application as discrete product manufacturing.

07/01/1998


Considering the globalization trend of industry, a completed control system may be shipped anywhere in the world. To discover what users want and need in enclosures, Control Engineering surveyed 1,500 readers. Of the 465 responses, 50% work in process industries while 39% identified their primary application as discrete product manufacturing.

More than 48% of those responding specify, recommend, and/or purchase enclosures for in-plant requirements. Another 40% are involved in OEM (original equipment manufacturing) applications.

Availability most needed

Availability from stock was rated the most important characteristic by 81% of the respondents, with ease-of-use second (74%), and cost third (70%).

Interestingly, while ability-to-customize was ranked the fourth most important characteristic (64%), respondants purchased customized enclosures only 26% of the time during the past 5 years, and see no significant change in the next 5 years.

Modular enclosure design is viewed by manufacturers such as Hoffman (Anoka, Minn.) and Rittal (Springfied, O.) as a way to meet customer requirements.

Modular enclosure systems offer initial and retrofit flexibility when placing control systems on the plant floor. Modular enclosures are based on a frame and skin concept for creating a range of enclosure possibilities around a single platform. Once a frame size is determined, door options, side-, back-, top-, and base-panels are selected to fit the application.

Modularity is not confined to enclosures. Rittal offers a modular power distribution system consisting of busbar holders, busbars, fused disconnect switches, busmounted fuse bases, connection systems, and system covers. Widely utilized in Europe and the Pacific Rim, modular power distribution systems are gaining North American acceptance. Two major advantages to modular power distribution systems, is the 30% footprint savings it can provide, and its inherent "touch safety" provided by covered components.

EMI/RFI protection important

User-developed enclosure specifications indicate an increasing awareness of the importance to protect electronic equipment from stray electromagnetic/radio frequency interference (EMI/RFI). Led by recent European requirements for systems to be electromagnetically compatible (EMC) with one another has prompted enclosure manufacturers to introduce enclosures that feature electromagnetic shielding.

Enclosures designed to provide EMC protection utilize a combination of plating, shielding, conductive gasketing, and latching technologies. Rittal recently converted from painted to zinc-plated mounting panels and components to improve EMC protection.

Locating control system electronics on the plant floor requires two protection considerations. Electronics must be protected against the harsh elements of the plant floor (i.e., dust, moisture, heat, cold, etc.); and where explosive gases or dust are present, the environment must be protected against the electronics serving as an ignition point.

In North America NEMA, UL, and CSA ratings are used to specify enclosure protection requirements against dust and moisture. For example, NEMA type 4X enclosures are intended for indoor or outdoor use to provide a degree of protection against corrosion, windblown dust, rain and snow, splashing and hose-directed water, and should remain undamaged by the formation of ice. But, in other parts of the world, NEMA, UL, and CSA are not the recognized compliance agencies. Currently the IEC (International ElectrotechnicalCommission) standard 529 defines ingress protection (IP) classes for enclosures in the form of two digit numbers. The first number identifies the protection against solid objects and the second number identifies the protection against liquids.

Another European standard for enclosures, EN 60439-6, is used to define enclosure protection against mechanical impacts (IK values). Efforts are underway to establish a single set of international enclosure standards but for now, users specifying enclosures for different world areas must rely on cross-reference tables that are available in the technical information section of most enclosure catalogs.

Thirty-two percent of the time, enclosures are being placed in areas classified as "hazardous environments." Like the NEMA and IP consolidation efforts, the NEC (National Electric Code) and CEC (Canadian Electrical Code) electrical classification ratings are being reviewed to achieve international standardization. Where the current North American (NA) system uses Class and Division, the European system uses Classes and Zones. The major difference is that Division 1 is divided into two Zones (0 and 1). Division 2 and Zone 2 are basically the same. The 1996 NEC (Article 505) and the 1998 CEC system allow NA industry to select how they classify hazardous areas. Companies doing business around the world can use this opportunity to standardize on the same equipment and installation standards, regardless of where they build the facility.

Regardless of the method used to classify a hazardous area, methods of protection remain the same: intrinsic safety barriers, explosion proof enclosures, and purging technologies.

Intrinsic safety limits the amount of energy a particular electrical device can generate or store, typically in the 5 to 24 V range. Control systems seldom fall in this category.

Explosion proof enclosures are designed to contain an explosion, not pre- vent one. They are generally constructed of cast iron or aluminum, and provide limited access usually rendering them unsuitable as control system enclosures.

Purging technologies are frequently used when installing a control system in a hazardous environment. The control system enclosure is usually rated NEMA 4 or 4X and is fitted with a means of providing a continuous flow of inert gas or instrument air which pressurizes the enclosure and prevents the intrusion of explosive substances. A pressure switch is installed to monitor for enclosure pressurization. Enclosures installed in areas assigned a Class I - Division 2 rating, require a Type Z purge system where the pressure switch generates an alert when enclosure pressurization is not present. Enclosures installed in Class I - Division 1 areas require the additional protection provided by Type X purge systems. The loss of enclosure pressurization requires an interruption of all electrical power to the enclosure. Before electrical power can be reenergized, the purge system must perform a "rapid exchange purge" in which four volumes of air are passed through the enclosure.

Heat is an electronic system's worst enemy. Studies have demonstrated that electronic reliability decreases 50% for each 10 8F the electronics are operated above their upper temperature design limit. Since heat is so destructive to electronics, a process engineer's knee-jerk reaction often is to "throw a lot of cooling" into an enclosure. However too much cooling can create condensate problems. The Genesis line of air conditioners from McLean Midwest (Champlin, Minn.) provides a condensate management system, eliminating the need for external drains or an optional condensate evaporator.

Enclosure manufacturers are able to offer a variety of cooling options that can be grouped into one of three categories.

  • Thermal/convection cooling is accomplished by heat being dissipated through the skin of the enclosure, using a heat-sink arrangement, or by some form of fan/filter system.

  • Near-ambient cooling is achieved using active air-to-air heat exchangers. This is a medium-cost solution and should be considered when the outside ambient temperature is 10 8F below the maximum operating temperature of the electronics to be protected.

xBelow-ambient cooling can be accomplished by traditional air-conditioner systems, solid state air conditioners, water-to-air heat exchangers, and vortex-style air coolers.

The use of thermoelectric cooling, or "solid-state-cooling," is gaining in popularity. These coolers are suitable for washdown, feature a smaller footprint than conventional air conditioning systems, and with no compressor, evaporator, or condenser, result in less maintenance. Solid state cooling uses dissimilar conductors of doped Bismuth Telluride to replace "refrigerant." The compressor is replaced by dc power, supplied from an internal ac to dc power supply, which "pumps" electrons from one semiconductor to the other. The use of heat sinks and fans, both inside and out, replaces the "condenser" fins and enhances the cooling effect to achieve the required cooling.

Diverse deployment of electronic devices increases the need to provide protection against dynamic stress influences such as shock, including earthquakes, and vibration. Dynamic stress can be divided into short- and long-term stress. Short-term stress results when shock conditions are encountered in shelters or machines. Long-term stress occurs during stationary operations when machines are in the vicinity, during periods of transportation, and during intermittent disturbances such as when generators stop and start. Mission critical electronic installations, such as hospitals, airport operations, police and fire dispatching systems, and mass transit systems should consider enclosures that can minimize the stress influences of shocks and vibrations. Hoffman and Rittal offer enclosure shock and vibration damping solutions. Electronics Corp. (Aurora, Ill.) and Hoffman each manufacture enclosures designed to withstand the most severe (Zone 4) earthquakes.

Control systems are changing in a host of ways. Enclosure manufacturers are working hard to keep up with the changes and to have solutions ready when users need them.

VXI mainframe enclosure

Wallingford, Conn. —Designed for "plug and play" applications, the VXI chassis features a 13-slot configuration and can be mounted in a 19-in. rack. Designed to be FCC Class A and Class B compliant after integration, the VXI is also UL/CUL and CE (low voltage directive) approved. The 1,600 W power supply ensures that the maximum current cited in the VMIbus specification can be met. Vero Electronics Inc.

Modular electronics housings

Harrisburg, Pa. —Provided with DIN rail mounting, metal latching feet, pluggable connections, earth ground contact, ventilation for heat dissipation, and full or partial PCB removal, the Smart Housing provides OEMs a pre-engineered enclosure for custom printed wiring boards. Phoenix Contact

Adapter kit saves space

Rochester, N.Y. —Designed to increase an enclosures capacity for cabling and wiring, this kit allows mounting of 19-in. EIA rails in a 24-in. enclosure. The extra space is available for cable intensive applications, such as telecommunications and networking, to safely house components and cabling in the same enclosure. EMCOR Products

New flat-panel workstation

Atlanta, Ga. —When installed with the optional "Flex-Arm" assembly for wall, column, or pedestal mounting, the Profile Series of flat panels can be pushed around to meet varying operator requirements. Featuring an active matrix display with 1,024 3 768 resolution, standard video interface with touchscreen, membrane keyboard, and rated NEMA 4/4X, this workstation is designed for harsh, factory floor assignments. Pro-Tech

ICE/Station PC enclosures

ICE/Stations (Internally Contained Environment Stations) have become the benchmark by which all other industrial PC enclosures are measured, according to Integrated Technology. Rated NEMA 12, 4 & 4X and UL listed, ICE/STATION enclosures protect against dust, dirt and splashing fluids. With an extensive list of standard features, heating and cooling options and accessories, ICE/STATION enclosures meet or exceed any requirements. All ICE/Station enclosures are shipped fully assembled and are Guaranteed Forever against defects in materials and workmanship. Integration Technology (ICE/Stations)

CompactPCI packaging

Designed to comply with the IEEE 1101.10 mechanical standards, the Vanguard chassis features hot-swapable power supplies suitable for operation world-wide. The 10-layer backplane construction allows for low crosstalk and consistent impedance. Layers dedicated to power and grounding improve distribution. Electronic Solutions

Hostile environment enclosures

Elkridge, Md. —Available in polycarbonate, ABS, or polyester, the CAB series of wall mounting cabinets are manufactured in thirteen sizes. Internal doors of aluminum or plastic can be hinged on either side or mounted with quick turn screws. The internal doors allow push buttons, displays, and switches to be mounted inside. Fibox Inc.

New PC industrial enclosures

Springfield, O.— Rittal has designed a new series of enclosures with a new rounded-front design to create a contemporary look for manufacturing and office environments. They are a bullet-style latching system, fold-down or pull-out keyboard drawers and an optional consolette. The half-height version can be placed on a table. Both the full-size and half-height units provide NEMA 12 protection, even when the keyboard housing is open. Rittal Corp.

Fieldbus component enclosure

Anoka, Minn. —Designed to accomodate components for popular device networks, including DeviceNet, Profibus, and Seriplex, Inline enclosures are available in a variety of sizes and materials, including Type 304 and 316L stainless steel. The DIN rail mounting system simplifies component installation. Hoffman

Fault-tolerant PC enclosure

Vista, Calif. —Equipped with a 20-slot ISA passive backplane, redundant 300 W ac hot-swapable power supplies, filtered hot-swapable cooling fans, and washable filters the FTU-1 rackmount PC enclosure is designed for mission critical applications. Available with a variety of drive bays, the FTU-1 can be fitted with a RAID controller. SBS Micro Alliance

NEMA 4 flat-panel browser

Medfield, Md. —Featuring a 13.8-in. active matrix display with 1024 3 768 resolution and 256 colors, the 6800 Browser workstation can survive the shock (upto 10 Gs) and vibration (up to 2 Gs) of plant floor applications. Features include an aluminum housing, epoxy finish, tilt/swivel mounting, silicone rubber covered keyboard, and on-board Ethernet communications. Comark Corp.



When to consider a custom enclosure

Anytime the word "custom" enters a conversation, the first thought is "higher cost" but according to Dick Dooley, vice president of Panelmatic (Fairfield, O.), free-standing, custom enclosures with two or more doors typically cost no more than pre-fabricated enclosures, and may actually cost less.

Custom enclosures offer an almost countless number of options that may be difficult to obtain in a standard, pre-fabricated enclosure. For example, different metal thickness can be used to accommodate special mounting requirements; where large or several cutouts are clustered together, special stiffeners can be installed; hidden or lift-off hinges can be used; special latching mechanisms can be added; doors can be designed to accommodate restricted swing spaces; and unique sub-panels can be incorporated. Many standard enclosure manufacturers offer customizing services, however this is not what they do best and some "customized" services are likely to cost more and extend delivery times.

When the available floor space is an odd size or configuration, a custom enclosure can be designed to take full advantage of the space; something even the most modular pre-fabricated enclosures may be unable to achieve.

Regarding enclosure standards and certifications, Mr. Dooley says, "High quality custom enclosure manufacturers are able to construct enclosures to meet the same standards and certifications as prefabricated manufacturers. Meeting UL, NEMA, CSA, and IEC standards or providing union constructed labeling is provided when that is what the customer requires."

A recent example of where a custom enclosure was best suited for the application was demonstrated when the 5,300 year-old Man of Similaun went on display in Bolzano, Italy. Discovered trapped in a glacier in the Otzaler Alps in 1991, the frozen mummy—a hunter of the Neolighic period—will be displayed in a dual-redundant environmentally controlled chamber, designed by Solartron (Allentown, Pa.).

Most applications are less complex than protecting a 5,300 year-old human, but custom enclosures can meet requirements that standard enclosures cannot and should remain a legitimate part of control system enclosure considerations.



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