Electronic Design

What Should You Look For When Specifying An Electronics Enclosure?

Standards Compliance, Ruggedness, Aesthetics, And Accessibility Must All Be Weighed Against Cost In The Final Analysis.

The electronics market is constantly evolving. PC clock speeds are rapidly approaching 500 MHz. Telecom bandwidth is increasing to handle voice, video, and data. Both changes are impacting electronics packaging through their need for higher power, more efficient cooling schemes, and tighter shielding from the negative effects of electromagnetic interference (EMI) and radio-frequency interference (RFI). As a result, systems designers looking for optimum enclosures are challenged daily as they balance performance requirements against cost and manufacturability.

The word "enclosure" means different things to various market segments. It could be an outdoor enclosure or an indoor cabinet. Or, it could also be a rack-mounted microcomputer system secured inside either of the above. Each of these has its own unique features and functions to fit a specific application and environment. Still, they must all provide the desired level of protection for the enclosed electronics (Fig. 1).

There are several steps in specifying an enclosure. First, determine if it should conform to a standard mechanical format described by industry or international specifications. For example, the 19-in. mechanical format used for indoor cabinets and subracks is defined by the International Electrotechnical Commission (IEC) 297 provision, while its metric equivalent is defined by IEC 917. There also is an IEC qualification under development for outdoor enclosures (IEC SC48D).

Other specifications exist as well. Telcordia Technologies, formerly known as Bellcore, has its own set of guidelines for network equipment building systems (NEBS) in the U.S. telecommunications market segment. NEBS-compliant enclosures are usually custom made for particular applications. The European Telecommunications Standards Institute (ETSI) also has its own set of enclosure conditions for public and private network installations in Europe. While many enclosure suppliers offer standard products conforming to the IEC regulations, special applications may warrant a custom-engineered mechanical format.

Defining the electronics layout inside the enclosure is the next critical step. For outdoor enclosures and indoor cabinet enclosures, define the size, location, and weight of the rack-mounted equipment. Include the power supplies, fan trays, and cable management. This information will help determine the overall size and type of enclosure needed.

The approach for defining the elec tronics layout is different for microcomputer enclosures. The primary concern here is the mechanical format of the subrack, or card-cage area. The backplane bus structure often defines the subrack's mechanicals. For instance, VME and CompactPCI both require the Euroboard mechanical format governed by IEC 297-3. The location and size of the power supply, fans, and cable management, combined with the subrack, should define the system enclosure's overall size.

Selecting the proper structure and material is another critical step. It depends on the application's environmental requirements. Outdoor enclosures are typically constructed of aluminum, desirable for its light weight and corrosion resistance. EMI/RFI shielding and shock/vibration requirements often determine indoor cabinet enclosures' structural design and material. Welded-steel cabinets are usually used for high static and dynamic loading conditions. They can be found in indoor cabinet applications that have to meet the seismic requirements of NEBS Level 3 (GR-63-CORE, Earthquake Zone 4). Euroboard mechanical packaging is recommended for microcomputer systems in shock and vibration environments. (For more on cabinet materials, see the sidebar.)

EMI/RFI concerns require the enclosure to act like a Faraday cage, which surrounds electronics and protects them from external EMI. The shield also should prevent the enclosed electronics from emitting RF signals that could interfere with adjacent equipment. Welded-steel cabinets that use EMI gasketing are common in EMI/RFI-shielded applications. This is because of their increased stiffness with reduced seams, which minimize RF-signal leakage and penetration.

To determine the enclosure-shielding performance, consider the range of frequencies over which the product must be tested for radiated emissions and immunity. Ascertain the highest clock rate or frequency generated in the system to prepare for the worst-case encounter, with emissions up to the tenth harmonic. A 300-MHz computer may need to be tested up to 3 GHz. Telecom applications refer to the NEBS Level 3 (GR-1089-CORE) specification for its range of test frequencies. Others may refer to MIL-STD-285, or be application-specific (Fig. 2).

Viewing windows in enclosure doors and panels should be made from a metal-impregnated acrylic material, or contain a transparent, thin-film conductive coating. All enclosure openings for intake/exhaust air flow and cable management must be shielded at a level dependent on the system's frequency range. Honeycomb EMI filters are common on air intake and exhaust areas. Cable management and I/O panels are problem areas for RF leakage and penetration. Shielded connectors attached to conductive surfaces will improve the shielding effectiveness in these suspect areas. So will shielded cables or ferrite beads around cables.

Standard products with integral shielding are also being offered by leading-edge suppliers. Conductive plating is being applied to covers, horizontal rails, and aluminum side plates. EMI gasketing is being applied between all mating parts, including front panels on circuit boards. There are tradeoffs between shielding at the cabinet level or microcomputer level, depending on price, performance, and aesthetics.

The power levels routinely depend on the system-level requirements. In the cabinet enclosure, the concern for power is mainly focused on the location and style of the power-inlet area. A managed-distribution scheme is defined throughout the cabinet to all rack-mounted equipment for power and ground. System-power monitoring also is important to determine any fault conditions within the enclosure. For microcomputer systems, the backplane bus structure, fans, and disk drives define most of the output-voltage requirements. The input voltage depends on the application: −48 V dc is common in telecommunications, while 125 V ac is common in the commercial-electronics market. Power supplies come in many different shapes and sizes, depending on the input/output voltages and EMI-filtering requirements.

The commercial electronics market has traditionally used fixed, frame-type power supplies. But today's trend is toward plug-in power-supply modules, which are easily accessible and serviceable from the front of an enclosure. Telecom applications demand high reliability with N+1 redundant power supplies that are hot swappable for fast replacement in the event of a power-supply failure.

Higher-power requirements demand increased cooling. Thermal management is arguably the most important aspect of the enclosure design to ensure the reliable operation of its integrated electronics. Outdoor enclosures deploy heat exchangers, air conditioners, and removable solar shields to handle extreme environments. Indoor cabinet enclosures use many different forms of cooling.

Natural convection cooling is common in server cabinets, which customarily are low-power applications with perforated front/rear doors and top covers. Forced-air conduction cooling, however, is the most common form of thermal management for indoor cabinet enclosures and microcomputer systems. Some applications deploy evacuation, or "pull," cooling. Others use a "push" system. And, some combine the push and pull systems with air movers at the enclosure's intake and exhaust ports. The high reliability required by telecom systems generally demands that redundant, plug-in fan trays, which are hot swappable, be used.

Alarm monitoring for notification of fan failures also is very common in telecom systems. Air filters, mandated by telecom companies, must meet the NEBS Level 3 (GR-63-CORE) standards for airborne contaminants. Thermal-simulation software is available to accurately model electronics systems to determine cooling requirements.

Other important enclosure items to consider have to do with ease of movement and setting the enclosure in its final resting place. High-strength eye-bolts for lifting are commonly available at the enclosure's top four corners. Castors are provided for moving the enclosure without lifting. In some applications, removable castor assemblies are used to move it into position. Then, they're detached.

Also, ganging kits are available for joining enclosures side by side. Mechanical fixing of the enclosure to the installation site can be accomplished by providing an add-on base equipped with retractable castors and leveling feet. There's a standard floor-mounting pattern for telecom enclosures defined in NEBS GR-63-CORE. Anti-tip brackets are common for fixing server cabinets to the floor. More importantly, they prevent enclosure tip-over from telescoping heavy equipment.

While it may sound like you need an experienced packaging engineer on staff to specify the right enclosure for the application, a good idea is to partner early with a leading-edge supplier.

References:
Vitale, Pat, "Fitting Shielded Enclosures Into the Solution," Evaluation Engineering Magazine, p. 52, Feb. 1999. *

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