Survival of the Fittest

>From the cold tundra of Alaska to the sweltering South, outdoor enclosures >must withstand the harshest weather and last, unattended, for years. >Outdoor conditions vary greatly, but some wireless carriers install the >same type of enclosure at all sites across the nation. The problem with a >uniform design is that some enclosures either end up over-designed to >handle the harshest weather, which is an unnecessary expense in mild >climates, or they are under-designed and can't withstand the worst >possible conditions.

To ensure long-term durability for your equipment, you need to consider the regional climate along with the localized weather extremes that your site will encounter such as a thunderstorm, a hurricane or debris propelled by high straight-line winds.

CLIMATE CONTROLBecause enclosures protect your valuable equipment, a consistent internal environment is crucial. Outdoor cabinets have become fully controlled "environmental chambers" capable of withstanding most anything the outdoors has to offer. This internal environment must maintain uniform temperature and humidity. The equipment inside generates heat, and this heat needs to be extracted from the cabinet in warm climates and used as supplemental heat in colder climates.

Air conditioners, heat exchangers and heaters with temperature sensors are located in strategic areas of the cabinet to control heat, cold and humidity. Air conditioners are necessary when the internal cabinet temperature needs to be below the outside ambient temperature. Heat exchangers extract heat when the internal cabinet temperature is higher than the outside ambient environment. Air conditioners are perceived as higher long-term maintenance items, and the industry trend is to use heat exchangers when the application warrants. Double-wall construction and insulation also control temperature and humidity inside the cabinet and assist in buffering rapid outdoor environmental swings.

You also should consider the effects of sunlight on the cabinet's internal environment, especially in roof-mounted applications in direct summer sunlight or those located on the north side of a building. The solar gain could create a few hundred watts of additional heat. The exterior finish should be UV resistant. Lighter colors are preferred, as solar heat gain is directly related to the cabinet's outside color.

In situations such as hurricanes, the enclosure's gasket system is important. The gasket must be seamlessly designed and remain in intimate contact with its mating surface to ensure a watertight cabinet. Hurricanes have been known to create a vacuum, causing gaskets to lose contact. In one field installation, the enclosure leaked water on the backside of the cabinet as the wind pulled at the door. The door facing the high wind-driven rain actually increased the gasket contact from the wind force. To maintain a seal, the sealing geometry should employ a flange-trough design and a knife-edge interface. The flange trough is a small gutter around the door, used to channel water away. The knife edge maintains even gasket contact around the door opening. Multipoint latching systems also ensure a strong gasket seal.

In some cases, enclosures are in earthquake-prone areas, and the fully equipped enclosure must withstand the rigors of a seismic event. According to Bellcore standards, such forces could reach 5Gs (five times the earth's gravitational force), which would destroy an unreinforced cabinet. Make sure your cabinets undergo a seismic-qualifying test to ensure survival in the field.

BEYOND WEATHERBesides climate extremes, you also must protect your interior electronics from other outdoor factors. Will your cabinet be in a remote location or in a public area? Each location has its own criteria to meet. In remote locations, a cabinet could become a target for bored hunters, so the enclosure's skin should endure a shotgun blast. In other rural areas, it must survive grass fires, insect infestations and animal entry. In public locations, the cabinet is a convenient perch for parade watchers and should endure the weight of a 200-pound individual.

In any location, vandal resistance is key. The cabinet should have few gaps between panels, and the handle/locking system should be tamperproof to discourage would-be perpetrators from prying open a door. The cabinet should not have any exposed fasteners, or you risk disassembly and unauthorized entry.

In some cases, a cabinet must be hoisted onto a concrete pad with a crane or onto a high-rise building's roof with a helicopter. The cabinet should be strong enough to bear its entire weight on one of its lifting points. Bellcore standards state that a cabinet should endure a lifting test of six times the cabinet weight to ensure that it is not dropped during installation. These requirements may sound extreme, but wind gusts during an installation could cause a hoisted cabinet to sway. All the weight could be intermittently subjected to one lifting point.

Another area of concern is the cabinet's EMI/RFI shielding effectiveness in both emissive and immune situations. In most situations, you can take care of the shielding at the board or sub-system level of the equipment. This shielding solution is more cost-effective than a 2-gasket system. In some cases, you need shielding at the cabinet level, which entails a conductive gasket material on the door mated to a conductive surface on the enclosure body, with intimate contact between them. Today's conductive gasket materials are not good for outdoor environmental seals and require a dual-gasket system -- one for shielding, surrounded by one for sealing.

Compartmentalizing your cabinets also can help protect your equipment. Typical cabinet configurations offer separate compartments for incoming power, wireless equipment and batteries. The cabinet's doors also are keyed or padlocked differently, allowing each tradesman to access only the devices pertinent to his area of expertise. For example, the battery-maintenance technician only can access the low-voltage section. Isolating each area of the cabinet also prevents a problem in one section from affecting other areas such as battery acid/gas leaks affecting the circuitry. The power and wireless compartments typically are sealed environments, while the ventilated battery compartment allows hydrogen gas to escape. Hydrogen gas is a by-product of battery charge and discharge cycles; gel-cell batteries typically do not out-gas.

SETTING THE STANDARDEstablished and proven governing standards provide guidance and standardization in determining proper outdoor cabinet performance. Among these standards are Bellcore GR-487-CORE (generic requirements for electronic equipment cabinets) and Bellcore GR-63-CORE (network equipment-building system, generic equipment-building requirements), which primarily are used for the mechanical, seismic, shielding and integrity-related enclosure attributes. UL 50 (enclosures for electrical equipment) entails the sealing and structural integrity. Among other criteria, these standards encompass corrosion resistance, shipping vibration, marking and labeling. The IEC has coordinated an international effort, which is in the final stages of development, to globally standardize outdoor enclosures and subsystems.

Don't take cabinet integrity for granted. The cabinet's performance and durability are just as important as the equipment inside. Without the proper protection from outdoor elements, you might find yourself with more downtime and less revenue-generating uptime.

To find the best environmental protection system for your sites, look for the following characteristics:

Reliable cooling in limited space. Equipment shelters have limited internal space. To ensure adequate cooling capacity regardless of size, look for shelters designed to ride out the hottest seasons in the smallest space. Look for features such as oversized evaporators and condenser coils to provide a high sensible heat ratio in virtually any structure.

Durability. To lower maintenance costs, your protection system should withstand exposure to extreme weather fluctuations. The assembled unit should be an airtight fit. All aspects of the unit, not just the cabinet itself, should resist corrosion.

Efficient, reliable operation. The system you choose to protect equipment should keep energy costs down, without constant maintenance. Some systems let you use ambient air cooling of the critical space when appropriate. Ask about independent efficiency rating of the unit and inquire if the protection system comes with any remote monitoring or control functions. Advanced features notify you of emergencies, allows maintenance crews to remotely check status, and balance runtimes to reduce system wear.

Easy installation and quick maintenance. Pre-assembled and pre-wired units go a long way toward easy installation. Youcan further simplify site wiring with built-in circuit breakers. The overall system's weight and the ease with which you can connect it to the nearest power source play a part in making the installation a one-trip, single-installer event.