– Eric Ziemer, design engineering technician at Crenlo (http://www.crenlo.com/), says:
In the aftermath of a bank robbery, a local TV news station provides live coverage by rushing a broadcast truck to the scene. Meanwhile, three states away, law-enforcement officials activate their mobile command center to re-establish communications and stability following a natural disaster.
Down South, an oil-field worker monitors productivity from a van outfitted with test-and-measurement instruments. Off the coast, a ship captain navigates volatile waters with the aid of state-of-the-art radar equipment.
Protecting vital electronic equipment in a static — or immobile — environment is a chal
lenge, but set that equipment in motion and the degree of difficulty rises exponentially. Enclosure solutions for mobile applications like those described require some special layout and design considerations not necessary with stationary operations.
There are many factors that decision-makers should examine when planning enclosure solutions for mobile applications in fields such as broadcast, security, test and measurement and datacom/telecom. Some of the most critical factors include:
1. Project footprint
2. Weight load/center of gravity
4. Shock and vibration
Neglecting to carefully consider these five elements in the design process could have serious repercussions, including an unsafe work environment, productivity losses and increased costs. Let’s explore these five factors and detail how they contribute to effective electronics protection in dynamic applications.
Bear in mind that every application is different and design requirements may vary greatly. Most standards and regulations associated with each factor are specific to electronic equipment and industry and will demand appropriate testing.
When sizing electronics protection for a mobile environment, bigger is simply better.
Examine floor-space and height constraints and maximize the footprint of the project. A larger footprint provides an enclosure or group of ganged enclosures with a stouter stance that helps minimize swaying and the likelihood of tipping.
While determining the largest possible project dimensions, however, do not overlook mounting, cable routing and access needs that could impact the size and location of the footprint.
Enclosures protecting equipment in a dynamic state need additional stability and should be mounted to structural members — not merely tied down to a surface that could tear, such as sheet metal. The need to secure enclosures to structural members could affect exactly how large and where the footprint can be configured.
Throughout the planning process, ensure that there is adequate space above, beneath and around the enclosure to route, connect and access cables, service equipment without inflicting damage and provide shock-and-vibration attenuation. Make certain that swinging panels and doors do not collide with each other, obstruct aisles or impede efficient maintenance.
Sometimes an entryway is not large enough to permit passage of a fully assembled enclosure to the project site. (Envision attempting to transport a cabinet through a submarine hatch.) Meet such a challenge by selecting a bolt-together enclosure that can be efficiently assembled inside the vehicle and still withstand all the environmental forces encountered in the application.
WEIGHT LOAD/CENTER OF GRAVITY
Enclosure weight-load capacities are usually determined in a static environment, so it is important to consider that a dynamic load exerts more force on a cabinet than a stationary load. Prioritize frame-channel construction and weld-joint design when seeking an enclosure, but take into account the vehicle’s weight capacity and restrictions when mulling cabinet strength and weight options.
When it comes to loading an enclosure for a mobile application, the question is: How low can you go?
Not a primary concern in a static environment, the center of gravity is a major factor in designing for a dynamic application. A higher center of gravity makes a cabinet more vulnerable to swaying and overturning, jeopardizing vehicle performance in the process. Therefore, it is critical to keep a dynamic load as low as possible. There is no universal standard for the location of a center of gravity — one application may demand situating the load below 30 percent of cabinet height, while loading below 60 percent may suffice for another — but in general, lower is better.
If an application requires additional stability after the center of gravity is bottomed out, add tie-down points. Extra tie-down points typically include the upper rear of a stand-alone cabinet and along a channel running across the top of ganged cabinets. High-grade fasteners are often ideal for attachment to structural members.
Lock-in and lock-out features will be necessary in a mobile scenario if a sliding chassis, drawer or door is utilized. These features hold a sliding chassis or component in place — either “in” or “out” on a track — enabling routine passage through aisles around the cabinet and safe, efficient electronic service behind the enclosure.
Cables require special attention in a dynamic environment, where they are more at risk of incurring productivity-stalling damage. Shock and vibration could cause chafing if the motion induces harsh interaction between cables and edges.
Anywhere cables meet an edge — such as the cables’ entry point into the cabinet — the cables should be tied down, and the edge should be conditioned with strain relief. Tying down cables restricts their movement, while incorporating edge protection such as a plastic grommet softens any friction that could damage a cable’s coating.
Sometimes cables are run through tie-down channels that brace the assembly to the mobile unit. If not, be wary of establishing cabinet tie-downs on structural members where they might interfere with crucial cable routing.
SHOCK AND VIBRATION
Before selecting an enclosure solution for a dynamic environment, conduct testing to establish the maximum amount of shock and vibration that the electronic equipment can sustain. Then, determine the shock and vibration inputs — pulse, vibration and G level — that will occur in the application.
Neutralizing shock and vibration in static environment is often simple, as many stationary environments include little more movement than that generated by internal equipment vibration and cooling fans. The attenuation challenge intensifies in a mobile setting. There, equipment must be protected from the substantial shock and vibration associated with, for example, a vehicle negotiating jagged terrain or a submarine bracing for a depth charge.
Satisfy shock-and-vibration attenuation needs and requirements — which vary according to application and industry — by selecting from a variety of vibration mounts and shock isolators. Vibration mounts such as rubber washers dampen high-frequency, low-magnitude vibration, while shock isolators like cable mounts counteract high-magnitude, low-frequency motion. Many applications also demand internal isolators, which provide shock absorption for equipment on the interior of an enclosure.
Constant shock and vibration threaten to loosen fasteners and cause them to fail, with potentially serious consequences such as injury and extensive equipment damage. Such danger makes locking hardware a must-have for virtually all enclosures in mobile applications. The failure of one small fastener could result in a major problem that locking hardware could have prevented.
Enclosures in dynamic operations often must protect electronic equipment not only from shock and vibration but also from the elements. Complicating matters, the elements cabinets must combat could change dramatically over the course of an application.
Prior to selecting an enclosure, identify all the environmental conditions that could adversely impact the performance of electronic equipment and prepare for withstanding each of them. Examples include water, salt, dust and extreme temperatures.
In the United States, the National Electrical Manufacturers Association (NEMA) publishes ratings of enclosures’ ability to protect against specific environmental conditions when completely and properly installed. These ratings can guide decision-makers to the enclosure type that best fits their application.
Each of NEMA’s 20 enclosure types is designed to protect against a different combination of environmental conditions. For example, a NEMA Type 4 enclosure is suitable for a nonhazardous location and is constructed “for either 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 the enclosure against ingress of solid foreign objects (falling dirt and windblown dust); to provide a degree of protection with respect to harmful effects on the equipment due to the 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.”
Other NEMA nonhazardous enclosure types are designed to provide a degree of protection against such conditions as oil and coolant seepage, corrosive agents and ingress of water (occasional prolonged submersion). NEMA hazardous enclosure types require shielding in atmospheres including the likes of acetylene, hydrogen, gasoline and metal dust.
No matter how mild or harsh the environmental conditions involved with an application, rely on NEMA ratings while also addressing thermal management needs.
Decision-makers in industries such as broadcast, security, test and measurement and datacom/telecom face unique layout and design challenges when generating enclosure solutions for mobile applications. Every application is different, with different standards and regulations for protecting electronic equipment. Choose a manufacturer capable of producing a tailored solution through engineering expertise and standard, modified and custom product options.
Although applications vary, by accounting for five general factors — project footprint, weight load/center of gravity, cabling, shock and vibration and environment — decision-makers will be better able to safeguard their electronic equipment from the rigors of a mobile environment while protecting their productivity, bottom line and safety record.