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 challenge, but set that equipment in motion and the degree of difficulty rises exponentially. Enclosure and console solutions for mobile applications, like those described above, require some special layout and design considerations not associated with stationary operations. Decision-makers should carefully examine many factors when planning enclosure and console solutions for mobile applications in fields such as broadcast, security, test and measurement, and datacom/telecom. While every scenario is different, failing to heed any one of these general tips may be a detriment to safety and productivity.
Project Footprint
Simply put, the larger the space the better. Unfortunately, mobile data centers are often in tight spaces and require a great deal of layout planning prior to installation of the enclosures and electronics. In doing so, it is best to work backwards, first accounting for the space needed around the enclosures in order to determine what size cabinet will be feasible. Doing so will help avoid mistakes on the front end as opposed to realizing that the footprint has to be redesigned due to inadequate space once everything is installed. Some important questions to consider include:
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Will there be adequate room for cabling? The size of the enclosure and its placement must allow sufficient space to run cabling either behind, above, between, or below the enclosures.
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Will there be adequate room to access important parts? Imagine how the equipment will be used once installed. Make sure there are adequate access points for things like switches, power management, cabling, cooling fans, and anything else that may need to be accessed. This includes adequate aisle space for a person to perform the anticipated tasks as well as adequate space for swinging doors and panels to fully open, unobstructed.
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How will the enclosures get inside the vehicle? In mobile settings, it is common for the project footprint to be larger than the door to the vehicle (imagine a submarine hatch). In this instance, a bolt-together solution will be necessary, and it will be important to ensure there is adequate space for assembly once inside the vehicle.
- Once the necessary space around the enclosures has been allocated, their appropriate size and placement can be determined. When considering placement, the structure of the vehicle and its framing should be taken into account.
Center of Gravity/Weight Load
Center of gravity is not particularly important in a stationary environment, but when in motion, a high center of gravity could mean catastrophic damage to equipment due to instability. Generally speaking, the lower the center of gravity in a mobile application, the less likely it will be to tip over.
One way to reduce the likelihood of tipping over would be to bolt multiple enclosures together. By securing the enclosures together, the combined footprint of the enclosures becomes wider, thus making them more stable versus each enclosure standing independently.
Because weight shifts while in motion, so does the center of gravity. In order to keep the center of gravity stable, the enclosure should have multiple tie-down points, each on a structural member of the vehicle as opposed to the sheet metal or plastic that lines the walls. This is the same concept as attaching a shelf to a wall in a house. It’s best to attach the shelf to the wall where there is a stud, rather than just the drywall. In a static environment, the drywall would most likely hold the shelf, but imagine the house driving over hills and rocks, and the likelihood of the shelf ripping out of the drywall becomes much higher as its weight shifts.
Weight load capacities for enclosures are generally measured in static environments, but weight in motion can shift suddenly. The internal pressure on a shelf caused by 400 pounds of electronics in motion could be many times greater than the weight of the electronics in a static environment. If the vehicle will experience rapid acceleration, deceleration, and turning, an enclosure with a higher weight load capacity would be the smart choice.
In addition, any sliding chassis, drawers, or doors should have locking features so that they can be secured in place (whether in or out) while being accessed. Going around a curve or hitting a bump could cause a drawer to slam shut or come flying out and injure someone.
Shock and Vibration
While shock and vibration in a stationary environment often equates only to the mild and consistent vibration from cooling fans and internal equipment, a mobile environment generally requires a much more substantial solution for absorption of shock and vibration caused by the vehicle being in motion. The first step in determining what is needed is to estimate the consistency and intensity of the shock and vibration to which the mobile data center will be subjected. This will vary dramatically based upon the intended use for each application and the type of vehicle in which the data center will be housed.
Situations of consistent and low-intensity vibration could require something as simple as rubber washers inserted between steel members. Sudden and intense shock, like that of equipment attached to a high mobility multipurpose wheeled vehicle (Humvee), could require several inches of shock isolation cables from bracket to bracket on the inside and/or outside of the enclosure. At the very minimum, even in the mildest mobile environments, locking hardware should be employed to keep everything fastened tight. If even one fastener rattles apart, it could create a devastating domino effect.
While every scenario is unique, the military has created standards requiring electronics to be cushioned by isolators that meet certain attenuation levels. Although the military standards were created specifically for Department of Defense (DoD) applications, they are often used as guidelines for testing and rating electronics and enclosures for commercial and industrial use. Some of the more commonly used standards are MIL-S-167 for vibration and MIL-S-901 for shock. They are both meant as guidelines for protecting electronics on shipboard applications but are applied to other mobile applications in commercial and industrial settings.
Environment
As with stationary data centers, mobile data centers can be subjected to harsh environments, and protecting the electronics from the environment is an important consideration. What makes a mobile application unique is that the environment to which it is subjected is ever changing. The enclosure must withstand every type of environmental scenario it could possibly encounter while in transit. There are a number of standards that can be used to rate an enclosure’s ability to withstand various environmental conditions.
In the United States, the National Electrical Manufacturers Association (NEMA) provides a set of standards by which to rate enclosures on their ability to keep foreign objects from getting through the enclosure and into the electronics.
As demonstrated by tables 1 and 2, NEMA ratings are only in relation to solids and liquids entering the enclosure. There are, however, other factors associated with environment, including but not limited to electromagnetic interference, temperature, altitude, solar radiation, and humidity.
Electromagnetic interference (EMI) is common in military settings, and even the slightest interference levels can destroy or impede the performance of electronics. The Institute of Electrical and Electronics Engineers (IEEE) has put forth a standard by which to rate enclosures for their ability to withstand varying levels of EMI, from 9 kilohertz (kHZ) to 18 gigahertz (GHz). An enclosure should meet IEEE Std 299-2006 when it is anticipated that the electronics could encounter EMI in their environment. EMI can be caused by such events as nuclear explosions, power line surges, lightning, and any other sudden surges of power and/or changes in radio frequency.
Many other environmental factors are encompassed by a broad set of testing standards under MIL-S-810. Such factors include high and low temperatures, rapid temperature fluctuation, humidity, solar radiation, fungus, and various corrosive environments tested by salt fog, and acoustic noise.
Because environmental factors can rapidly change in a mobile application, it is important to anticipate what the data center will be subjected to and test the equipment and enclosure according to the applicable set of standards.
Cabling
Protecting cabling in a dynamic environment requires some special considerations when compared to a static environment. The vibration and movement can cause cables to chafe against surfaces, which can lead to damage to the cable sheathing and may eventually cause them to short out.
When cables are being run through a cabinet, they should be tied down with a strain release at the point of entry to prohibit them from moving around and scraping against the metal edge. Any sort of metal edge that the cabling could come into contact with needs to be covered with plastic edge protection. Additionally, the cabling should not run across any structural member of the vehicle to which the enclosures are tied. Doing so could cause friction between the cabling and the tie-down materials, and thus, damage the cabling.
Bottom Line
Every scenario is different. Each market has its own standards and regulations, every brand and type of electronics are built to withstand different levels of abuse, and every vehicle has a different layout and will be subjected to different environments. In order to determine what is needed to protect a mobile data center, it will first be necessary to determine every possible environmental scenario the data center could encounter while in transit, and what standards and regulations will apply. As a set of generalities, the five factors are intended to guide those designing a mobile data center take the necessary steps to protect their electronics from the rigors of a mobile environment.
