Whole lotta shaking goin' on

In the aftermath of the North- ridge Earthquake, which struck the San Fernando, CA area on January 17, 1994, people had no power, no gas, no water. But they could call 911.

The fact that Pacific Bell reported virtually no service interruptions after one of this century's worst natural disasters--6.7 on the Richter Scale--testifies to the ability of engineers to design telecom equipment to survive even violent tremors. Now, like seismic sleuths, they're using their experience to design equipment for other environments that shake, rattle, and roll--including over-the-road transit and mobile communications.

Shock stoppers. "Seismic enclosures protect the electronics from harsh environments and shock conditions, enabling telecommunications to continue uninterrupted and data transmissions to remain intact," says General Devices Corp.'s Ashok Mehta, field applications engineering manager. Recent engineering efforts have focused on the design of the metal enclosures used to house telecom equipment.

And no wonder. While the concept of seismically hardened enclosures is not entirely new--General Devices Corp. has been making its Series 7700 seismic cabinet for 25 years--many believe the interest in them is growing because the stakes are getting higher. "In the past, when you lost a rack of equipment, you lost what--a few lines? Today you would lose hundreds or thousands of lines of communications, due to the degree of miniaturization and computerization," says Rod Thornberry. He's a test engineer at Wyle Labs, one of the largest environmental testing agencies in the country specializing in seismic analysis.

"Obviously, seismic enclosures are ideal for fire and police communications, gas and electric, and other critical applications. But no one wants their communications to go down," says enclosure maker Hoffman's Todd Mickely, product development manager. He estimates that sales of Hoffman enclosures, rated for Zone 4 seismic events (see map), are growing at 50% a year.

Seismic enclosure ratings are based on vibration tests defined in the 1995 Bellcore GR-63-CORE standard for the telecommunications industry. This voluntary standard is part of the Network Equipment Building Systems Classification, which addresses a host of other calamities that can potentially cripple electronic equipment, including fire, EMI, and airborne contaminants.

Testing, which is performed on shaker tables that generate accelerations up to 8g's, are designed to simulate conditions ranging from the low-level vibrations of a truck bumping along a pothole-laden road to an earthquake measuring 8.3 on the Richter Scale.

Minimum motion key. Displacement is the main parameter of interest, and Bellcore requirements call for a maximum movement of just 2 inches. "The seismic range is actually between 1 and 33 Hz, but basically what we're doing is simulating the low frequencies, from about 1 to 5 Hz," says Thornberry. "That's where you are going to get the greatest degree of motion and greatest damage to the structure."

Displacement is also the reason that engineers are interested in resonance, which occurs when an applied pulse (in this case the vibration) is synchronous with the natural frequency at which the enclosure moves in response. Enclosure designers would like to avoid this condition because it amplifies the response, producing rapid vertical movements that cause the greatest of damage.

Engineers who have watched this accelerated motion during testing describe the enclosure as looking like "Jell-O quivering on a plate." In other cases, side panels come loose and start flapping, hardware breaks off, frames distort and the doors pop off, or frames fatigue and fail at the connection points. Any of these conditions can render the electronics inside inoperable.

A stiffer frame is advantageous because it has a higher natural frequency at which it will move in response to the low-frequency, side-to-side motion of an earthquake. The Bellcore standard requires that fully-loaded enclosures have a natural frequency of more than 2 Hz, but Thornberry points out that designing to a minimum of 6 Hz avoids the problem of resonance altogether.

Design engineers use various schemes for stiffening the enclosure's frame, which carries the majority of the load. Some of the more common strategies include strengthening the enclosure's corner posts, attaching channel sway bracing, reinforcing the vertical and horizontal members, and adding extra longitudinal and diagonal members to limit the vertical movement. A stiffer base achieves better transmission of energy to the frame. Designers also strive to achieve a geometric configuration that provides a higher moment of inertia for greater stability.

The weight of the electronics inside the enclosure and their distribution plays a role in seismic design. In general, the more weight, the more stress on the frame. And each time the configuration changes, so does the performance of the enclosure.

"We test the enclosure fully-loaded and provide the customer with baseline test data for a specific configuration," says Michael Murphy, industrial product manager at Rittal Corp. "Later, if he puts an extra 10 lbs in or takes 15 lbs out of the bottom, that enclosure may no longer meet the seismic rating of the original design."

With their high center of gravity, enclosures that are top-heavy are particularly problematic. Even in the absence of external forces, such designs are automatically rendered less stable. "The rule of thumb is to keep your mass as low in your equipment as possible," says Thornberry.

Because seismic enclosures can be rated to different levels and every application is unique, prices vary tremendously. According to manufacturers, seismic enclosures can cost anywhere from 20% to 50% more than a standard enclosure. Even so, companies are finding that the added cost does pay off. "Given the cost of the electronics alone inside of these cabinets," says Thornberry, "the economic case is pretty overwhelming."

Seismic enclosures: not just for earthquakes

The same principles that allow seismic enclosures to survive earthquakes make them ideal for other hostile environments, including:

Applications with heavy shock loads to the base of the enclosure

Enclosure standards

Seismic standards aren't the only ones designers of enclosures have to worry about. A long list of standards-writing groups has produced an abundance of diverse enclosure standards. The most common types of standards deal with protection issues, including an enclosure's ability to withstand corrosion, mechanical impact, and exposure to water, dust, and icing. More esoteric topics include explosion protection and protection against vermin. Here's a sampling of some of the organizations with enclosure standards.

American National Standards Institute 1430 Broadway, New York, NY 10018; www.ansi.org