To protect your systems from power failures, protect your UPSs from module failures

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Network administration typically doesn't leave much time for UPS testing and maintenance. But neglected and aging UPSs can fail when your organization needs them most. The Windows 2000 Magazine Lab recently experienced a UPS failure that was of minor consequence, but such a problem could have happened with mission-critical hardware. UPSs with failover capabilities are fairly new products that offer more reliable protection than older UPSs.

Many vendors, including Invensys, Liebert, and American Power Conversion (APC), sell redundant UPSs. Although power capacities and form factors vary, these products typically provide modular designs with redundant power rectification, inversion logic, and battery modules. Redundant UPSs also typically offer high-end benefits, such as double-conversion power circuitry. During brownouts, double-conversion circuitry provides better voltage regulation than that of cheaper line-interactive models. Double-conversion circuitry also improves the AC sine wave purity of UPS battery power.

APC recently sent the Lab a Symmetra Rack-Mount (RM) UPS. Its power modules evenly share the electrical load, and if one module fails, the others take up the slack without power interruption.

Although redundancy sets these new devices apart from other UPSs, their scalability features can also be attractive. The Symmetra RM supports as many as four 2KVA power modules (VA is the product of volts and amps). After reserving enough power to cover a module failure, the Symmetra RM is scalable to a 6KVA power capacity. Another Symmetra model has four 4KVA power modules with a maximum capacity of 12KVA. To increase reliability and extend the Symmetra RM's operation during power outages, you can also install as many as five 7.2 ampere hour (Ah) battery modules.

When you're comparison-shopping for a redundant UPS device, your key considerations are your systems' power requirements, manageability features (e.g., remote management), physical form factors (e.g., rack mountability), and cost. Most vendor Web sites have a calculator that helps you define the UPS power capacity that your server models and configuration require. I recommend adding a 10 to 20 percent safety margin to the calculators' KVA figures. To determine your battery needs, check the Web sites for estimates of battery runtimes under your projected loads.

To ensure that a device will scale to your future needs, consider its maximum capacity. When you compare costs, also compare the cost of adding power modules. For example, at about $1500 apiece, additional 2KVA power modules for the Symmetra RM seem expensive.

Redundant UPSs can cost as much as 15 percent more than nonredundant double-conversion models and as much as 30 percent more than line-interactive models. However, a redundant UPS's scalability and reliability can outweigh these cost differences. When you consider what you invest in building a high-availability computing platform, the cost of a UPS that helps you preserve that availability is insignificant.