PC Hardware in a Nutshell, 3rd Edition

What BPSs Protect Against

Most electric utilities supply consistent, well-regulated power. But as that power moves from the generating plant through the distribution grid to you, the power company gradually loses control of its quality. A good BPS protects against all of the following power problems:

Blackout

A blackout is a sudden, complete loss of voltage, which may be accidental (a tree falling on a power line) or intentional (the power company shedding load during a power emergency). Blackouts are the reason most people consider buying a BPS, but they are the least-common power problem. Blackouts of very short duration, called drops, occur frequently and often pass unnoticed. Drops may be so short that the lights may not flicker. High-quality PC power supplies have enough inertia to continue supplying power to the PC during short drops. Lower-quality PC power supplies have much shorter hold-up times, so even very short drops may cause the PC to lock up. In fact, this is one of the most common causes of PC lockups, and installing a BPS eliminates them.

Brownout

A brownout is a significant reduction in voltage lasting from seconds to days. Short brownouts, called sags, are usually caused by a sudden load on the line, such as a high-amperage motor being turned on. Longer brownouts may be caused by the utility intentionally reducing voltage in response to demands heavier than they can meet. Utilities supply a nominal standard voltage, which in the U.S. normally ranges from 108V ...