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:
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.
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 to 125V, with 110 to 115V most common. During a brownout, voltage may drop from nominal to 90V or less. Brownouts can damage equipment because as voltage drops, equipment draws more current to compensate, which increases heat production.
During a surge, delivered voltage is substantially (20% to 100%) higher than nominal. Surges may last from a fraction of a second to several seconds, and often result when a heavy load is suddenly removed from the circuit. Surges are relatively common, and all but the most extreme are relatively benign. Despite claims made by the manufacturers of so-called "surge protectors," most equipment takes normal surges in stride. A good BPS does, however, prevent them from reaching the equipment in the first place.
A spike, also called a transient, is an extreme overvoltage of very short duration. Spikes originate from various sources, including voltages induced by remote lightning strikes, transformer failures, and nonbrushless motors turning on and off. Although spikes may carry 50,000 V or more, most are of such short duration (milliseconds or less) that they deliver very little electrical energy. PC power supplies themselves protect against most spikes. A good PC power supply smothers typical short spikes of up to 5,000 V without affecting system operation. Spikes of higher voltage or longer duration are stopped by a good BPS. The worst spikes, those that result from a direct "bolt on copper" lighting strike nearby, cannot be stopped by any power protection equipment.
Most people don't realize that damage from electrical problems, particularly spikes, is incremental and cumulative. That is, a computer may absorb a severe spike and continue to operate normally. But that spike may have caused invisible damage to the chips, down almost at the quantum level. Computer chips, including memory and CPUs, typically use 12 V or less. A spike at even 10,000 times that voltage may simply lock up the system with no other obvious effects or apparent damage, but leave the system teetering on the edge. A subsequent spike, even a small one, may be the straw that breaks the camel's back. Little Spike finishes the job that Big Spike started, causing the system to fail for no apparent reason. A good BPS prevents such problems.