Dell Precision 410 User Manual page 199

A number of devices are available that protect against power problems, such as
power surges, transients, and power failures. The following subsections describe
some of these devices.
Surge protectors are available in a variety of types and usually provide a level of pro-
tection commensurate with the cost of the device. Surge protectors prevent voltage
spikes, such as those caused during an electrical storm, from entering a system
through the AC power source. Surge protectors, however, do not offer protection
against brownouts, which occur when the voltage drops more than 20 percent below
the normal AC line voltage level.
Line conditioners go beyond the overvoltage protection of surge protectors. Line con-
ditioners keep a computer's AC power source voltage at a fairly constant level and,
therefore, can handle brownouts. Because of this added protection, line conditioners
cost more than surge protectors—up to several hundred dollars. However, these
devices cannot protect against a complete loss of power.
Uninterruptible power supply (UPS) systems offer the most complete protection
against variations in power because they use battery power to keep the system run-
ning when AC power is lost. AC power, while available, charges the battery; when AC
power is lost, the battery can provide power to the system for a limited amount of
time—from 15 minutes to an hour or so—depending on the UPS system.
UPS systems range in price from a few hundred dollars to several thousand dollars,
with the more expensive units allowing you to run larger systems for a longer period
of time when AC power is lost. UPS systems that provide only 5 minutes of battery
power let you conduct an orderly shutdown of the system, but are not intended to
provide continued operation. Surge protectors should be used with all UPS systems,
and the UPS system should be Underwriters Laboratories (UL) safety-approved.
Maintaining the System D-9