Cisco Systems 566 Manual De Usuario

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 
protection 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 electrical outlet. 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 
conditioners keep a system’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 
running when AC power is lost. The battery is charged by the AC power while it 
is available, so once 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.