Page 696 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

Power supplied by electric companies is not always consistent and
clean. Most electronic equipment demands clean power to function

properly. Equipment damage from power fluctuations is a common
occurrence. Many organizations opt to manage their own power
through various means. An uninterruptible power supply (UPS) is a
type of self-charging battery that can be used to supply consistent
clean power to sensitive equipment. A UPS functions by taking power
in from the wall outlet, storing it in a battery, pulling power out of the
battery, and then feeding that power to whatever devices are

connected to it. By directing current through its battery, it is able to
maintain a consistent clean power supply. This concept is known as a
double conversion UPS. A UPS has a second function, one that is often
used as a selling point: it provides continuous power even after the
primary power source fails. A UPS can continue to supply power for

minutes or hours, depending on its capacity and how much power the
equipment attached to it needs. The switching from power grid to
battery-supplied power occurs instantaneously with no interruption of
power supplied to the equipment.

Another form of UPS is the line-interactive UPS. This type of system
has a surge protector, battery charger/inverter, and voltage regulator
positioned between the grid power source and the equipment. The

battery is not in-line under normal conditions. If the grid fails, the
power is pulled from the battery inverter and voltage regulator to
provide uninterrupted power to the equipment.

A battery backup or fail-over battery is not a form of UPS as there is
usually a period of time (even if just a moment) of complete power loss
to the equipment as the grid source of power fails and a switching
event occurs to retrieve power from a battery.

Another means to ensure that equipment is not harmed by power

fluctuations requires use of power strips with surge protectors. A surge
protector includes a fuse that will blow before power levels change
enough to cause damage to equipment. However, once a surge
protector’s fuse or circuit is tripped, current flow is completely
interrupted. Surge protectors should be used only when instant
termination of electricity will not cause damage or loss to the

equipment. Otherwise, a UPS should be employed instead.