2 cissp ® Official Study Guide Eighth Edition

 Protecting Power Sources 
Fault tolerance can be added for power sources with an
uninterruptible power supply
(UPS), a generator, or both. In general, a UPS provides battery-supplied power for a short 
period of time between 5 and 30 minutes, and a generator provides long-term power. The 
goal of a UPS is to provide power long enough to complete a logical shutdown of a system, 
or until a generator is powered on and providing stable power. 
Ideally, power is consistently clean without any fl uctuations, but in reality, commercial 
power suffers from a wide assortment of problems. A
spike
is a quick instance of an 
increase in voltage whereas a
sag
is a quick instance of a reduction in voltage. If power 
stays high for a long period of time, it’s called a
surge
rather than a spike. If it remains low 
for a long period of time, it’s called a
brownout
. Occasionally, power lines have noise on 
them called
transients
that can come from many different sources. All of these issues can 
cause problems for electrical equipment. 
A very basic UPS (also called an offl ine or standby UPS) provides surge protection and 
battery backup. It is plugged into commercial power, and critical systems are plugged into 
the UPS system. If power fails, the battery backup will provide continuous power to the 
systems for a short period of time. Line-interactive UPS are becoming popular, and they 
provide additional services beyond a basic UPS. They include a variable-voltage transformer 
that can adjust to the overvoltage and undervoltage events without draining the battery. 
When power is lost, the battery will provide power to the system for a short period of time. 
Generators provide power to systems during long-term power outages. The length of 
time that a generator will provide power is dependent on the fuel, and it’s possible for a 
site to stay on generator power as long as it has fuel and the generator remains functional. 

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Chapter 18 
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Disaster Recovery Planning
In the lengthy aftermath of Hurricane Irma in Puerto Rico in 2017, generators were called 
on to operate for extended periods and began to fail after weeks and months of continuous 
operation. 
Generators also require a steady fuel supply—they commonly use diesel fuel, natural 
gas, or propane. In addition to making sure that you have suffi cient fuel on hand, you 
should also take steps to ensure that you can be delivered fuel on a regular basis in the 
event of an extended emergency. Remember, if the disaster is widespread, there will be 
signifi cant demand for a limited fuel supply. If you have contracts in place with suppliers, 
you’re much more likely to receive fuel in a timely manner.