ity, are easily located either on the ground or from system maps. Detailed maps of U.S. power systems were once readily available in the public domain and on the Internet. Despite attempts to control access to such maps, they can still be easily obtained. Commercially available satellite data, as well as direct observation on the ground, can also be used to readily update and confirm system map information for potential attackers.

Facilities and equipment can be damaged or destroyed by a variety of means well known to international terrorists, surrogate agents, and special operations military forces. Physical facilities are vulnerable to mechanical intervention or from serious physical damage from stand-off attack projectiles and explosive devices. In addition some choke points on the electric systems of the modern world are vulnerable to cyber incursion. Chapter 4 discusses the cyber threat. Any attack could be considerably amplified if aided by insiders, whether voluntary or coerced. The insider issue is discussed in Chapter 5.

Most utilities are well prepared to handle outages caused by all but the largest natural events. However, the power industry is not capable of reliable performance if major components are severely damaged on a widespread basis by deliberately planned terrorist acts or natural phenomena. Virtually no utilities are equipped or staffed to mitigate the consequences of multiple attacks against major critical components or from widespread impacts of natural phenomena like Hurricane Katrina. National security planners have devoted insufficient attention to this fact or to the fact that electricity must be produced and delivered, through highly complex technological systems, at the instant of demand, and cannot be easily stored.

Points of Vulnerability

Specific points of vulnerability can be better understood by considering briefly each major element of power systems: generators, substations, transmission towers, distribution components, system control centers, and customers or users.


Although this report focuses on the power delivery system, it is important to note that in some parts of the world generators have been targets of terrorist attacks. In the United States generator units and ancillary equipment are installed within a power house that is manned by operational personnel, giving them some protection. Some are inside a perimeter fence with physical security equipment and trained security forces, and others are being upgraded. However, most generating stations except nuclear plants have very limited in-place security measures which could be circumvented by expert saboteurs, and lack supporting contingency plans to coordinate with local authorities.

BOX 3.1
Security Criteria to Be Considered in Evaluating Substation Security

•  Potential threat and probability of attack

•   Frequency and duration of past security breaches

•   Severity of damage

•   Cost of breaches

•   Safety hazards in the substation

•   Equipment types and design

•   Number and types of customers served

•   Substation location

•   Criticality of load

•   Overall cost of facility

•   Quality of service at existing substations

•   Exposure to vandalism, sabotage, and terrorist attack of control houses, control equipment, and key electrical system components

Transmission Substations

Bulk Transmission Substations have unique security concerns in that they are relatively soft targets; they are vulnerable to stand-off attack as well as penetration attacks by adversaries compromising the substation’s perimeter fences. There is general agreement among security planners that key high-voltage substations are the most worrisome terrorist targets within the power transmission system. They are also difficult to protect. Their replacement parts are difficult to obtain, and damage to substations can separate customers from generation for long periods.

Box 3.1 lists security criteria that may be considered in evaluating substation security.

Transmission Lines and Towers

Transmission lines have been a desirable terrorist target in countries suffering from insurrection or civil unrest. A circuit can be temporarily disabled by fairly simple means. Shooting insulators on a tower can short a line. Severing the legs of the tower with explosives can bring it down, shorting all the lines it carries. On some transmission lines, taking out a tower can cause a domino effect, resulting in a cascade collapse of several adjacent towers.5 Taking out a tower where two lines cross can disable both circuits at once.


5Transmission lines normally consist mainly of suspension towers that are intended to support the conductors, which are under tension to minimize sagging. These towers are held in place by the conductors and require little horizontal bracing under normal conditions. If the lines break in one direction, however, the tower may be pulled down by the tension on conductors in the other direction. Thus a cascading failure of towers can occur up to a

The National Academies of Sciences, Engineering, and Medicine
500 Fifth St. N.W. | Washington, D.C. 20001

Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement