FIGURE 1.3 Relative frequency of electrical outages in the United States between 1984 and 2000. Of the 533 transmission or generation events shown, 324 involved a power loss of 1 MW (average of once every 19 days), and 46 involved a power loss of 1,000 MW (average of 3 per year). Dots indicate actual outage events. The dashed line is an exponential (Weibull) distribution fit to the failures below 800 MW loss. The solid line is a power law fit to the NERC data over 500 MW loss. SOURCE: Data compiled by NERC DAWG, plotted by Jay Apt, Carnegie Mellon University, 2006.


FIGURE 1.4 Frequency of electrical outages in the United States over time. Note that while there is significant year-to-year variability, there is no long-term trend. SOURCE: Data compiled by NERC DAWG, plotted by Paul Hines, Carnegie Mellon University, 2006.

attacks more likely, but many of the steps that should be taken to strengthen the system against terrorists will help against these attacks also. This section briefly describes the types of attacks that may be encountered.

Individuals or small low-tech groups with limited resources who want to kill people or cause widespread societal damage could pose a serious threat, but the amount of harm that one or a few such people could do to the electric system is probably limited. Individuals or groups that want to harm the power system but not kill a lot of people or cause widespread societal damage or harm might include people angry at the power company, bored hunters taking pot shots at insulator strings, or individuals who view the power company as an important symbol of something they oppose. For example, the Earth Liberation Front has reportedly been involved in a plot to bring down high-voltage power lines.4 Any such attack could be serious, especially if undertaken by a current or former employee with detailed insider knowledge. Between 1984 and 2000, approximately 3 percent of major disturbances in the United States were attributed to sabotage.5 The authors of Making the Nation Safer note that sabotage of individual components has “posed a nuisance, but the impacts have generally been manageable” (NRC, 2002, p. 177). Pernicious hackers are people whose primary motivation is not to kill people or cause specific damage, but rather to test limits and perhaps gain recognition within a subculture by demonstrating technical prowess by disrupting the operation of an important and highly visible societal system. Their motivation would be similar to that of computer hackers who release computer viruses and worms, or disrupt corporate and government computer sites. It is likely that such attacks would come from lone individuals or small groups.


FIGURE 1.5 Annual number of transmission loading relief events since 1997. The substantial increase indicates that over the past decade the level of stress on the system has grown considerably. SOURCE: NERC data plotted by Jay Apt, Carnegie Mellon University, 2006.

Finally, harmful activity could be motivated by commercial benefit. A power company seeking a competitive


4See “11 Indicted in Eco-terror Arsons,” available at

5Based on NERC Disturbance Analysis Working Group (DAWG) data available at

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