“The transmission network is the part of the power system of greatest concern because it is highly vulnerable to attack, and the consequences can be great. The lines themselves are essentially impossible to protect because they extend many thousands of miles, often in sparsely populated areas.”
Transmission lines can be broadly categorized into two types: Overhead and underground. Virtually all transmission lines in the United States are overhead, primarily because underground lines are far more expensive to install.
Overhead transmission lines are composed of two major components, the cables through which electricity is conducted and the towers which support the cables. For clarity, the term “transmission line” will refer to the combination of cables and towers. The vulnerabilities of each component are examined below.
Transmission Cable Characteristics and Vulnerabilities
Transmission cables are typically constructed of an inner core of steel wires surrounded by an outer layer of aluminum alloy wires. The inner steel core provides structural support for the outer layer of aluminum wires, which serve as conductors for the electricity.
Transmission cables are sized according to the amount of electricity that will be conducted through them. As a rule, larger amounts of electricity require larger diameter cables. In some cases the capacity of a transmission line is increased by using multiple cables. This is known as a ‘bundle’ and usually consists of two, three or occasionally four or more cables.
Whether single or multiple, cables are highly vulnerable to a surprisingly low-tech method of attack: they can simply be shot down with an ordinary rifle. Ironically, the larger the capacity of a transmission line, the more vulnerable it is to being shot down. This is because large-capacity power lines use large-diameter cables which present a relatively easy target to hit.
A sampling of the technical literature on high voltage engineering considerations reveals that these publications typically address natural threats to extra high voltage transmission lines such as earthquakes, wind, lightning strikes and forest fires, but do not address this specific vulnerability or the threat of sabotage in general.
A web search of national security publications revealed only one reference to this potential threat:
“…even greatly enhanced resistance to sabotage is likely to simply move the problem somewhere else. For instance, small groups deterred from attacking substations could simply shoot transmission lines out. While the impact of a single incident would be much less dramatic and lasting [than] that of blowing up several substations, it could be repeated frequently over a wide geographic area, achieving much of the same disruption.”
Considering the potentially profound implications of such a low-tech method of attack, it seems too simple to be effective. Furthermore, if it actually were so simple and effective, it is difficult to believe that transmission lines would be routed across remote landscapes in which opportunities for sabotage are abundant. The technique does appear to present several potential problems, including the danger to the saboteur from electrocution, the degree of marksmanship required to shoot a transmission cable, and whether a bullet could inflict sufficient damage to disable a cable.
The following 3 subsections examine each of these issues, followed by a 4th subsection describing the testing of this method.
Electrocution: The most plausible electrical paths potentially causing electrocution are either through the air (arcing) from a cable that has been shot but hasn’t fallen, or through the ground from an already fallen cable.
The danger from arcing to people in the vicinity of live transmission cables is addressed in the National Electrical Safety Code. Known as the “minimum approach distance,” it is the closest distance that a worker can safely approach a live transmission line. The higher the voltage, the greater that distance is, because a higher voltage will arc across a longer gap. As an example, the minimum approach distance for a large 500 kilovolt transmission line is approximately 11 feet. A saboteur would be safe from electricity arcing through the air provided the minimum approach distance was maintained.
The second type of electrocution danger is from a live cable that has fallen to the ground. A live cable can energize the ground around it, possibly causing electrocution to anyone standing in the vicinity. Soil is a poor conductor of electricity, but since water is a relatively good conductor, soil moisture is the most significant variable determining the minimum safe distance to downed power lines.
One utility company recommends staying at least 10 feet away from fallen power lines after a storm. Presumably this assumes a saturated soil condition. Although this may be referencing lower voltage distribution lines, it suggests that under dry soil conditions, even a downed 500 kilovolt cable would be unlikely to cause electrocution to a saboteur standing 100 feet or so away.
It can thus be inferred that the danger from electrocution is an unlikely deterrent to a potential saboteur.
Marksmanship: To evaluate the difficulty of shooting a transmission cable, two variables need to be estimated: The size of the target (diameter of the cable) and the range (distance from shooter to cable).
The size of the target is of course the diameter of the transmission cable. Among the most common sizes of extra high voltage transmission lines, and thus the sizes most likely to be targeted, are those operating at 345 kilovolts to 500 kilovolts. The cables used for these lines are typically around 1 to 1-1/2 inches in diameter, with the larger diameter being more common.
From a saboteur’s perspective, the ideal range would be as short as possible in order to increase the accuracy of the shot and to maximize the damage potential of the bullet, but not so short as to risk electrocution.
An approximate range can be calculated by first determining the vertical height of the target cable above the ground, then by determining the minimum safe horizontal distance between the cable and the saboteur. Range is then derived from a simple trigonometric calculation.
Transmission cables hang downward in a shallow curve between each tower with the lowest point typically being about midway between towers. This downward curve is known in the transmission trade as sag. Sag is a dynamic characteristic. It increases as cables heat up and thus elongate. Heating is caused by both higher ambient air temperatures and by heat generated from electricity passing through a cable. The greater the sag, the closer a cable will be to the ground. The following discussion assumes a condition of maximum sag for reasons explained later.
In the interest of safety and reliability, the National Electrical Safety Code requires a minimum vertical clearance between the lowest point of sag and the ground. Its purpose is to insure that transmission lines are high enough to prevent electricity from arcing through the air to people, vehicles, trees, etc. and the consequent danger of electrocution, fires and disabled transmission lines.
The amount of clearance depends on several variables such as the voltage the transmission line is designed to carry and what lies below it, such as roads, railroad tracks, pipelines, etc. As an example, the minimum vertical clearance to ground for 500 kilovolt extra high voltage transmission lines over roads is 28 feet. For remote areas in which no roads exist, the clearance is 24 feet. These figures are corroborated by an online transmission line clearance calculator.
Since a remote, roadless area would be the most likely site for an attack, it can be assumed that a typical cable in such a location would be approximately 25 to 30 feet above the ground, depending on sag and terrain. A horizontal distance from the cable to the saboteur of 100 feet would afford an ample margin of safety against electrocution from arcing and probably from conduction through the ground as well. A horizontal distance of 100 feet and a vertical height of 25 feet results in a range of just slightly over 100 feet.
It should be noted that higher cables are unlikely to deter an attack. For example, if a cable is 40 feet above the ground, a 100 range could be maintained by simply shortening the horizontal distance to about 90 feet, still an ample margin of safety for a saboteur.
Damage: A rifle bullet hitting a transmission cable can be expected to do considerable damage, not only to the relatively soft aluminum conductor wires, but also to the inner steel support wires. Even an indirect hit that damages only a portion of the cable will result in electricity arcing around the damaged area, causing temperatures high enough to melt the remaining conductor wires as well as the inner support wires.
Testing: Given the significant implications of such a low-tech method of sabotage, two tests were performed to verify the efficacy of using a rifle to disable an extra high voltage transmission line. The first test was designed to establish the difficulty of hitting a cable-sized target at a range of 100 feet and the second to examine the damage caused by a bullet hitting a transmission cable at that range.
As noted below, values for several of the variables in each test were chosen so as to make the tests more challenging than would be the case in an actual attack. These included target size and skill level of the shooter.
The firearm used in the tests was a .270 caliber rifle. This type of rifle is commonly used for deer hunting. The rifle was equipped with a low-end 4 power scope. Prior to the test, the rifle had been sighted in at a range of 100 feet. The ammunition used was commonly available Winchester 150gr. Super-X Power-Point Soft Point.
Since I performed the tests, a comment on my familiarity with firearms and level of marksmanship is appropriate. Although I own a rifle, I am neither a gun enthusiast nor a marksman. Frequently, a year or more will pass without my firing it. When I do, it is invariably to put down a neighbor’s suffering horse or mule, a task requiring no particular marksmanship.
The first test was performed by setting up a target consisting of two parallel horizontal lines drawn on the side of a cardboard box. These represented a transmission cable. As noted above, extra high voltage transmission cables are typically about 1 to 1-1/2 inches in diameter. To make the test more challenging than would be the case in a real-life attack, the lines on the box were drawn 1 inch apart.
The rifle was positioned 100 feet from the target box. A prone position was assumed and the forestock of the rifle was rested on a cardboard box to steady it. Four shots were fired. Each of the four bullets entered the target box cleanly between the lines. This test supported the concept that an extra high voltage transmission cable could be hit at a range of 100 feet with relative ease.
In the second test, the effect of a bullet hitting a transmission cable was evaluated. A short length of transmission cable was attached to the side of the target box. The cable used in this test was a piece discarded during the construction of a 115 kilovolt transmission line. It was ¾ of an inch in diameter, which is a smaller and thus more challenging target than the cables used for most of the 80,000 miles of extra high voltage transmission lines in the United States.
Firing was performed using the same method and range as in the first test. Only one bullet was fired. It hit and essentially destroyed the cable, as seen in the photo below:
Those familiar with firearms will note that although a prone position was used in the test due to the target being at ground level, such a position could not be effectively used by a saboteur aiming at an elevated target such as a transmission cable. The following photo suggests the ease with which saboteurs could attain effective shooting positions for higher angle shots using commonly available materials.
Transmission Tower Characteristics and Vulnerabilities
A significant aspect of an attack on transmission lines is the probability that it would result in tower failure, significantly compounding the time and expense required for repairs.
Transmission towers depend to a large extent on tension in the transmission cables for structural support. During an attack, if cables were severed by bullets, the towers adjacent to the targeted span would become progressively less stable and could collapse.
“It is to be noted that the weight and strength of the tower is dwarfed by the weight and strength of the wire system attached to it. This is a normal characteristic of EHV [extra high voltage] lines.
Given the strength and weight of the wires compared to the strength and weight of a tower, it should be recognized that a tower will fall where the (very powerful) wire system permits it to fall.” 
Collapse of one tower can then lead to cascading tower collapse.
In addition to the possibility of tower collapse caused by severed transmission cables, certain types of towers are highly vulnerable to a different kind of attack.
Extra high voltage transmission towers can be broadly divided into two categories: self-supporting and guyed. Self-supporting towers stand on their own whereas guyed towers depend on support cables (guys) for structural strength. Within each category are a wide variety of configurations.
The majority of towers used in the United States are self-supporting lattice-type towers.
However, a significant minority of extra high voltage transmission lines utilize guyed towers. The relative ease with which guyed towers can be collapsed could make them a particularly attractive target for saboteurs.
The weak point of guyed towers is the support cables. Most guyed towers are supported by 4 steel cables. The upper end of a support cable is fastened near the top of the tower and the lower end to an anchor buried in the ground. The lower end is easily accessible and thus the most likely target for sabotage. Support cables are vulnerable to being cut by saboteurs using an ordinary hacksaw.
By cutting most of the way through the support cables on the same side of each tower, all of the towers within a several-mile-long section of a transmission line could be jeopardized by one saboteur in just a day. If the support cables of just one of those towers was then cut all the way through, that tower would begin to collapse away from the side on which its support cables had been cut. As that tower began to fall, it would transfer excess tension through the transmission cables to the 2 adjacent towers, causing the compromised support cables on those towers to break. The entire section of compromised towers would fall, one after another. Thus, in a relatively short time, a saboteur equipped with little more than a hacksaw could cause extensive damage to a transmission line supported by guyed towers.
A full description of the weaknesses of every transmission line configuration is beyond the scope and intent of this website, but it should be abundantly clear that extra high voltage transmission lines are inherently vulnerable to attack.
The Role of Remote Locations in Compounding the Vulnerabilities of Extra High Voltage Transmission Lines
Locating transmission lines in remote areas significantly undermines national energy security by enhancing the ease and effectiveness of attacks in several ways. Lines traversing vast, sparsely populated expanses, which are typical in the American West, allow saboteurs the luxury of attacking far from watchful eyes. An abundance of time in which to carry out attacks permits saboteurs to inflict extensive damage prior to detection. Additionally, an attack in a remote area would greatly increase the time, difficulty and expense of performing subsequent repairs.
As noted in the U.S. Congress report quoted at the beginning of this chapter, effective security for transmission lines routed across vast, rugged and remote landscapes is essentially impossible.
 U.S. Congress, Office of Technology Assessment, “Physical Vulnerability of Electric Systems to Natural Disasters and Sabotage,” P. 47 (Washington, DC: U.S. Government Printing Office, OTA-E-453, June, 1990)
 Mazen Abdel-Salam, Hussein Anis, Ahdab El-Morshedy, Roshdy Radwan, High-Voltage Engineering, Second Edition, 2000; R.D. Begamudre, Extra High Voltage A.C. Transmission Engineering, 2007; Subir Ray, An Introduction To High Voltage Engineering, 2004)
U.S. Congress, Office of Technology Assessment, “Physical Vulnerability of Electric Systems to Natural Disasters and Sabotage,” P. 63 (Washington, DC: U.S. Government Printing Office, OTA-E-453, June, 1990)
 Occupational Health and Safety Administration, “Minimum Approach Distances” http://www.osha.gov/SLTC/etools/electric_power/illustrated_glossary/substation_equipment/approach_distance.html
 Center Point Energy, “Downed Power Lines and Safety After a Storm”, 2012 http://www.centerpointenergy.com/newsroom/stormcenter/7ee0f25efaaa4110VgnVCM10000001a10d0aRCRD/
 Alcan Cable, “Aluminum Conductor Steel Reinforced (ASCR) Cables” P. 15 http://ece.citadel.edu/mckinney/elec403/ACSR.pdf
 PPL Corporation, “PPL Design Criteria and Safety Practices, Appendix E-7”, P. 3 http://www.pplreliablepower.com/NR/rdonlyres/DE65C1DF-A8FD-4B84-B26A-39CB554006B7/0/E7PPLDesignCriteriaandSafetyPractices.pdf
 Marne and Associates,’Transmission Voltage Clearance Calculator” http://www.codehandbook.com/codestore/LineClearanceTool.html
 Faustino L. Quintanilla, “Palo Verde to Westwing Line 2 and Palo Verde to Rudd Double Line Outage Probability Analysis SRP (Failure Mode Analysis of SRP 500kV 5T2 and 5T3 Transmission Towers, P. 3)” Salt River Project https://www.wecc.biz/committees/BOD/12082010/Lists/Minutes/1/Performance%20Category%20Upgrade%20Requests%20for%20Palo%20Verde%20to%20Westwing%20Line%202%20and%20Palo%20Verde%20to%20Rudd%20Double%20Line%20Outage.pdf
 SRP Memorandum “Report No: CE-573”, August 17, 2010, P. 2 https://www.wecc.biz/committees/BOD/12082010/Lists/Minutes/1/Performance%20Category%20Upgrade%20Requests%20for%20Palo%20Verde%20to%20Westwing%20Line%202%20and%20Palo%20Verde%20to%20Rudd%20Double%20Line%20Outage.pdf
© David Omick and Operation Circuit Breaker, 2012. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, as well as photos by the author, provided that full and clear credit is given to David Omick and Operation Circuit Breaker with appropriate and specific direction to the original content.