AC power is used for long-distance transmission because its voltage can be easily manipulated using transformers. AC involves the periodic reversal of current flow, allowing transformers to efficiently step up the voltage for travel. High voltage is necessary to minimize resistive power loss, which is proportional to the square of the current. To deliver the same power with a lower current, the voltage must be significantly increased, often reaching hundreds of thousands of volts.
How Alternating Current Affects the Human Body
The immediate threat from an electrical incident is the current, or amperage, that flows through the body, not the voltage. Current passing through human tissue can disrupt the heart’s natural electrical rhythm, causing ventricular fibrillation. This chaotic quivering prevents the circulation of oxygenated blood, leading to death if not immediately corrected.
AC is particularly dangerous compared to direct current (DC) because its oscillating nature interferes with the heart’s electrical cycle. A current pathway crossing the chest can induce fibrillation with an amperage as low as 50 to 60 milliamperes (mA) at the common 60-hertz frequency. This sensitivity is heightened during the heart’s vulnerable repolarization phase.
Another danger is the “let-go current,” the maximum current a person can tolerate and still voluntarily release a conductor. For a healthy man, the average 60-Hz AC let-go threshold is around 15.5 mA. Above this level, the current causes tetanic muscle contraction, freezing the hand onto the conductor.
This inability to let go increases the duration of exposure, allowing a small current to cause extended damage. Prolonged contact increases internal heating, increasing the probability of fatal ventricular fibrillation. Extended exposure can also cause the skin’s electrical resistance to break down, allowing more current to pass through the body.
The Danger of Heat and Arcing
Electrical transmission systems move large amounts of energy, creating thermal hazards when released improperly. When high current encounters resistance, such as in a fault, the energy is instantly dissipated as heat, a process called Joule heating. This rapid heating can melt conductors and ignite surrounding materials.
A more violent thermal event is an arc flash, occurring when a fault current flashes through the air between conductors or to the ground. This explosive release of electrical energy generates temperatures up to 35,000 degrees Fahrenheit, several times hotter than the sun’s surface. This heat causes severe external burns and vaporizes metal components.
The rapid expansion of air and vaporized metal results in a supersonic pressure wave known as an arc blast. This explosive force propels molten metal and shrapnel at speeds exceeding 700 miles per hour, causing blunt force trauma, internal injuries, and hearing damage. Fatalities are often due to the thermal burns rather than the electrocution itself. Additionally, the intense light from the arc flash, including ultraviolet radiation, can cause temporary or permanent blindness.
High Voltage Proximity Hazards
The high voltage levels used in AC transmission create dangers in the air and ground, even without direct contact. Air, normally an excellent insulator, can break down and become a conductor when the voltage gradient is high enough. This causes electricity to “jump” the air gap, creating an arc to a nearby object or person.
The distance electricity can arc increases with voltage, making proximity to high-voltage equipment hazardous if proper clearance is not maintained. For instance, a 500,000-volt line can arc across seven feet or more in certain conditions. Approaching a downed line or working near a substation can result in fatal contact without touching the wire.
Another danger is Ground Potential Rise (GPR), which occurs when a fault, like a broken line hitting the earth, dumps current into the ground. Since the earth has resistance, this current elevates the voltage of the surrounding soil, creating a hazardous voltage gradient that spreads outward.
A person standing in this gradient can be exposed to a “step potential,” the voltage difference between their two feet, causing current to flow up one leg and down the other. The farther apart the feet, the greater the voltage difference. Similarly, touching a conductive object connected to the earth creates a “touch potential,” the voltage difference between the object and the ground where the person is standing.
Finally, the alternating nature of the current creates a fluctuating electromagnetic field that can induce voltage in nearby parallel metallic objects. Long structures like pipelines or metal fences running parallel to the transmission line can act as antennas, picking up a charge through electromagnetic induction. This induced voltage can be high enough to cause a painful “startle shock” or hazardous current flow when the object is touched.