Encyclopedia
The term
high voltage characterizes electrical circuits, in which the voltage used is the cause of particular safety concerns and insulation requirements. High voltage is used in
electrical power distribution, in
cathode-ray tubes, to generate
X-rays and particle beams, to demonstrate
arcing, for ignition, in
photomultiplier tubes, and high power amplifier
vacuum tubes.
The International Electrotechnical Commission and its national counterparts define
high voltage circuits as those with at least 1000
V for
alternating current and at least 1500 V for
direct current, and distinguish it from low voltage and extra low voltage circuits.
Other definitions exist as well. For example, in the
United States 2005
National Electrical Code ,
high voltage is any voltage over 600 V . Laypersons may consider household
mains circuits , which carry the highest and most dangerous voltages they normally encounter, to be
high voltage. In
digital electronics, a
high voltage is the one that represents a logic 1 .
Safety and insurance industry
Whilst mains voltages are capable of delivering fatal shocks and may constitute
high-voltage hazards, they cannot jump significant distances, so they are dangerous only if touched. Therefore standards bodies do not generally refer to them as high voltages.
Various
safety and insurance organizations consider anything outside of the ELV range to be dangerous and in need of regulation. Voltages above this range are capable of producing heart fibrillation if they produce electric currents in body tissues which happen to pass through the
chest area. The electrocution danger is mostly determined by the low conductivity of dry human skin. If skin is wet or if there are wounds, or if the voltage is applied to electrodes which penetrate through the skin, then even voltages far below 40 V can be lethally high. On the other hand, voltages above approximately 500 V have a natural
defibrillating effect, so sometimes a higher voltage can be safer than a lower voltage, though by no means safe. A
DC circuit may be especially dangerous because it will cause muscles to lock around the wire. It has also been noted that accidental contact with high voltage power lines has not always been fatal because sometimes the victim is thrown clear of the power line by the intensity of the arc that is created and has survived, although with extremely severe injuries.
Sparks in air
The
dielectric breakdown strength of dry air, at Standard Temperature and Pressure , between spherical electrodes is approximately 33 kV/cm. This value should be used as a rough guide since the actual breakdown voltage is highly dependent upon the electrode shape and size. High voltages, i.e. strong
electric fields, often produce violet-colored corona discharges in air, as well as visible sparks. Voltages below about 500-700 volts cannot produce easily visible
sparks or glows in air at atmospheric pressure, so by this rule these voltages are 'low.' However, under conditions of low atmospheric pressure , or in an environment of noble gas such as
argon,
neon, etc., sparks will appear at much lower voltages. Five hundred to 700 volts is not a fixed minimum for producing spark breakdown, but it is a rule of thumb. For air at STP, the minimum sparkover voltage is around 380 volts.
While lower voltages will not generally jump a gap that is present before the voltage is applied, interrupting an existing current flow often produces a low voltage
spark or
arc. As the contacts are separated, a few small points of contact become the last to separate. The current becomes constricted to these small
hot spots, causing them to become incandescent, so that they emit electrons . Even a small
9V battery can spark noticeably by this mechanism in a darkened room. The ionized air and metal vapour form a plasma which temporarily bridges the widening gap. If the power supply and load allow sufficient current to flow, a self-sustaining
arc may form. Once formed, an arc may be extended to a significant length before breaking the circuit. Attempting to open an inductive load facilitates the formation of an arc since the inductance provides a high voltage pulse whenever the current is interrupted.
AC systems make sustained arcing somewhat less likely since the current returns to zero twice per cycle. The arc is extinguished every time the current goes through a
zero crossing, and must reignite during the next half cycle in order to maintain the arc.
Science classroom devices
A high voltage is not necessarily dangerous. Physics demonstration devices such as
Van de Graaff generators and
Wimshurst machines can produce voltages approaching one million volts, yet at worst they deliver a brief sting. These devices have a limited amount of stored energy, so the current produced is low and usually for a short time. During the discharge, these machines apply high voltage to the body for only a millionth of a second or less. The discharge may involve extremely high power over very short periods, but in order to produce heart fibrillation, an electric
power supply must produce a significant current in the heart muscle continuing for many milliseconds, and must deposit a total energy in the range of at least millijoules or higher. Alternatively, it must deliver enough energy to damage tissue through heating. Since the duration of the discharge is brief, it generates far less heat than a mobile phone.
Note that
Tesla coils are a special case, and touching them is not recommended. Among other issues, they have a tendency to arc to their own bottom-end circuitry, which can introduce powerline frequency currents at lethally high voltages to the body.
Electrostatic attraction/repulsion
The terminals of DC high voltage machines can attract dust, lint, and bits of paper. On an everyday scale, voltages higher than a few thousand volts are required in order to create an
electric field with a
gradient large enough to produce nontrivial forces. On the other hand, the forces depend on the distance from the electrodes and the electrode shapes, and at the microscopic scale of
MEMS, even a few tens of volts acts like a high voltage.
Power lines
Electrical transmission and distribution lines for
electric power always use voltages significantly higher than 50 volts, so contact with or close approach to the line conductors presents a danger of electrocution. Contact with
overhead wires is a frequent cause of injury or death. Metal ladders, farm equipment, boat masts, construction machinery, television antennas, and similar objects are frequently involved in fatal contact with overhead wires. Digging into a buried cable can also be dangerous to workers at the excavation site. Digging equipment that contacts a buried cable may energize piping or the ground in the area, resulting in electrocution of nearby workers. Unauthorized persons climbing on power pylons or electrical apparatus are also frequently the victims of electrocution. At very high transmission voltages even a close approach can be hazardous since the high voltage may spark across a significant air gap.
For high voltage and extra-high voltage transmission lines, specially trained personnel use so-called "live line" techniques to allow hands-on contact with energized equipment. In this case the worker is electrically connected to the high voltage line so that he is at the same electrical potential. Since training for such operations is lengthy, and still presents a danger to personnel, only very important transmission lines are the objects of live-line maintenance practices. Outside these specialized situations, one should not assume that being ungrounded allows one to safely touch energized objects; grounding, or arcing to ground, can occur in unexpected ways, and high-frequency currents can cause burns even to an ungrounded person .
Normally protective equipment on high-voltage transmission lines prevents formation of an unwanted arc, or insures it is de-energized within tens of milliseconds. Electrical apparatus designed to interrupt high-voltage circuits is designed to safely direct the resulting arc so that it dissipates without damage. High voltage
circuit breakers often use a blast of high pressure air, a special
dielectric gas , or immersion in mineral oil to quench the arc when the high voltage circuit is broken.
Arc flash hazard
Depending on the short circuit current available at a switchgear line-up, a hazard is presented to maintenance and operating personnel due to the possibility of a high-intensity
electric arc. Maximum temperature of an arc can exceed 10,000
kelvins, and the radiant heat, expanding hot air, and explosive vaporization of metal and insulation material can cause severe injury to unprotected workers. Such switchgear line-ups and high-energy arc sources are commonly present in electric power utility substations and generating stations, industrial plants and large commercial buildings. In the
United States the National Fire Protection Association, has published a guideline standard NFPA 70E for evaluating and calculating
arc flash hazard, and provides standards for the protective clothing required for electrical workers exposed to such hazards in the workplace.
Explosion hazard
Even voltages insufficient to break down air can be associated with enough energy to ignite atmospheres containing flammable gases or vapours, or suspended dust. For example, air containing
hydrogen gas or
natural gas or
gasoline vapor can be ignited by sparks produced by electrical apparatus. Examples of industrial facilities with hazardous areas are petrochemical refineries, chemical plants, grain elevators, and some kinds of coal mines.
Measures taken to prevent such explosions include:
- Intrinsic safety, which is apparatus designed to not accumulate enough stored energy to touch off an explosion
- Increased safety, which applies to devices using measures such as oil-filled enclosures to prevent contact between sparking apparatus and an explosive atmosphere
- Explosion-proof enclosures, which are designed so that an explosion within the enclosure cannot escape and touch off the surrounding atmosphere .
In recent years standards for explosion hazard protection have become more uniform between European and North American practice. The "zone" system of classification is now used in modified form in
U.S. National Electrical Code and in the Canadian electrical code. Intrinsic safety apparatus is now approved for use in North American applications, though the explosion-proof enclosures used in North America are still uncommon in Europe.
Toxic gases
Electrical discharges, including partial discharge and corona, can produce small quantities of toxic gases, which in a confined space can prove a serious health hazard. These gases include
ozone and various oxides of nitrogen.
Lightning
The largest-scale sparks are those produced naturally by
lightning. An average bolt of negative lightning carries a current of 30-to-50 kiloamperes, and transfers a charge of 5 coulombs, and disssipates 500 megajoules of energy . However, an average bolt of positive lightning may carry a current of 300-to-500 kiloamperes, transfer a charge of up to 300 coulombs, have a potential difference up to 1 gigavolt , and may dissipate enough energy to light a 100 watt lightbulb for up to 95 years. A negative lightning stroke typically lasts for only tens of microseconds, but multiple strikes are common. A positive lightning stroke is typically a single event. However, the larger peak current may flow for hundreds of milliseconds, making it considerably hotter and more dangerous than negative lightning.
Hazards due to lightning obviously include a direct strike on persons or property. However, lightning can also create dangerous voltage gradients in the earth, and can charge extended metal objects such as
telephone cables, fences, and pipelines to dangerous voltages that can be carried many miles from the site of the strike. These transferred potentials are dangerous to people, livestock, and electronic apparatus. Lightning strikes also start fires and explosions which result in fatalities, injuries, and property damage. For example, each year in North America, thousands of
forest fires are started by lightning strikes.
Measures to control lightning can mitigate the hazard; these include lightning rods, shielding wires, and bonding of electrical and structural parts of buildings to form a continuous enclosure.
Lightning discharges in the atmosphere of
Jupiter are thought to be the source of that planet's powerful
radio frequency emissions.
See also
References
External links
;Guidelines, Catalog Standards and codes
