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 tubeThe cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen, with internal or external means to accelerate and deflect the electron beam, used to create images in the form of light emitted from the fluorescent screen...
s, to generate X-rays and
particle beamA particle beam is an accelerated stream of charged particles or neutrons which may be directed by magnets and focused by electrostatic lenses, although they may also be self-focusing ....
s, to demonstrate arcing, for ignition, in
photomultiplier tubesPhotomultiplier tubes , members of the class of vacuum tubes, and more specifically phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum...
, and high power
amplifierGenerally, an amplifier or simply amp, is any device that changes, usually increases, the amplitude of a signal. The relationship of the input to the output of an amplifier—usually expressed as a function of the input frequency—is called the transfer function of the amplifier, and the magnitude of...
vacuum tubeIn electronics, a vacuum tube, electron tube , thermionic valve, or valve is a device used to amplify, switch, otherwise modify, or create an electrical signal by controlling the movement of electrons in a low-pressure space...
s and other industrial and scientific applications.
Definition
The numerical definition of
high voltage depends on the context of the discussion. Two factors considered in the classification of a "high voltage" are the possibility of causing a spark in air, and the danger of electric shock by contact or proximity. The definitions may refer either to the voltage between two conductors of a system, or between any conductor and
groundIn electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
.
In
electric power transmissionElectric power transmission is the bulk transfer of electrical energy, a process in the delivery of electricity to consumers. A power transmission network typically connects power plants to multiple substations near a populated area...
engineering, high voltage is usually considered any voltage over approximately 35,000 volts. This is a classification based on the design of apparatus and insulation.
The
International Electrotechnical CommissionThe International Electrotechnical Commission is a not-for-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...
and its national counterparts (
IETThe Institution of Engineering and Technology is a British professional body for those working in engineering and technology in the United Kingdom and worldwide. It was formed from two separate institutions: the Institution of Electrical Engineers, dating back to 1871, and the Institution of...
, IEEE,
VDEVDE is a three-letter abbreviation with multiple meanings, as described below:*Verband der Elektrotechnik, Elektronik und Informationstechnik*Stock ticker symbol for the Vangaurd Energy VIPERS ETF* VDE-Gallo, a Swiss record label...
, etc.) define
high voltage circuits as those with more than 1000
VThe volt is the SI derived unit of electromotive force, commonly called "voltage". It is also the unit for the related but slightly different quantity electric potential difference...
for
alternating currentIn alternating current the movement of electric charge periodically reverses direction. An electric charge would for instance move forward, then backward, then forward, then backward, over and over again...
and at least 1500 V for
direct currentDirect current is the undirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also be through...
, and distinguish it from
low voltageLow voltage is an electrical engineering term that broadly identifies safety considerations of an electricity supply system based on the voltage used. While different definitions exist for the exact voltage range covered by "low voltage", the most commonly used ones include "mains voltage"...
(50–1000 V AC or 120–1500 V DC) and
extra low voltageIn electricity supply, using extra-low voltage is one of several means to protect against electrical shock. The International Electrotechnical Commission and its member organizations define an ELV circuit as one in which the electrical potential of any conductor against earth is not more than...
(<50 V AC or <120 V DC) circuits. This is in the context of building wiring and the safety of electrical apparatus.
In the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
2005 National Electrical Code (NEC),
high voltage is any voltage over 600 V (article 490.2). British Standard
BS 7671British Standard BS 7671 "Requirements for electrical installations" is the national standard in the United Kingdom for low voltage electrical installations. It is also used as a national standard by Mauritius, St Lucia, Saint Vincent and the Grenadines, Sierra Leone, Sri Lanka, Trinidad and...
:2008 defines
high voltage as any voltage difference between conductors that is higher than 1000 V AC or 1500 V ripple-free DC, or any voltage difference between a conductor and Earth that is higher than 600 V AC or 900 V ripple-free DC.
ElectricianAn electrician is a tradesman specializing in electrical wiring of buildings, stationary machines and related equipment. Electricians may be employed in the installation of new electrical components or the maintenance and repair of existing electrical infrastructure...
s may only be licensed for particular voltage classes, in some jurisdictions. For example an electrical license for a specialized sub-trade such as installation of
HVACHVAC is an acronym that stands for the closely related functions of "Heating, Ventilating, and Air Conditioning"-the technology of indoor environmental comfort...
systems, fire alarm systems, closed circuit television systems may only be authorized to install systems energized up to 30 volts between conductors, and may not be permitted to work on mains-voltage circuits.
The general public may consider household
mainsMains is the general-purpose alternating current electric power supply. The term is not often used in the United States and Canada. In the US, mains power is referred to by a variety of formal and informal names, including household power, household electricity, domestic power, wall power, line...
circuits (100–250 V AC), which carry the highest voltages they normally encounter, to be
high voltage.
Voltages over approximately 50 volts can usually cause dangerous amounts of current to flow through a human being touching two points of a circuit, so safety standards generally are more restrictive where the chance of contact with such high voltage circuits exists.
In digital electronics, a
high voltage is the one that represents a logic 1; this may be only several hundred millivolts for some
logic familiesIn computer engineering, a logic family may refer to one of two related concepts. A logic family of monolithic digital integrated circuit devices is a group of electronic logic gates constructed using one of several different designs, usually with compatible logic levels and power supply...
.
The definition of
extra high voltage (EHV) depends on the context of the discussion. In electric power transmission engineering this refers to equipment designed for more than 345,000 volts between conductors. In electronics systems, a power supply that provides greater than 275,000 volts is known as an "EHV Power Supply". It is often used in experiments in physics.
The accelerating voltage for a television cathode ray tube may be described as "extra high voltage" or "extra-high tension" (EHT), as compared to other voltage supplies within the equipment. This type of supply ranges from >5 kV to about 50 kV.
Safety
Voltages of greater than 50 V applied across dry unbroken human skin are capable of producing heart
fibrillationFibrillation is the rapid, irregular, and unsynchronized contraction of muscle fibers. An important occurrence is with the heart.-Cardiology:There are two major classes of cardiac fibrillation: atrial fibrillation and ventricular fibrillation....
if they produce
electric currentElectric current can mean, depending on the context, a flow of electric charge or the rate of flow of electric charge ....
s in body tissues which happen to pass through the
chestThe chest is a part of the anatomy of humans and various other animals sometimes referred to as the thorax.-Chest anatomy - Humans and other hominids:...
area. The
electrocutionAn electric shock can occur upon contact of a human body with any source of voltage high enough to cause sufficient current through the muscles or hair. The minimum current a human can feel is thought to be about 1 milliampere ....
danger is mostly determined by the low
conductivityConductivity may refer to:*Electrical conductivity, a measure of a material's ability to conduct an electric current*Conductivity , a measurement of an electrolytic solution, such as water...
of dry human skin. If skin is wet, or if there are wounds, or if the voltage is applied to
electrodeAn electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s which penetrate the skin, then even voltage sources below 40 V can be lethal if contacted.
Accidental contact with high voltage supplying sufficient energy will usually result in severe injury or death. This can occur as a person's body provides a path for current flow causing tissue damage and heart failure. Other injuries can include burns from the arc generated by the accidental contact. These can be especially dangerous if the victim's airways are affected. Injuries may also be suffered as a result of the physical forces exerted as people may fall from height or be thrown a considerable distance.
Low-energy exposure to high voltage may be harmless, such as the spark produced in a dry climate when touching a doorknob after walking across a carpeted floor.
Sparks in air
The dielectric breakdown strength of dry air, at Standard Temperature and Pressure (STP), between spherical electrodes is approximately 33 kV/cm. This is only as a rough guide since the actual breakdown voltage is highly dependent upon the electrode shape and size. Strong
electric fieldIn physics, the space surrounding an electric charge or in the presence of a time-varying magnetic field has a property called an electric field. This electric field exerts a force on other electrically charged objects...
s (from high voltages applied to small or pointed conductors), often produce violet-colored
corona dischargeIn electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor, which occurs when the potential gradient exceeds a certain value, but conditions are insufficient to cause complete electrical breakdown or arcing.- Introduction :A corona...
s in air, as well as visible sparks. Voltages below about 500–700 volts cannot produce easily visible
sparkA spark is a small airborne ember or particle of red-hot matter.Spark may also refer to:
In science:* An electric spark, usually with a flash and a sharp noise, may be:** A momentary electrostatic discharge...
s or glows in air at atmospheric pressure, so by this rule these voltages are "low". However, under conditions of low atmospheric pressure (such as in high-altitude
aircraftAn aircraft is a vehicle which is able to fly by being supported by the air, or in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil An aircraft is a vehicle which is able to fly by being supported...
), or in an environment of
noble gasThe noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with a very low chemical reactivity...
such as
argonArgon is a chemical element designated by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is present in the Earth's atmosphere at 0.94%. Terrestrially, it is the most abundant and most frequently used of the noble gases...
,
neonNeon is the chemical element that has the symbol Ne and atomic number 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in discharge tubes and neon lamps...
, etc., sparks will appear at much lower voltages. 500 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
sparkA spark is a small airborne ember or particle of red-hot matter.Spark may also refer to:
In science:* An electric spark, usually with a flash and a sharp noise, may be:** A momentary electrostatic discharge...
or
arcAn electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. The phenomenon was first described by Vasily V. Petrov, a Russian scientist who discovered...
. 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 (through
thermionic emissionThermionic emission is the heat-induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the carrier overcomes the forces restraining it. The charge carriers can be electrons or ions, and in older literature are sometimes...
). Even a small 9 V battery can spark noticeably by this mechanism in a darkened room. The ionized air and metal vapour (from the contacts) form plasma, which temporarily bridges the widening gap. If the power supply and load allow sufficient current to flow, a self-sustaining
arcAn electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. The phenomenon was first described by Vasily V. Petrov, a Russian scientist who discovered...
may form. Once formed, an arc may be extended to a significant length before breaking the circuit. Attempting to open an inductive circuit often forms an arc since the
inductanceInductance is the property in an electrical circuit where a change in the electric current through that circuit induces an electromotive force that opposes the change in current ....
provides a high voltage pulse whenever the current is interrupted.
ACIn alternating current the movement of electric charge periodically reverses direction. An electric charge would for instance move forward, then backward, then forward, then backward, over and over again...
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 crossingZero-crossing is a commonly used term in electronics, mathematics, and image processing. In mathematical terms, a "zero-crossing" is a point where the sign of a function changes Zero-crossing is a commonly used term in electronics, mathematics, and image processing. In mathematical terms, a...
, and must reignite during the next half cycle in order to maintain the arc.
Unlike an ohmic conductor, the voltage at the ends of an arc decreases as the current increases. This makes unintentional arcs in electrical apparatus dangerous since once even a small arc is initiated, if sufficient current is available, the arc will grow. Such arcs can cause great damage to equipment and present a severe fire hazard. Intentionally produced arcs, such as used in lighting or
weldingWelding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...
, require some element in the circuit to stabilize the arc's current/voltage characteristics.
Electrostatic devices and phenomena
A high voltage is not necessarily dangerous if it cannot deliver substantial current. The common
static electric sparksElectrostatics is the branch of science that deals with the phenomena arising from stationary or slow-moving electric charges.Since classical antiquity it was known that some materials such as amber attract light particles after rubbing. The Greek word for amber, ήλεκτρον , was the source of the...
seen under low-humidity conditions always involve voltage buildups well above 700 V. For example, sparks to car doors in winter can involve voltages as high as 20,000 V. Also, physics demonstration devices such as
Van de Graaff generatorA Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate very high electrostatically stable voltages on a hollow metal globe on the top of the stand. Invented in 1929 by American physicist Robert J. Van de Graaff, the potential differences achieved in modern...
s and
Wimshurst machineThe Wimshurst machine is an electrostatic device for generating high voltages developed between 1880 and 1883 by British inventor James Wimshurst ....
s 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 supplyPower supply is a reference to a source of electrical power. A device or system that supplies electrical or other types of energy to an output load or group of loads is called a power supply unit or PSU...
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 (spread over time) than a mobile phone.
Note that
Tesla coilA Tesla coil is a type of resonant transformer circuit invented by Nikola Tesla around 1891. It is used to produce high voltage, low current, high frequency alternating current electricity. A Tesla coil consists of two, or sometimes three, coupled resonant electric circuits, but is difficult to...
s 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 (50 Hz or 60 Hz, and capable in any case of depolarizing cells and stopping the heart) currents at lethally high voltages to the body.
Power lines
Electrical transmission and distribution lines for
electric powerElectric power is defined as the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt.When electric current flows in a circuit, it can transfer energy to do mechanical or thermodynamic work...
always use voltages significantly higher than 50 volts, so contact with or close approach to the line conductors presents a danger of
electrocutionAn electric shock can occur upon contact of a human body with any source of voltage high enough to cause sufficient current through the muscles or hair. The minimum current a human can feel is thought to be about 1 milliampere ....
. Contact with overhead wires is a frequent cause of injury or death. Metal ladders, farm equipment, boat masts, construction machinery, aerial 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 an excavation site. Digging equipment (either hand tools or machine driven) that contacts a buried cable may energize piping or the ground in the area, resulting in electrocution of nearby workers. A fault in a high-voltage transmission line or substation may result in high currents flowing along the surface of the earth, producing an
earth potential riseIn electrical engineering, Earth Potential Rise also called Ground Potential Rise occurs when a large current flows to earth through an earth grid impedance. The potential relative to a distant point on the Earth is highest at the point where current enters the ground, and declines with distance...
that also presents a danger of electric shock.
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 lineIn electrical engineering, live-line working is the maintenance of electrical equipment, often operating at high voltage, while still energised, that is while it is "live" or "hot". The first techniques for live-line working were developed in the early years of the twentieth century, and both...
" techniques to allow hands-on contact with energized equipment. In this case the worker is electrically connected to the high voltage line but thoroughly insulated from the earth so that he is at the same electrical potential as the line. Since training for such operations is lengthy, and still presents a danger to personnel, only very important transmission lines are subject to maintenance while live. Outside these properly engineered situations, it should not be assumed that being insulated from earth guarantees that no current will flow to earth as grounding, or arcing to ground, can occur in unexpected ways, and high-frequency currents can cause burns even to an ungrounded person (touching a transmitting
antennaAn antenna is a transducer designed to transmit or receive electromagnetic waves. In other words, antennas convert electromagnetic waves into electrical currents and vice versa. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, wireless...
is dangerous for this reason, and a high-frequency
Tesla CoilA Tesla coil is a type of resonant transformer circuit invented by Nikola Tesla around 1891. It is used to produce high voltage, low current, high frequency alternating current electricity. A Tesla coil consists of two, or sometimes three, coupled resonant electric circuits, but is difficult to...
can sustain a spark with only one endpoint).
Protective equipment on high-voltage transmission lines normally prevents formation of an unwanted arc, or ensures that it is quenched within tens of milliseconds. Electrical apparatus which interrupts 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
dielectricA dielectric is a nonconducting substance, i.e. an insulator. The term was coined by William Whewell in response to a request from Michael Faraday...
gas (such as SF
6 under pressure), or immersion in
mineral oilMineral oil or liquid petroleum is a by-product in the distillation of petroleum to produce gasoline and other petroleum based products from crude oil. It is a transparent, colorless oil composed mainly of alkanes and cyclic paraffins, related to petroleum jelly . It has a density of around 0.8...
to quench the arc when the high voltage circuit is broken.
Arc flash hazard
Depending on the prospective short circuit current available at a
switchgearThe term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults...
line-up, a hazard is presented to maintenance and operating personnel due to the possibility of a high-intensity
electric arcAn electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. The phenomenon was first described by Vasily V. Petrov, a Russian scientist who discovered...
. Maximum temperature of an arc can exceed 10,000
kelvinThe kelvin is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic temperature scale where absolute zero, the theoretical absence of all thermal energy, is zero kelvin...
, 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 StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
the
National Fire Protection AssociationThe National Fire Protection Association is a U.S. organization charged with creating and maintaining minimum standards and requirements for fire prevention and suppression activities, training, and equipment, as well as other life-safety codes and standards...
, has published a guideline standard
NFPA 70ENFPA 70E, Electrical Safety in the WorkplaceNFPA 70E is a standard of the National Fire Protection Association. NFPA is a membership organization whose headquarters is located in Quincy, Massachusetts. Its members include experts on electrical safety...
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
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
gas,
natural gasNatural gas is a gas consisting primarily of methane. It is found associated with fossil fuels, in coal beds, as methane clathrates, and is created by methanogenic organisms in marshes, bogs, and landfills...
, or
gasolineGasoline or petrol is a petroleum-derived liquid mixture, primarily used as fuel in internal combustion engines...
vapor mixed with air can be ignited by sparks produced by electrical apparatus. Examples of industrial facilities with hazardous areas are
petrochemicalPetrochemicals are chemical products made from raw materials of petroleum or other hydrocarbon origin. Although some of the chemical compounds that originate from petroleum may also be derived from coal and natural gas, petroleum is the major source...
refineries,
chemical plantA chemical plant is an industrial process plant that manufactures chemicals, usually on a large scale. The general objective of a chemical plant is to create new material wealth via the chemical or biological transformation and or separation of materials. Chemical plants use special equipment,...
s,
grain elevatorGrain elevators are buildings or complexes of buildings for storage and shipment of grain. They were invented in 1842-43 in Buffalo, New York, by a local merchant named Joseph Dart, Jr. and an engineer named Robert Dunbar...
s, and coal mines.
Measures taken to prevent such explosions include:
- Intrinsic safety
Intrinsic safety is a protection technique for safe operation of electronic equipment in explosive atmospheres. The concept was developed for safe operation of process control instrumentation in hazardous areas, particularly North Sea gas platforms...
by the use of apparatus designed not to accumulate enough stored electrical energy to trigger an explosion
- Increased safety, which applies to devices using measures such as oil-filled enclosures to prevent sparks
- Explosion-proof (flame-proof) enclosures, which are designed so that an explosion within the enclosure cannot escape and ignite a surrounding explosive atmosphere (this designation does not imply that the apparatus will survive an internal or external explosion).
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.The United States of America is a federal constitutional republic comprising fifty states and a federal district...
National Electrical CodeThe National Electrical Code , or NFPA 70, is a United States standard for the safe installation of electrical wiring and equipment. It is part of the National Fire Codes series published by the National Fire Protection Association . "National Electrical Code" and "NEC" are registered trademarks...
and in the
Canadian Electrical CodeThe Canadian Electrical Code, CE code, or CSA C22.1 is a standard published by the Canadian Standards Association pertaining to the installation and maintenance of electrical equipment in Canada....
. Intrinsic safety apparatus is now approved for use in North American applications, though the explosion-proof (flame-proof) enclosures used in North America are still uncommon in Europe.
Toxic gases
Electrical discharges, including partial discharge and
coronaIn electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor, which occurs when the potential gradient exceeds a certain value, but conditions are insufficient to cause complete electrical breakdown or arcing.- Introduction :A corona...
, can produce small quantities of toxic gases, which in a confined space can be a serious health hazard. These gases include
ozoneOzone or trioxygen is a simple triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic O
2. Ground-level ozone is an air pollutant with harmful effects on the respiratory systems of animals...
and various
oxides of nitrogenThe term nitrogen oxide typically refers to any binary compound of oxygen and nitrogen, or to a mixture of such compounds:* Nitric oxide , nitrogen oxide* Nitrogen dioxide , nitrogen oxide...
.
Lightning
The largest-scale sparks are those produced naturally by
lightningLightning is an atmospheric discharge of electricity accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...
. An average bolt of negative lightning carries a current of 30 to 50 kiloamperes, transfers a charge of 5
coulombThe coulomb is the SI derived unit of electric charge. It is named after Charles-Augustin de Coulomb.- Definition :If 2 like point charges of equal magnitudes are placed in a vacuum at a distance of 1 metre away from each other and if they repel each other with a force of 9*1000000000 Newton, then...
s, and dissipates 500 megajoules of energy (enough to light a 100 watt light bulb for 2 months). However, an average bolt of positive lightning (from the top of a thunderstorm) 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 (a billion volts), and may dissipate enough energy to light a 100 watt light bulb 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
telephoneThe telephone is a telecommunications device that transmits and receives sound, most commonly the human voice. It is one of the most common household appliances in the developed world, and has long been considered indispensable to business, industry and government...
cables, fences, and pipelines to dangerous voltages that can be carried many miles from the site of the strike. Although many of these objects are not normally conductive, very high voltage can cause the
electrical breakdownThe term electrical breakdown has several similar but distinctly different meanings. The term can apply to the failure of an electric circuit....
of such insulators, causing them to act as conductors. 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 rodA lightning rod or lightning conductor is a metal rod or conductor mounted on top of a building and electrically connected to the ground through a wire, to protect the building in the event of lightning...
s, shielding wires, and bonding of electrical and structural parts of buildings to form a continuous enclosure.
High-voltage lightning discharges in the atmosphere of
JupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with a mass slightly less than one-thousandth that of the Sun but is two and a half times the mass of all of the other planets in our Solar System combined. Jupiter is classified as a gas...
are thought to be the source of the planet's powerful
radioRadio is the transmission of signals by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
frequency emissions.
See also
- Electrical engineering
Electrical engineering, sometimes referred to as electrical and electronic engineering, is a field of engineering that deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after...
- Electric power transmission
Electric power transmission is the bulk transfer of electrical energy, a process in the delivery of electricity to consumers. A power transmission network typically connects power plants to multiple substations near a populated area...
(includes a 'Health concerns' section)
- Lock and tag
Lockout-Tagout or lock and tag is a safety procedure which is used in industry and research settings to ensure that dangerous machines are properly shut off and not started up again prior to the completion of maintenance or servicing work. It requires that hazardous power sources be "isolated and...
Safety Procedures (As required by OSHAOSHA may refer to:* Occupational Safety and Health Administration, an American federal agency* Occupational Safety and Health Act of 1970, a federal law in the United States, the act that created the Occupational Safety and Health Administration mentioned above* Occupational Safety and Health Act...
and NFPA 70ENFPA 70E, Electrical Safety in the WorkplaceNFPA 70E is a standard of the National Fire Protection Association. NFPA is a membership organization whose headquarters is located in Quincy, Massachusetts. Its members include experts on electrical safety...
in the USA)
- People : Nikola Tesla
Nikola Tesla was an inventor and a mechanical and electrical engineer. He is frequently cited as one of the most important contributors to the birth of commercial electricity and is best known for his many revolutionary developments in the field of electromagnetism in the late 19th and early 20th...
, Robert J. Van de GraaffRobert Jemison Van de Graaff, was an American physicist and instrument maker, and professor of physics at Princeton University.-Biography:...
, Thomas Burton Kinraide
- Devices : Tesla coil
A Tesla coil is a type of resonant transformer circuit invented by Nikola Tesla around 1891. It is used to produce high voltage, low current, high frequency alternating current electricity. A Tesla coil consists of two, or sometimes three, coupled resonant electric circuits, but is difficult to...
, spark gapA spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air. When a suitable voltage is supplied, a spark forms, ionizing the gas and drastically reducing its electrical resistance...
- Other: voltage
Voltage is commonly used as a short name for electrical potential difference. Its corresponding SI unit is the volt...
, 25 kV ACThe AC, railway electrification system is commonly used in railways worldwide, especially on high-speed lines.-Overview:This electrification system is ideal for railways that cover long distances and/or carry heavy traffic...
- Detecting and measuring internal discharges is extremely important in H V technique, see Partial discharge#Discharge detection and measuring systems
- Capacitive voltage transformer
External links