International safety symbol "Caution, risk of electric shock" (ISO 3864), colloquially known as high voltage symbol. Hazard symbols are easily recognizable symbols designed to warn about hazardous materials or locations

High voltages may lead to electrical breakdown resulting in an electrical discharge as illustrated by the plasma filaments streaming from a Tesla coil. The term electrical breakdown has several similar but distinctly different meanings Electrostatic discharge ( ESD) is the sudden and momentary Electric current that flows between two objects at different Electrical potentials The term is In Physics and Chemistry, plasma is an Ionized Gas, in which a certain proportion of Electrons are free rather than being bound A Tesla coil is a type of resonant transformer circuit invented by Serbian-American scientist Nikola Tesla around 1891

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 and other industrial and scientific applications. Electricity distribution is the penultimate stage in the delivery (before retail) of Electricity to end users The cathode ray tube (CRT is a Vacuum tube containing an Electron gun (a source of electrons and a Fluorescent screen with internal or X-radiation (composed of X-rays) is a form of Electromagnetic radiation. A particle beam is an accelerated stream of Charged particles or Neutrons (often moving at very near the Speed of light) which may be directed by Magnets An electric arc is an Electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive Photomultiplier tubes ( photomultipliers or PMT s for short members of the class of Vacuum tubes and more specifically Phototubes are extremely Generally an amplifier or simply amp, is any device that changes usually increases the amplitude of a signal. This article is about the electronic device not an evacuated pipe used for experiments in Free-fall.

Definition

The 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.

In electric power transmission engineering, high voltage is usually considered any voltage over approximately 35,000 volts. Electric power transmission, a process in the delivery of Electricity to consumers is the bulk transfer of electrical power This is a classification based on the design of apparatus and insulation.

The International Electrotechnical Commission and its national counterparts (IET, IEEE, VDE, etc. The International Electrotechnical Commission ( IEC) is a not-for-profit, non-governmental international Standards organization that prepares and publishes The Institution of Engineering and Technology ( IET) is a British Professional body for those working in Engineering and Technology The Institute of Electrical and Electronics Engineers or IEEE (read eye-triple-e) is an international Non-profit, professional organization ) define high voltage circuits as those with more than 1000 V for alternating current and at least 1500 V for direct current, and distinguish it from low voltage (50–1000 V AC or 120–1500 V DC) and extra low voltage (<50 V AC or <120 V DC) circuits. The volt (symbol V) is the SI derived unit of electric Potential difference or Electromotive force. An alternating current ( AC) is an Electric current whose direction reverses cyclically as opposed to Direct current, whose direction remains constant Direct current ( DC) is the unidirectional flow of Electric charge. Low voltage is an Electrical engineering term that broadly identifies safety considerations of an Electricity supply system based on the Voltage used In Electricity supply, using extra-low voltage ( ELV) is one of several means to protect against Electrical shock. This is in the context of the safety of electrical apparatus.

In the United States 2005 National Electrical Code (NEC), high voltage is any voltage over 600 V (article 490. The United States of America —commonly referred to as the The National Electrical Code ( NEC) or NFPA 70, is a United States standard for the safe installation of Electrical wiring and equipment 2). British Standard BS 7671:2008 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. British Standard BS 7671 "Requirements for electrical installations" is the national standard in the United Kingdom for Low voltage electrical installations

The general public may consider household mains circuits (100–250 V AC), which carry the highest voltages they normally encounter, to be high voltage. Mains is the general-purpose Alternating current (AC Electric power supply (as in "I've connected the appliance to the mains" For example, an installer of heating, ventilation and air conditioning (HVAC) equipment may be licensed to install 24 Volt control circuits, but may not be permitted to connect the 240 volt power circuits of the equipment.

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 families. Digital electronics are Electronics systems that use Digital signals Digital electronics are representations of Boolean algebra also see In Computer engineering, a logic family may refer to one of two related concepts

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 235,000 volts between conductors.

A power supply that provides an e.m.f. greater than 275,000 volts is known as an "EHV Power Supply". Electromotive force ( emf, \mathcal{E} is a term used to characterize electrical devices such as Voltaic cells thermoelectric devices electrical 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 15 kV to 35 kV.

Safety

Voltages of greater than 50 V are capable of producing heart fibrillation if they produce electric currents in body tissues which happen to pass through the chest area. Fibrillation is the rapid irregular and unsynchronized contraction of the muscle fibers of the Heart. Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. The chest is a part of the Anatomy of humans and various other animals sometimes referred to as the Thorax. The electrocution danger is mostly determined by the low conductivity of dry human skin. "Electrocute" redirects here For the band see Electrocute (band. If skin is wet, or if there are wounds, or if the voltage is applied to electrodes which penetrate the skin, then even voltage sources below 40 V can be lethal if contacted. An electrode is an Electrical conductor used to make contact with a nonmetallic part of a circuit (e

Accidental contact with high voltage 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 victims 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.

Sparks in air

Long exposure photograph of a Tesla coil showing the repeated electric discharges. A Tesla coil is a type of resonant transformer circuit invented by Serbian-American scientist Nikola Tesla around 1891 Electrostatic discharge ( ESD) is the sudden and momentary Electric current that flows between two objects at different Electrical potentials The term is

The dielectric breakdown strength of dry air, at Standard Temperature and Pressure (STP), between spherical electrodes is approximately 33 kV/cm. The term electrical breakdown has several similar but distinctly different meanings In Physical sciences standard conditions for temperature and pressure are Standard sets of conditions for experimental measurements to allow comparisons to be made ^[1] This is only as a rough guide since the actual breakdown voltage is highly dependent upon the electrode shape and size. Strong electric fields (from high voltages applied to small or pointed conductors), often produce violet-colored corona discharges in air, as well as visible sparks. In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can In Electricity, a corona discharge is an Electrical discharge brought on by the Ionization of a Fluid surrounding a conductor, which 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 (such as in high-altitude aircraft), or in an environment of noble gas such as argon, neon, etc. History Noble gas is translated from the German noun de ''Edelgas'' first used in 1898 by Hugo Erdmann to indicate their extremely low level of reactivity This article pertains to the chemical element For other uses see Argon (disambiguation. Neon (ˈniːɒn is the Chemical element that has the symbol Ne and Atomic number 10 , 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 spark or arc. An electric arc is an Electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive 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 emission). Thermionic emission is the flow of Charge carriers from a surface or over some other kind of Electrical potential barrier caused by thermal vibrational energy Even a small 9 V battery can spark noticeably by this mechanism in a darkened room. A PP3 battery, commonly referred to simply as a "nine-volt battery" is shaped as a rounded rectangular prism and has a nominal output of nine Volts 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 arc may form. An electric arc is an Electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive 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 inductance provides a high voltage pulse whenever the current is interrupted. In Electrical circuits, any Electric current i produces a Magnetic field and hence generates a total Magnetic flux \Phi acting AC systems make sustained arcing somewhat less likely since the current returns to zero twice per cycle. An alternating current ( AC) is an Electric current whose direction reverses cyclically as opposed to Direct current, whose direction remains constant 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. Zero crossing is a commonly used term in electronics mathematics and image processing

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 welding, require some element in the circuit to stabilize the arc's current/voltage characteristics. Welding is a fabrication process that joins materials usually Metals or Thermoplastics by causing coalescence.

Electrostatic devices and phenomena

A high voltage is not necessarily dangerous if it cannot deliver substantial current. Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. The common static electric sparks seen under low-humidity conditions always involve voltage buildups well above 700V. Electrostatics is the branch of Science that deals with the Phenomena arising from what seems to be stationary Electric charges Since Classical For example, sparks to car doors in winter can involve voltages as high as 20,000V^[2]. Also, 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. A Van de Graaff generator is an electrostatic machine which uses a moving belt to accumulate very high electrostatically stable Voltages on a hollow metal globe The Wimshurst machine is an electrostatic device for generating high Voltages developed between 1880 and 1883 by British Inventor James Wimshurst These devices have a limited amount of stored energy, so the current produced is low and usually for a short time. ^[3] 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. Power 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 A millisecond (from Milli- and Second; abbreviation ms is one thousandth of a Second. 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 coils are a special case, and touching them is not recommended. A Tesla coil is a type of resonant transformer circuit invented by Serbian-American scientist Nikola Tesla around 1891 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

High tension 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. Electric power is defined as the rate at which Electrical energy is transferred by an Electric circuit. "Electrocute" redirects here For the band see Electrocute (band. Contact with overhead wires is a frequent cause of injury or death. Overhead lines or overhead wires are used to transmit Electrical energy to Trams Trolleybuses or Trains at a distance from the Metal ladders, farm equipment, boat masts, construction machinery, aerial antennas, and similar objects are frequently involved in fatal contact with overhead wires. An antenna is a Transducer designed to transmit or Receive electromagnetic waves In other words antennas convert electromagnetic waves into 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. Unauthorized persons climbing on power pylons or electrical apparatus are also frequently the victims of electrocution. ^[4] 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 but thoroughly insulated from the earth so that he is at the same electrical potential as the line. Electric power transmission, a process in the delivery of Electricity to consumers is the bulk transfer of electrical power 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 antenna is dangerous for this reason, and a high-frequency Tesla Coil can sustain a spark with only one endpoint). An antenna is a Transducer designed to transmit or Receive electromagnetic waves In other words antennas convert electromagnetic waves into A Tesla coil is a type of resonant transformer circuit invented by Serbian-American scientist Nikola Tesla around 1891

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 dielectric gas (such as SF₆ under pressure), or immersion in mineral oil to quench the arc when the high voltage circuit is broken. A circuit breaker is an automatically-operated electrical Switch designed to protect an electrical circuit from damage caused by overload or A dielectric is a nonconducting substance ie an insulator. The term was coined by William Whewell in response to a request from Michael Faraday. Sulfur hexafluoride is an Inorganic compound with the formula. Mineral oil or liquid Petroleum is a By-product in the Distillation of Petroleum to produce Gasoline and other petroleum

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. The term switchgear, used in association with the electric power system or grid refers to the combination of electrical disconnects fuses and/or Circuit breakers An electric arc is an Electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive Maximum temperature of an arc can exceed 10,000 kelvin, and the radiant heat, expanding hot air, and explosive vaporization of metal and insulation material can cause severe injury to unprotected workers. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic 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. The United States of America —commonly referred to as the The National Fire Protection Association (NFPA is a US organization (albeit with some international members charged with creating and maintaining minimum standards and requirements NFPA 70E ( Electrical Safety in the Workplace) is a standard of the National Fire Protection Association that addresses electrical safety requirements for employees And even then, workers must still be careful.

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 hydrogen gas, natural gas, or gasoline vapor mixed with air can be ignited by sparks produced by electrical apparatus. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Natural gas is a Gaseous Fossil fuel consisting primarily of Methane but including significant quantities of Ethane, Propane, 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 by the use of apparatus designed not to accumulate a static charge sufficient to trigger an explosion
• Increased safety, which applies to devices using measures such as oil-filled enclosures to prevent sparks
• Explosion-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). Intrinsic safety ( IS) is a protection technique for safe operation of electronic equipment in explosive atmospheres

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. The United States of America —commonly referred to as the The National Electrical Code ( NEC) or NFPA 70, is a United States standard for the safe installation of Electrical wiring and equipment 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 be a serious health hazard. In Electricity, a corona discharge is an Electrical discharge brought on by the Ionization of a Fluid surrounding a conductor, which These gases include ozone and various oxides of nitrogen. OZONE is an object oriented Operating system written in the C programming language. The term nitrogen oxide typically refers to any Binary compound of Oxygen and Nitrogen, or to a mixture of such compounds Nitric

Lightning

The largest-scale sparks are those produced naturally by lightning. Lightning is an atmospheric discharge of Electricity, which typically occurs during Thunderstorms and sometimes during volcanic eruptions or An average bolt of negative lightning carries a current of 30-to-50 kiloamperes, transfers a charge of 5 coulombs, and dissipates 500 megajoules of energy (enough to light a 100 watt light bulb for 2 months). The coulomb (symbol C) is the SI unit of Electric charge. It is named after Charles-Augustin de Coulomb. The joule (written in lower case ˈdʒuːl or /ˈdʒaʊl/ (symbol J) is the SI unit of Energy measuring heat, Electricity 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 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. Basic principle A traditional landline telephone system also known as "plain old telephone service" (POTS, commonly handles both signaling and audio information Although many of these objects are not normally conductive, very high voltage can cause the electrical breakdown 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. A wildfire, also known as a wildland fire, forest fire, brush fire, vegetation fire, grass fire, Peat fire,

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. A lightning rod (USA or lightning conductor (UK is a single component in a Lightning protection system.

Lightning discharges in the atmosphere of Jupiter are thought to be the source of the planet's powerful radio frequency emissions. Radio is the transmission of signals by Modulation of electromagnetic waves with frequencies below those of visible Light.

See also

• Electrical engineering
• Electric power transmission (includes a 'Health concerns' section)
• Lock and tag Safety Procedures (As required by OSHA and NFPA 70E in the USA)
• People : Nikola Tesla, Robert J. Van de Graaff, Thomas Burton Kinraide
• Devices : Tesla Coil, spark gap
• Other: voltage, 25 kV AC

References

• High Voltage Engineering - Dr M S Naidu & Dr V Kamaraju, Publisher: McGraw-Hill

• [2]Vladimir Gurevich "Protection Devices and Systems for High-Voltage Applications", CRC Press, London - New York, 2003, 292 p.

External links

• Prevention of High Voltage Accidents
• NFPA 70E: Electrical Safety in the Workplace, USA
• Mike Holt NEC in the USA
• USA Department of Energy electrical safety handbook
• High voltage glossary
• [3] Vladimir Gurevich "Protection Devices and Systems for High Voltage Applications", CRC Press, London - New York, 2003, 292 p.