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85-500: Southwestern operates and maintains 1,380 miles of high-voltage transmission lines , 24 substations, and 46 microwave and VHF radio sites from field offices. Around-the-clock power scheduling and dispatching is conducted from the Operations Center. Southwestern's rates, by law, are designed to recover the costs of producing the power. These costs include repayment of the dams' initial construction, with interest; repayment of

170-424: A glow discharge , an arc has little discernible structure, since the positive column is quite bright and extends nearly to the electrodes on both ends. The cathode fall and anode fall of a few volts occur within a fraction of a millimeter of each electrode. The positive column has a lower voltage gradient and may be absent in very short arcs. A low-frequency (less than 100 Hz) alternating current arc resembles

255-403: A short circuit and tripping protective devices ( fuses and circuit breakers ). A similar situation may occur when a lightbulb burns out and the fragments of the filament pull an electric arc between the leads inside the bulb, leading to overcurrent that trips the breakers. An electric arc over the surface of plastics causes their degradation. A conductive carbon-rich track tends to form in

340-461: A 'mini- Tennessee Valley Authority ' system for the states of Texas, Arkansas, Oklahoma, and Missouri (and a few more states who would be customers to these dams' energy production) but only for the federally-built dams and their hydroelectric power plants. Other power plants like fossil fuel plants were not going to be owned and managed by the SWPA like Tennessee's power plants. The reason for the creation of

425-541: A 100-watt light bulb for approximately 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 300 GJ of energy (72 tons TNT, or enough energy to light a 100-watt light bulb for up to 95 years). A negative lightning strike typically lasts for only tens of microseconds, but multiple strikes are common. A positive lightning stroke

510-497: A blast of high pressure air, a special dielectric gas (such as SF 6 under pressure), or immersion in mineral oil to quench the arc when the high voltage circuit is broken. Wiring in equipment such as X-ray machines and lasers requires care. The high voltage section is kept physically distant from the low voltage side to reduce the possibility of an arc forming between the two. To avoid coronal losses, conductors are kept as short as possible and free of sharp points. If insulated,

595-402: A bypass switch engaged), the arc will not reignite. The arc can be also broken by a blast of compressed air or another gas. An undesirable arc can also occur when a high-voltage switch is opened and is extinguished in similar ways. Modern devices use sulphur hexafluoride at high pressure in a nozzle flow between separated electrodes within a pressurized vessel. The arc current is interrupted at

680-536: A direct current arc; on each cycle, the arc is initiated by breakdown, and the electrodes interchange roles, as anode or cathode, when current reverses. As the frequency of the current increases, there is not enough time for all ionization to disperse on each half cycle, and the breakdown is no longer needed to sustain the arc; the voltage vs. current characteristic becomes more nearly ohmic. The various shapes of electric arcs are emergent properties of non-linear patterns of current and electric field . The arc occurs in

765-413: A direct strike on persons or property. However, lightning can also create dangerous voltage gradients in the earth, as well as an electromagnetic pulse , 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. Although many of these objects are not normally conductive, very high voltage can cause

850-417: 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 ). 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

935-497: A fixed-voltage supply until the apparatus is destroyed. Industrially, electric arcs are used for welding , plasma cutting , for electrical discharge machining , as an arc lamp in movie projectors , and spotlights in stage lighting . Electric arc furnaces are used to produce steel and other substances. Calcium carbide is made in this way as it requires a large amount of energy to promote an endothermic reaction (at temperatures of 2500 °C). Carbon arc lights were

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1020-475: A glow discharge or by momentarily touching the electrodes then separating them), increased current results in a lower voltage between the arc terminals. This negative resistance effect requires that some positive form of impedance (as an electrical ballast ) be placed in the circuit to maintain a stable arc. This property is the reason uncontrolled electrical arcs in apparatus become so destructive, since once initiated an arc will draw more and more current from

1105-477: A glow discharge, and current is higher. An arc in gases near atmospheric pressure is characterized by visible light emission, high current density, and high temperature. An arc is distinguished from a glow discharge partly by the similar temperatures of the electrons and the positive ions; in a glow discharge, the ions are much colder than the electrons. A drawn arc can be initiated by two electrodes initially in contact and drawn apart; this can initiate an arc without

1190-518: 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

1275-572: A hazard to people and equipment. Undesired arcing in electrical contacts of contactors , relays and switches can be reduced by devices such as contact arc suppressors and RC snubbers or through techniques including: Arcing can also occur when a low resistance channel (foreign object, conductive dust , moisture...) forms between places with different voltage. The conductive channel then can facilitate formation of an electric arc. The ionized air has high electrical conductivity approaching that of metals, and it can conduct extremely high currents, causing

1360-421: A high-voltage transmission line or substation may result in high currents flowing along the surface of the earth, producing an earth potential rise that also presents a danger of electric shock. For high voltage and extra-high voltage transmission lines, specially trained personnel use " live line " techniques to allow hands-on contact with energized equipment. In this case the worker is electrically connected to

1445-406: A human body will be relatively constant as long as contact is maintained, unlike with electrostatic machines which generally take longer to build up charges, and the voltage will be much higher than the break-down voltage of human skin. As a consequence, the output of a Tesla coil can be dangerous or even fatal. Depending on the prospective short-circuit current available at a switchgear line-up,

1530-486: A leg. Electricity can flow between two conductors in high voltage equipment and the body can complete the circuit. To avoid that from happening, the worker should wear insulating clothing such as rubber gloves, use insulated tools, and avoid touching the equipment with more than one hand at a time. An electrical current can also flow between the equipment and the earth ground. To prevent that, the worker should stand on an insulated surface such as on rubber mats. Safety equipment

1615-457: A method to control the path of an arc between two electrodes by firing laser beams at the gas between the electrodes. The gas becomes a plasma and guides the arc. By constructing the plasma path between the electrodes with different laser beams, the arc can be formed into curved and S-shaped paths. The arc could also hit an obstacle and reform on the other side of the obstacle. The laser-guided arc technology could be useful in applications to deliver

1700-467: 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 burns can be especially dangerous if the victim's airway is affected. Injuries may also be suffered as a result of the physical forces experienced by people who fall from a great height or are thrown a considerable distance. Low-energy exposure to high voltage may be harmless, such as

1785-465: A relatively few electrons move. These devices have a limited amount of stored energy, so the average current produced is low and usually for a short time, with impulses peaking in the 1 A range for a nanosecond. The discharge may involve extremely high voltage over very short periods, but to produce heart fibrillation, an electric power supply must produce a significant current in the heart muscle continuing for many milliseconds , and must deposit

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1870-408: A short distance apart. The demonstration produced a "feeble" arc, not readily distinguished from a sustained spark , between charcoal points. The Society subscribed for a more powerful battery of 1,000 plates, and in 1808 he demonstrated the large-scale arc. He is credited with naming the arc. He called it an arc because it assumes the shape of an upward bow when the distance between the electrodes

1955-420: A small-scale arc flash , is the foundation of exploding-bridgewire detonators . Electric arcs are used in arcjet , a form of electric propulsion of spacecraft. They are used in the laboratory for spectroscopy to create spectral emissions by intense heating of a sample of matter . Arc is still being used in high voltage switchgear for protection of extra high voltage transmission networks. To protect

2040-404: A spark gap can be fitted with arcing horns − two wires, approximately vertical but gradually diverging from each other towards the top in a narrow V shape. Once ignited, the arc will move upwards along the wires and will break down when the distance between the wires will become too large. If the arc is extinguished and the original trigger condition no longer exists (a fault has been resolved or

2125-411: A spark of electricity to a precise spot. Undesired or unintended electric arcing can have detrimental effects on electric power transmission , distribution systems and electronic equipment. Devices which may cause arcing include switches, circuit breakers, relay contacts, fuses and poor cable terminations. When an inductive circuit is switched off, the current cannot instantaneously jump to zero:

2210-617: A total energy in the range of at least millijoules or higher. Relatively high current at anything more than about fifty volts can therefore be medically significant and potentially fatal. During the discharge, these machines apply high voltage to the body for only a millionth of a second or less. So a low current is applied for a very short time, and the number of electrons involved is very small. Despite Tesla coils superficially appearing similar to Van de Graaff generators, they are not electrostatic machines and can produce significant radio frequency currents continuously. The current supplied to

2295-517: A transient arc will be formed across the separating contacts. Switching devices susceptible to arcing are normally designed to contain and extinguish an arc, and snubber circuits can supply a path for transient currents, preventing arcing. If a circuit has enough current and voltage to sustain an arc formed outside of a switching device, the arc can cause damage to equipment such as melting of conductors, destruction of insulation, and fire. An arc flash describes an explosive electrical event that presents

2380-485: A transmitting antenna is dangerous for this reason, and a high-frequency Tesla coil 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 that interrupts high-voltage circuits is designed to safely direct the resulting arc so that it dissipates without damage. High voltage circuit breakers often use

2465-472: A unit (e. g., a series capacitor in a transmission line) against overvoltage, an arc-inducing device, so called spark gap , is connected in parallel to the unit. Once the voltage reaches the air-breakdown threshold, an arc ignites across the spark plug and short-circuits the terminals of the unit, thus protecting the latter from the overvoltage. For the reinsertion of a unit, the arc needs to be extinguished, this can be achieved in multiple ways. For example,

2550-912: A voltage as high voltage are the possibility of causing a spark in air, and the danger of electric shock by contact or proximity. The International Electrotechnical Commission and its national counterparts ( IET , IEEE , VDE , etc.) define high voltage as above 1000  V for alternating current , and at least 1500 V for direct current . In the United States, the American National Standards Institute (ANSI) establishes nominal voltage ratings for 60 Hz electric power systems over 100 V. Specifically, ANSI C84.1-2020 defines high voltage as 115 kV to 230 kV, extra-high voltage as 345 kV to 765 kV, and ultra-high voltage as 1,100 kV. British Standard BS 7671 :2008 defines high voltage as any voltage difference between conductors that

2635-481: Is U+26A1, the symbol "⚡︎" . The common static electric sparks seen under low-humidity conditions always involve voltage well above 700 V. For example, sparks to car doors in winter can involve voltages as high as 20,000 V. Electrostatic generators such as Van de Graaff generators and Wimshurst machines can produce voltages approaching one million volts at several amps, but typically don't last long enough to cause damage. Induction coils operate on

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2720-624: Is a device for producing a continuous train of electric arcs that rise upwards. The device is named for the Jacob's Ladder leading to heaven as described in the Bible. Similarly to the arcing horns, the spark gap is formed by two wires diverging from the base to the top. When high voltage is applied to the gap, a spark forms across the bottom of the wires where they are nearest each other, rapidly changing to an electric arc. Air breaks down at about 30 kV/cm, depending on humidity, temperature, etc. Apart from

2805-530: Is classified as voltages in the range of 345,000– 765,000 V. In electronics systems, a power supply that provides greater than 275,000 volts is called an EHV Power Supply , and 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), compared to other voltage supplies within the equipment. This type of supply ranges from 5 kV to about 30 kV. The Unicode text character representing "high voltage"

2890-399: Is dispersed rapidly to the heavier particles by elastic collisions , due to their great mobility and large numbers. Current in the arc is sustained by thermionic emission and field emission of electrons at the cathode. The current may be concentrated in a very small hot spot on the cathode; current densities on the order of one million amperes per square centimeter can be found. Unlike

2975-503: Is extinguished every time the current goes through a zero crossing , and must reignite during the next half-cycle to maintain the arc. Unlike an ohmic conductor, the resistance of an arc decreases as the current increases. This makes unintentional arcs in an electrical apparatus dangerous since even a small arc can grow large enough to damage equipment and start fires if sufficient current is available. Intentionally produced arcs, such as used in lighting or welding , require some element in

3060-675: Is higher than 1000 VAC or 1500 V ripple-free DC, or any voltage difference between a conductor and Earth that is higher than 600 VAC or 900 V ripple-free DC. Electricians 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 HVAC systems, fire alarm systems, closed-circuit-television systems may be authorized to install systems energized up to only 30 volts between conductors, and may not be permitted to work on mains-voltage circuits. The general public may consider household mains circuits (100 to 250 VAC), which carry

3145-442: Is initiated either by thermionic emission or by field emission . After initiation, the arc relies on thermionic emission of electrons from the electrodes supporting the arc. An arc discharge is characterized by a lower voltage than a glow discharge . An archaic term is voltaic arc , as used in the phrase "voltaic arc lamp". Techniques for arc suppression can be used to reduce the duration or likelihood of arc formation. In

3230-480: Is invisible but has a sweet odor. It oxidizes to nitrogen dioxide within a few minutes, which has a yellow or reddish-brown color depending on concentration and smells of chlorine gas like a swimming pool. Ozone is invisible but has a pungent smell like that of the air after a lightning storm. It is a short-lived species and half of it breaks down into O 2 within a day at normal temperatures and atmospheric pressure. Hazards due to lightning obviously include

3315-415: Is limited only by the external circuit, not by the arc itself. An arc between two electrodes can be initiated by ionization and glow discharge, when the current through the electrodes is increased. The breakdown voltage of the electrode gap is a combined function of the pressure, distance between electrodes and type of gas surrounding the electrodes. When an arc starts, its terminal voltage is much less than

3400-407: 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 327 volts, as noted by Friedrich Paschen . While lower voltages do not, in general, jump a gap that is present before the voltage is applied, interrupting an existing current flow with a gap often produces a low-voltage spark or arc . As the contacts are separated,

3485-460: Is not small. This is due to the buoyant force on the hot gas. The first continuous arc was discovered independently in 1802 and described in 1803 as a "special fluid with electrical properties", by Vasily V. Petrov , a Russian scientist experimenting with a copper-zinc battery consisting of 4200 discs. In the late nineteenth century, electric arc lighting was in wide use for public lighting . The tendency of electric arcs to flicker and hiss

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3570-674: 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. Electrical discharges, including partial discharge and corona , can produce small quantities of toxic gases, which in a confined space can be a health hazard. These gases include oxidizers such as ozone and various oxides of nitrogen . They are readily identified by their characteristic odor or color, and thus contact time can be minimized. Nitric oxide

3655-649: Is tested regularly to ensure it is still protecting the user. Test regulations vary according to country. Testing companies can test at up 300,000 volts and offer services from glove testing to Elevated Working Platform (or EWP) testing. Contact with or close approach to line conductors presents a danger of electrocution . Contact with overhead wires can result in 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. Unauthorized persons climbing on power pylons or electrical apparatus are also frequently

3740-404: Is typically a single event, but the larger peak current may flow for hundreds of milliseconds, making it considerably more energetic than negative lightning. The dielectric breakdown strength of dry air, at Standard Temperature and Pressure (STP), between spherical electrodes is approximately 33 kV/cm. This is only a rough guide, since the actual breakdown voltage is highly dependent upon

3825-400: Is used in electrical power distribution , in cathode-ray tubes , to generate X-rays and particle beams , to produce electrical arcs , for ignition, in photomultiplier tubes , and in high-power amplifier vacuum tubes , as well as other industrial, military and scientific applications. The numerical definition of high voltage depends on context. Two factors considered in classifying

3910-560: The chest area. The voltage at which there is the danger of electrocution depends on the electrical conductivity of dry human skin. Living human tissue can be protected from damage by the insulating characteristics of dry skin up to around 50 volts. If the same skin becomes wet, if there are wounds, or if the voltage is applied to electrodes that penetrate the skin, then even voltage sources below 40 V can be lethal. Accidental contact with any high voltage supplying sufficient energy may result in severe injury or death. This can occur as

3995-486: 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. Measures to control lightning can mitigate

4080-553: The high-voltage line but thoroughly insulated from the earth so that he is at the same electrical potential as that of 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, insulation from earth does not guarantee that no current flows to earth—as grounding or arcing to ground can occur in unexpected ways, and high-frequency currents can burn even an ungrounded person. Touching

4165-483: The 10,000–30,000-volt range, such as a neon sign transformer (5–15 kV) or a television picture tube circuit ( flyback transformer ) (10–28 kV), and two coat hangers or rods built into a V shape. For larger ladders, microwave oven transformers connected in series, voltage multipliers and utility pole transformers (pole pigs) run in reverse (step-up) are commonly used. [REDACTED] Media related to Jacob's ladder at Wikimedia Commons Scientists have discovered

4250-665: The House Sam Rayburn of Texas and Representative Clyde T. Ellis of Arkansas spearheaded the effort to make all federal dams constructed by the U.S. Army Corps of Engineers in the region to stay owned and operated by the military while the hydroelectric power stations were to be managed and marketed by the new Southwestern Power Administration. In the late 1930s Rayburn and Ellis were able to secure dams in Denison, TX and Norfork, AR in separate independent projects for their House district constituents. The two representatives pushed for

4335-458: The SWPA was the fear that the federal dams, which are multipurpose in terms of flood control and water storage for recreation and human use, would be poorly managed by private businesses and also because utility companies during the 1920s and 1930s hiked up electricity prices in rural areas far higher than in urban areas. The SWPA was a hallmark of Rayburn's campaign to provide electricity to all farmers and rural Americans. He previously succeeded in

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4420-1025: 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. Even voltages insufficient to break down air can supply enough energy to ignite atmospheres containing flammable gases or vapours, or suspended dust. For example, hydrogen gas, natural gas , or petrol/ gasoline vapor mixed with air can be ignited by sparks produced by electrical apparatus. Examples of industrial facilities with hazardous areas are petrochemical refineries, chemical plants , grain elevators , and coal mines . Measures taken to prevent such explosions include: In recent years, standards for explosion hazard protection have become more uniform between European and North American practice. The "zone" system of classification

4505-400: The anode and cathode voltage drops, the arc behaves almost as a short circuit , drawing as much current as the electrical power supply can deliver, and the heavy load dramatically reduces the voltage across the gap. The heated ionized air rises, carrying the current path with it. As the trail of ionization gets longer, it becomes more and more unstable, finally breaking. The voltage across

4590-413: The arc is continuous and in an enclosed space such as a room. An arc that occurs outside is less of a hazard because the heated ionized gases will rise up into the air and dissipate into the atmosphere. Spark gaps which only intermittently produce short spark bursts are also minimally hazardous because the volume of ions generated is very small. Arcs can also produce a broad spectrum of wavelengths spanning

4675-958: The arc path, called "carbon tracking", negatively influencing their insulation properties. The arc susceptibility, or "track resistance", is tested according to ASTM D495, by point electrodes and continuous and intermittent arcs; it is measured in seconds required to form a track that is conductive under high-voltage low-current conditions. Some materials are less susceptible to degradation than others. For example, polytetrafluoroethylene has arc resistance of about 200 seconds (3.3 minutes). From thermosetting plastics , alkyds and melamine resins are better than phenolic resins . Polyethylenes have arc resistance of about 150 seconds; polystyrenes and polyvinyl chlorides have relatively low resistance of about 70 seconds. Plastics can be formulated to emit gases with arc-extinguishing properties; these are known as arc-extinguishing plastics . Arcing over some types of printed circuit boards , possibly due to cracks of

4760-445: The circuit to stabilize the arc's current/voltage characteristics. Electrical transmission and distribution lines for electric power typically use voltages between tens and hundreds of kilovolts. The lines may be overhead or underground. High voltage is used in power distribution to reduce ohmic losses when transporting electricity long distance. It is used in the production of semiconductors to sputter thin layers of metal films on

4845-411: The continuous electric arc creates heat, which ionizes more gas molecules (where the degree of ionization is determined by temperature), and as per this sequence: solid-liquid-gas-plasma; the gas is gradually turned into a thermal plasma. A thermal plasma is in thermal equilibrium; the temperature is relatively homogeneous throughout the atoms, molecules, ions, and electrons. The energy given to electrons

4930-567: 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. 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 or neon , sparks appear at much lower voltages. 500 to 700 volts

5015-415: The electrodes then rises and the spark re-forms at the bottom of the device. This cycle leads to an exotic-looking display of electric white, yellow, blue or purple arcs, which is often seen in horror films and films about mad scientists . The device was a staple in schools and science fairs of the 1950s and 1960s, typically constructed out of a Model T spark coil or any other source of high voltage in

5100-433: The first electric lights. They were used for street lights in the 19th century and for specialized applications such as searchlights until World War II. Today, electric arcs are used in many applications. For example, fluorescent tubes , mercury, sodium, and metal halide lamps are used for lighting; xenon arc lamps are used for movie projectors and theatrical spotlights. Formation of an intense electric arc, similar to

5185-453: The flyback effect resulting in voltages greater than the turns ratio multiplied by the input voltage. They typically produce higher currents than electrostatic machines, but each doubling of desired output voltage roughly doubles the weight due to the amount of wire required in the secondary winding. Thus scaling them to higher voltages by adding more turns of wire can become impractical. The Cockcroft-Walton multiplier can be used to multiply

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5270-406: The gas-filled space between two conductive electrodes (often made of tungsten or carbon) and it results in a very high temperature , capable of melting or vaporizing most materials. An electric arc is a continuous discharge, while the similar electric spark discharge is momentary. An electric arc may occur either in direct current (DC) circuits or in alternating current (AC) circuits. In

5355-442: The hazard; these include lightning rods , shielding wires, and bonding of electrical and structural parts of buildings to form a continuous enclosure. Arcing An electric arc (or arc discharge ) is an electrical breakdown of a gas that produces a prolonged electrical discharge . The current through a normally nonconductive medium such as air produces a plasma , which may produce visible light . An arc discharge

5440-412: The high-voltage glow discharge. This is the way a welder starts to weld a joint, momentarily touching the welding electrode against the workpiece then withdrawing it until a stable arc is formed. Another example is separation of electrical contacts in switches, relays or circuit breakers; in high-energy circuits arc suppression may be required to prevent damage to contacts. Electrical resistance along

5525-525: 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 who touches two points of a circuit, so safety standards are more restrictive around such circuits. In automotive engineering , high voltage is defined as voltage in range 30 to 1000 VAC or 60 to 1500 VDC. The definition of extra-high voltage (EHV) again depends on context. In electric power transmission engineering, EHV

5610-658: The isolation and discovery of the element argon from atmospheric air. Induction coils powered early X-ray tubes. Moseley used an X-ray tube to determine the atomic number of a selection of metallic elements by the spectrum emitted when used as anodes. High voltage is used for generating electron beams for microscopy . Cockcroft and Walton invented the voltage multiplier to transmutate lithium atoms in lithium oxide into helium by accelerating hydrogen atoms. Voltages greater than 50 V applied across dry unbroken human skin can cause heart fibrillation if they produce electric currents in body tissues that happen to pass through

5695-519: The landmark Rural Electrification Act of 1936 to establish rural cooperatives to supply very cheap electricity to rural Americans. The SWPA (as well as the establishment of the other three Power Marketing Administrations ) and the guaranteed cheap electricity from dams helped the REA's goals to be realized. Rayburn first persuaded President Roosevelt to sign an executive order establishing the SWPA in July 1943. This

5780-494: The late 19th century, electric arc lighting was in wide use for public lighting . Some low-pressure electric arcs are used in many applications. For example, fluorescent tubes , mercury, sodium, and metal-halide lamps are used for lighting; xenon arc lamps have been used for movie projectors . Electric arcs can be utilized for manufacturing processes, such as electric arc welding , plasma cutting and electric arc furnaces for steel recycling. Sir Humphry Davy discovered

5865-438: The latter case, the arc may re-strike on each half cycle of the current. An electric arc differs from a glow discharge in that the current density is quite high, and the voltage drop within the arc is low; at the cathode , the current density can be as high as one megaampere per square centimeter. An electric arc has a non-linear relationship between current and voltage. Once the arc is established (either by progression from

5950-548: The latter method generally applies to devices such as electromechanical power switches, relays and contactors. In this context, arc suppression uses contact protection . Part of the energy of an electrical arc forms new chemical compounds from the air surrounding the arc: these include oxides of nitrogen and ozone , the second of which can be detected by its distinctive sharp smell. These chemicals can be produced by high-power contacts in relays and motor commutators, and they are corrosive to nearby metal surfaces. Arcing also erodes

6035-401: The moment within an AC cycle when the current goes to zero and the highly electronegative SF6 ions quickly absorb free electrons from the decaying plasma. The SF6 technology mostly displaced the similar air-based one because many noisy air-blast units in series were required to prevent the arc inside the switch from re-igniting. A Jacob's ladder (more formally, a high voltage travelling arc )

6120-405: The plastic coating should be free of air bubbles which result in coronal discharges within the bubbles. A high voltage is not necessarily dangerous if it cannot deliver substantial current . Despite electrostatic machines such as Van de Graaff generators and Wimshurst machines producing voltages approaching one million volts, they deliver a brief sting. That is because the current is low, i.e. only

6205-448: 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 circuit often forms 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

6290-609: The short-pulse electrical arc in 1800. In 1801, he described the phenomenon in a paper published in William Nicholson 's Journal of Natural Philosophy, Chemistry and the Arts . According to modern science, Davy's description was a spark rather than an arc. In the same year Davy publicly demonstrated the effect, before the Royal Society , by transmitting an electric current through two carbon rods that touched, and then pulling them

6375-444: The similar Southeastern Power Administration . From 1943 to 1984 twenty-four dams were built for the SWPA. High-voltage High voltage electricity refers to electrical potential large enough to cause injury or damage. In certain industries, high voltage refers to voltage above a certain threshold. Equipment and conductors that carry high voltage warrant special safety requirements and procedures . High voltage

6460-410: The spark produced in a dry climate when touching a doorknob after walking across a carpeted floor. The voltage can be in the thousand-volt range, but the average current is low. The standard precautions to avoid injury include working under conditions that would avoid having electrical energy flow through the body, particularly through the heart region, such as between the arms, or between an arm and

6545-477: The surface of the wafer . It is also used for electrostatic flocking to coat objects with small fibers that stand on edge. Spark gaps were used historically as an early form of radio transmission. Similarly, lightning discharges in the atmosphere of Jupiter are thought to be the source of the planet's powerful radio frequency emissions. High voltages have been used in landmark chemistry and particle physics experiments and discoveries. Electric arcs were used in

6630-436: The surfaces of the contacts, wearing them down and creating high contact resistance when closed. Exposure to an arc-producing device can pose health hazards. An arc formed in air will ionize oxygen and nitrogen, which then can re-form into reactive molecules such as ozone and nitric oxide . These products can be damaging to the mucous membranes . Plants are also susceptible to ozone poisoning. These hazards are greatest when

6715-698: The traces or the failure of a solder joint, renders the affected insulating layer conductive as the dielectric is combusted due to the high temperatures involved. This conductivity prolongs the arcing due to cascading failure of the surface. Arc suppression is a method of attempting to reduce or eliminate an electrical arc. There are several possible areas of use of arc suppression methods, among them metal film deposition and sputtering , arc flash protection , electrostatic processes where electrical arcs are not desired (such as powder painting , air purification , PVDF film poling) and contact current arc suppression. In industrial, military and consumer electronic design,

6800-530: The transmission system construction, with interest; major replacements; and both the Corps of Engineers' and Southwestern's annual operating and maintenance expenses. Annual revenues vary with water conditions, but generally average about US$ 100 million. Southwestern returns the revenues to the U.S. Treasury , and of that, about US$ 40 million is applied toward interest and repayment of the Federal investment. Speaker of

6885-443: The victims of electrocution. At very high transmission voltages even a close approach can be hazardous, since the high voltage may arc across a significant air gap. 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

6970-482: The voltage produced by an induction coil. It generates DC using diode switches to charge a ladder of capacitors. Tesla coils utilize resonance, are lightweight, and do not require semiconductors. The largest scale sparks are those produced naturally by lightning . 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 (120 kg TNT equivalent , or enough to light

7055-585: Was a major problem. In 1895, Hertha Marks Ayrton wrote a series of articles for the Electrician , explaining that these phenomena were the result of oxygen coming into contact with the carbon rods used to create the arc. In 1899, she was the first woman ever to read her own paper before the Institution of Electrical Engineers (IEE). Her paper was entitled "The Hissing of the Electric Arc". Shortly thereafter, Ayrton

7140-610: Was crucial because private utility companies were hoping for the dams to be turned over to the Federal Works Administration during World War II . Most public projects that were turned over to the FWA were scheduled to end up in private companies' hands after the war. Executive orders are only temporary so Rayburn was able to get the SWPA to be permanently established in the Flood Control Act of 1944 . This act also established

7225-517: Was elected the first female member of the IEE; the next woman to be admitted to the IEE was in 1958. She petitioned to present a paper before the Royal Society, but she was not allowed because of her gender, and "The Mechanism of the Electric Arc" was read by John Perry in her stead in 1901. An electric arc is the form of electric discharge with the highest current density. The maximum current through an arc

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