High voltage
High voltage is an electrical potential large enough to cause injury or damage. In certain industries, high voltage refers to voltage above a nominal threshold. Equipment and conductors that carry high voltage warrant special safety requirements and procedures.
High voltage 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.
Definition
The numerical definition of depends on context. Two factors considered in classifying 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 define high voltage as above 1000 V for alternating current, and at least 1500 V for direct current. in IEC 61140.
In the United States, the American National Standards Institute 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 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, 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 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 again depends on context. In electric power transmission engineering, EHV 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, 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" is U+26A1, the symbol "⚡︎".
Production
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 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 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. However, an average bolt of positive lightning may carry a current of 300 to 500 kiloamperes, transfer a charge of up to 300 coulombs, have a potential difference up to 1 gigavolt, and may dissipate 300 GJ of energy. A negative lightning strike typically lasts for only tens of microseconds, but multiple strikes are common. A positive lightning strike is typically a single event, but the larger peak current may flow for hundreds of milliseconds, making it considerably more energetic than negative lightning.
Sparks in air
The dielectric breakdown strength of dry air, at Standard Temperature and Pressure, between spherical electrodes is approximately 33 kV/cm. This is only a rough guide, since the actual breakdown voltage is highly dependent upon the electrode shape and size. Strong electric fields often produce violet-colored corona discharges in air, as well as visible sparks. Voltages below about 500–700 volts cannot produce easily visible sparks or glows in air at atmospheric pressure, so by this rule these voltages are "low". However, under conditions of low atmospheric pressure, or in an environment of noble gas such as argon or neon, sparks 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 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, a few small points of contact become the last to separate. The current becomes constricted to these small hot spots, causing them to become incandescent, so that they emit electrons. Even a small 9 V battery can spark noticeably by this mechanism in a darkened room. The ionized air and metal vapour 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. 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 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 the circuit to stabilize the arc's current/voltage characteristics.
Uses
Distribution
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.Industrial
It is used in the production of semiconductors to sputter thin layers of metal films on the surface of the wafer. It is also used for electrostatic flocking to coat objects with small fibers that stand on edge.Scientific
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 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.
Safety
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 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 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 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 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 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 testing.