High voltage
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).
The dielectric breakdown strength of dry air, at Standard Temperature and Pressure (STP), between spherical electrodes is approximately 33 kV/cm.
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.
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.
The current becomes constricted to these small hot spots, causing them to become incandescent, so that they emit electrons (through thermionic emission).
The ionized air and metal vapour (from the contacts) form plasma, which temporarily bridges the widening gap.
High voltage is used in power distribution to reduce ohmic losses when transporting electricity long distance.
Similarly, lightning discharges in the atmosphere of Jupiter are thought to be the source of the planet's powerful radio frequency emissions.
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.
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.
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.
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.
Metal ladders, farm equipment, boat masts, construction machinery, aerial antennas, and similar objects are frequently involved in fatal contact with overhead wires.
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.
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.
Touching 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.
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.
Despite Tesla coils superficially appearing similar to Van de Graaff generators, they are not electrostatic machines and can produce significant radio frequency currents continuously.
Depending on the prospective short-circuit current available at a switchgear line-up, a hazard is presented to maintenance and operating personnel due to the possibility of a high-intensity electric arc.
Maximum temperature of an arc can exceed 10,000 kelvins, and the radiant heat, expanding hot air, and explosive vaporization of metal and insulation material can cause severe injury to unprotected workers.
Such switchgear line-ups and high-energy arc sources are commonly present in electric power utility substations and generating stations, industrial plants and large commercial buildings.
Even voltages insufficient to break down air can supply enough energy to ignite atmospheres containing flammable gases or vapours, or suspended dust.
Examples of industrial facilities with hazardous areas are petrochemical refineries, chemical plants, grain elevators, and coal mines.
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.
