* #REDIRECT[[ High voltage]] ▼The difference between '''high voltage''' and '''low [[voltage]]''' depends on the situation and on the field of science or industry involved. Laypeople generally consider mains to be high voltage largely because it is the highest voltage they normally encounter. The UK's [[Institution of Electrical Engineers]] defines high voltage as more than 1kV, low voltage as above 50V but below 1kV and extra low voltage (ELV) as below 50V. These definitions will be used in the rest of this article except where otherwise stated. Whilst mains voltages are capable of delivering small sparks and fatal shocks, they cannot jump significant distances, so they are dangerous only if touched.
==Safety and insurance industry==
Various [[safety]] and [[insurance]] organizations consider anything outside of the ELV range to be dangerous and in need of regulation. Voltages above this range are sometimes capable of producing heart [[fibrillation]] if they produce [[electric current]]s in body tissues which happen to pass through the [[chest]] area. The [[electrocution]] danger is mostly determined by the low [[conductivity]] of dry human skin, and if skin is wet (especially with [[electrolyte]]s,) or if there are wounds, or if the voltage is applied to [[electrode]]s which penetrate through the skin, then even voltages far below 40V can be lethally high. On the other hand, voltages above approximately 500V have a naturally [[defibrillator|defibrillating]] effect, so sometimes a higher voltage can be safer than a lower voltage.
==Sparks in air==
High voltages make violet [[corona discharge]] and obvious sparks. Voltages below about 500-700 volts cannot produce easily visible [[spark]]s or glows, 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]], [[neon]], etc., sparks appear at much lower voltages. 500 to 700 volts is not a fixed minimum for producing sparks, but it is a rule of thumb.
==Science classroom devices==
A high voltage is not necessarily dangerous. Physics demonstration devices such as [[Van de Graaff generator]]s, [[Wimshurst machine]]s, and [[Tesla coil]]s can produce voltages approaching one million volts, yet at worst they deliver a brief sting. During the discharge, these machines apply high voltage to the body for only a millionth of a second or less. Since electrical power equals the voltage multiplied by the current, or P = V · A, in order to be dangerous, an electric [[power supply]] must also produce a significant current in the heart muscle continuing for many [[millisecond]]s, or it must deliver enough energy to damage tissue through heating. Since the current is only a few micro[[ampere]]s, the power is only a few milli[[watt]]s
==Electrostatic attraction/repulsion==
The terminals of DC high voltage machines can attract dust, lint, and bits of paper. On an everyday scale, voltages higher than a few thousand volts are required in order to create an [[electric field]] with a [[gradient]] large enough to produce obvious forces. On the other hand, the forces depend on the distance from the electrodes and the electrode shapes, and at the microscopic scale of [[MEMS]] machines, even a few tens of volts acts like a high voltage.
==Power lines==
In [[electric power transmission|power grids]], the optimum voltage for long distance distribution usually falls well into the 'high voltage' range. [[electric power|Electric energy flow]] (i.e. power) is the product of [[voltage]] and [[electric current|current]]. Higher voltage at lower current can give the same energy flow as lower voltage at higher current. However, because of [[Ohm's law]], the heating of the wires and the waste of energy is proportional only to current. Utility companies avoid wasting energy by transmitting it at low currents, but at high (sometimes extremely high) voltages. It is easy to trade voltage for current using a [[transformer]], but transformers require that [[alternating current]] is used. Today even DC voltages can be stepped up and down easily by using a [[switched-mode power supply]]. However these are a fairly recent introduction and still have some problems at extremely high powers and tend to be less efficient than transformers, so AC has remained the rule for power distribution. [[HVDC]] is now used to transmit electricity over long distances especially where underground or undersea cables with a relatively high capacitance are in use. It is also used to transmit power between unsynchronised power grids. Separately controlled grids help prevent cascading blackouts as well as having political advantages (countries generally prefer to control their own infrastructure).
==See also==
* '''People''': [[Nikola Tesla]], [[Robert J. Van de Graaff]]
[[category: electrical engineering]]
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