Voltage sparks and danger

From Academic Kids

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.

Contents

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 currents 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. If skin is wet (especially with electrolytes, including sea water) or if there are wounds, or if the voltage is applied to electrodes 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 natural defibrillating effect, so sometimes a higher voltage can be safer than a lower voltage. A DC circuit may be especially dangerous because it will cause muscles to lock around the wire.

Sparks in air

The dielectric breakdown strength of dry air at typical room temperature and sea-level pressure is about 33 kV per cm. [1] High voltages, i.e. strong electric fields produce violet-colored corona discharge 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, 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.

Note that small sparks caused by exploding metal surfaces can occur with much smaller voltages; try tapping the terminals of a 9V battery with a paper clip in a dark room. Electric arc welding is an example of an electric arc sustained by voltages less than 100 volts, but such arcs are initiated by contact between the welding rod and work piece. Hot metal surfaces and metal vapor in the arc allow it to persist with a low applied voltage.

Science classroom devices

A high voltage is not necessarily dangerous. 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. These devices have a limited amount of stored energy, so the current produced is low and usually for a short time. 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. 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. Among other issues, they have a tendency to arch to their own bottom-end circuitry, which can introduce powerline frequency (50 Hz or 60 Hz) currents at lethally high voltages to the body.

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, even a few tens of volts acts like a high voltage.

Power lines

Transmission lines for electric power always use voltages in excess of 50 volts, so contact with or close approach to the line conductors presents a danger of electrocution. Contact with overhead wires is a frequent cause of injury or death. Metal ladders, farm equipment, boat masts, construction machinery, televison antennas, and similar objects are frequently involved in fatal contact with overhead wires. Digging into a buried cable can also be dangerous, though often the operator of excavating machines survives such accidents. Unauthorized persons climbing on power pylons or electrical apparatus are also frequently the victims of electrocution. At very high transmission voltages even a close approach can be hazardous.

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. Since training for such operations is lengthy, and still presents a danger to personnel, only very important transmission lines are the objects of live-line maintenance practices.

Normally protective equipment on high-voltage transmission lines prevents formation of an arc, or insures it is de-energized within a few score milliseconds. Electrical apparatus designed to interrupt high-voltage circuits is designed to safely direct the arc so that it dissipates without damage.

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. Maximum temperature of an can exceed 10,000 degrees K, and the radiant heat, expanding hot air, and vaporized 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 the United States the NFPA has published a guideline standard for evaluating arc flash hazard, and provides standards for the protecctive clothing required for electrical workers exposed to such hazards.

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, air containing hydrogen gas or natural gas or gasoline vapor can be ignited by sparks produced by electrical apparatus. 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, which is apparatus designed to not accumulate enough stored energy to touch off an explosion
  • Increased safety, which applies to devices using measures such as oil-filled enclosures to prevent contact between sparking apparatus and an explosive atmosphere
  • Explosion-proof enclosures, which are designed so that an explosion within the enclosure cannot escape and touch off the surrounding atmosphere (this designation does not imply that the appartus will survive an internal or external explosion).

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. NEC and in the Canadian electrical code. 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.


Lightning

The largest-scale sparks are those produced naturally by lightning. Each stroke carries hundreds of thousands of amperes, at potentials of of upwards of a million volts, with hundreds of joules of energy released in each strike. Each stroke lasts for only tens of microseconds. Hazards due to lighting 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. These transferred potentials are dangerous to people, livestock, and electronic apparatus. Lighting 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 the hazard; these include lightning rods, shielding wires,and bonding of electrical and structural parts of buildings to form a continuous enclosure.

Lightning discharges in the atmosphere of Jupiter are thought to be the source of that planet's powerful radio frequency emissions.

See also

References

[1] A. H. Howatson, "An Introduction to Gas Discharges", Pergamom Press, Oxford, 1965, no ISBN page 67

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