The term high voltage usually means electrical energy at a high enough voltage to cause harm to living organisms. High voltage equipment and conductors guarantee certain safety requirements and procedures. In certain industries, high voltage means a voltage above a certain threshold (see below) . High voltages are used in the distribution of electric power, in cathode ray tubes, to produce X-rays and particle beams, to demonstrate curves, for ignition, in photomultiplier tubes, and in high power booster vacuum tubes and other industrial and scientific applications.
Video High voltage
Definisi
The "high voltage" numerical definition depends on the context. Two factors considered in classifying the voltage as "high voltage" are the possibility of causing splashes in the air, and the danger of electric shock due to contact or proximity. Definitions can refer to the voltage between two conductors of a system, or between conductors and ground.
In electric power transmission techniques, high voltage is usually considered any voltage of about 35,000 volts. This is a classification based on equipment design and isolation.
The International Electrotechnical Commission and its national counterparts (IET, IEEE, VDE, etc.) define high voltage as above 1000Ã, V for alternating current, and at least 1500 V for direct current - and distinguish it from low voltage (50 to 1000 VAC or 120-1500 VDC) and an extra voltage (& lt; 50 VAC or & lt; 120 VDC) circuit. This is in the context of building wiring and electrical equipment safety.
In the United States 2011 the National Electrical Code (NEC) is the standard that governs most electrical installations. There is no definition related to high voltage . NEC covers voltages of 600 volts and less and more than 600 volts. The National Electrical Manufacturers Association (NEMA) defines high voltages in excess of 100 to 230 kV. British Standard BS 7671: 2008 defines high voltage as any voltage difference between conductors higher than 1000 VAC or 1500Ã, V DC free ripple, or a voltage difference between conductors and Earth higher than 600 VAC or 900Ã, V DC-free ripple.
Electricity can only be licensed for certain voltage classes, in some jurisdictions. For example, electrical licenses for specific sub-trades such as the installation of HVAC systems, fire alarm systems, closed-circuit television systems may be permitted to install energy-only systems up to 30 volts between conductors, and may not be permitted for electrical voltage circuits. The general public can consider household electrical circuits (100 to 250 VAC), which carry the highest voltage they normally encounter, being high voltage .
Voltages over 50 volts can usually cause dangerous flowing currents through humans touching two circuit points - so safety standards are, in general, more limited around the circuit. The extrahigh voltage (EHV) definition again depends on the context. In electric power transmission techniques, EHV is classified as a voltage in the range of 345,000 - 765,000 volts. In electronic systems, a power supply that provides more than 275,000 volts is called EHV Power Supply , and is often used in experiments in physics.
The acceleration voltage for television cathode ray tubes can be described as extreme voltage or extrusion stress (EHT), as compared to other supply voltages in the equipment. This type of supply ranges from 5 kV to about 30 kV.
In automotive engineering, high voltage is defined as a voltage in the range of 30 to 1000 VAC or 60 to 1500 VDC.
In digital electronics, high voltage usually refers to something that represents logic 1 in positive logic and logic 0 in negative logic. This is not used to indicate the voltage and dangerous levels between ICs for standard TTL/CMOS and their modern derivatives are well below the dangerous level. The highest in mainstream usage is 15Ã,V for original CMOS and 5Ã,V for TTL but modern devices use 3.3Ã,V, with 1.8Ã,V or lower used in many applications.
Maps High voltage
Security
Voltages greater than 50Ã V, applied to the dry, unbroken human skin can cause cardiac fibrillation if they produce electrical current in body tissues that happen to pass through the chest area. The voltage at which there is a danger of electric shock depends on the electrical conductivity of the dry human skin. Human living tissue can be protected from damage by dry skin insulation characteristics up to about 50 volts. If the same skin gets wet, if there is a wound, or if a voltage is applied to an electrode that pierces the skin, then even a voltage source below 40 V can be deadly.
Inadvertent contact with high voltage that supplies sufficient energy can cause severe injury or death. This can happen because one's body provides a pathway for current flow, causing tissue damage and heart failure. Other injuries may be burns from arcs produced by accidental contact. This burn can be very dangerous if the victim's airway is affected. Injuries can also be suffered as a result of the physical strength experienced by people falling from a height or being thrown at a considerable distance.
Exposure to low energy to high voltage may be harmless, such as splashes produced in dry climates when touching the door knob after walking across carpeted floors. Voltage can be in the range of thousands of volts, but the current (charge transfer rate) is low.
Safety equipment used by electrical workers including rubber gloves and insulated mats. This protects the user from electric shock. Security equipment is tested regularly to ensure that it still protects the user. The test rules vary by country. Testing companies can test up to 300,000 volts and offer services from glove testing for Elevated Working Platform (or EWP) testing.
Sparks in the air
The dry air dielectric permeability, at Temperature and Standard Pressure (STP), between round electrodes is approximately 33 kV/cm. This is just a rough guide, because the translucency voltage actually depends on the shape and size of the electrode. Strong electric fields (from high voltage applied to small or pointed conductors) often produce a purple corona release in the air, as well as a visible spark. The voltage below about 500-700 volts can not produce sparks or sparks that are easily visible in the air at atmospheric pressure, so with this rule this voltage is "low". However, under conditions of low atmospheric pressure (such as in high-altitude planes), or in a noble gas environment such as argon or neon, sparks appear at much lower voltages. 500 to 700 volts is not a fixed minimum to produce splash damage, but this is a rule-of-thumb. For air on the STP, the minimum sparkover voltage is about 327 volts, as noted by Friedrich Paschen.
While lower voltages do not, in general, leap gaps that exist before the voltage is applied, disrupting current flow with gaps often results in low voltage sparks or arcs. When the contacts are separated, some small contact points become the last to be separated. The current becomes confined to these tiny hot spots, causing them to become incandescent, so they emit electrons (via thermionic emissions). Even a small 9 V battery can dazzle with this mechanism in a darkened room. Ionized air and metal vapor (from contact) form a plasma, which temporarily bridges an ever widening gap. If the power supply and load allow sufficient current to flow, the independent curvature may be formed. Once formed, the arc can be extended to a significant length before breaking the circuit. Trying to open an inductive circuit often forms an arc, because the inductance provides a high-voltage pulse every time the current is interrupted. The AC system makes the continuous arc somewhat less likely, since the current returns to zero twice per cycle. The arc is extinguished every time the current passes through the zero crossing, and must reignite for the next half cycle to maintain the arc.
Unlike ohmic conductors, arc resistance decreases with increasing currents. This makes the accidental bow in a dangerous electrical device because even a small bow can grow large enough to damage the equipment and start a fire if sufficient current is available. Intentionally generated curves, such as those used in lighting or welding, require several elements in the circuit to stabilize the current/arc voltage characteristics.
Electrostatic devices, natural static electricity, and similar phenomena
High voltage is not always dangerous if it can not produce large currents. The common static electric flame seen under low humidity conditions always involves a voltage above 700 V. For example, sparks to the car door in winter may involve a voltage as high as 20,000 V. Also, a physical demonstration device such as a Van de Graaff generator and a Wimshurst Machine can produce voltages approaching one million volts, but at worst they give a brief sting. That's because the number of electrons involved is not high. This device has a limited amount of stored energy, so the resulting average current is low and usually for a short time, with impulse peaking in the range of 1 A for the nanosecond. During discharge, this machine uses high voltage to the body for just 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.
Disposal may involve very high stresses in a very short time, but, to produce cardiac fibrillation, the supply of electricity must produce significant currents in the heart muscle that persists for several milliseconds, and must deposit the total energy in the range of at least millijoules. or higher. A relatively high current on anything more than about fifty volts can be medically significant and potentially fatal.
Tesla coils are not electrostatic machines and can produce significant current for sustainable intervals. Although their appearance in operation is similar to a high-voltage static electricity device, the current supplied to the human body will be relatively constant as long as contact is maintained, and the voltage will be much higher than the human skin's break-down voltage. As a result, the output of the Tesla coil can be dangerous or even fatal.
Power lines
Electrical transmission and distribution lines for electric power always use voltages significantly higher than 50 volts, so contact with or close approaches with line conductors presents the danger of electric shock. Contact with overhead cables is often the cause of injury or death. Metal stairs, agricultural equipment, mast ships, construction machinery, air antennas, and similar objects are often involved in fatal contact with wires overhead. Digging buried cables can also be dangerous for workers at the excavation site. Excavation equipment (either hand tools or powered machines) that contact the buried cable can energize the pipe or soil in the area, resulting in the stinging of nearby labor. Errors in high-voltage transmission lines or substations can cause high currents flowing along the earth's surface, resulting in an earth-increasing potential that also poses a hazard of electric shock.
People who are not authorized to climb electric poles or electrical equipment are also often the victims of electric shock. At very high transmission voltages even a close approximation can be dangerous, because high voltage can pass a significant air gap.
For high-voltage and extra-high voltage transmission lines, specially trained personnel use "lifecycle" techniques to enable direct contact with powered equipment. In this case the worker is electrically connected to a high voltage channel but completely isolated from the earth so that it is at the same electrical potential as that line. Since training for such operations is long-lasting, and still poses a danger to personnel, only very important transmission lines can be maintained while alive. Beyond this properly designed situation, the isolation of the earth does not guarantee that no current flows to the earth - such as ground or curved to the ground can occur in unexpected ways, and high-frequency currents can burn even those who are not stranded. Touching the transmitter antenna is dangerous for this reason, and the high frequency Tesla coil can sustain sparks with just one end point.
Protective equipment on high voltage transmission lines usually prevents the formation of unwanted arcs, or ensures that it is extinguished within tens of milliseconds. Electrical equipment that disrupts high-voltage circuits is designed to steer the resulting arc safely so that it disappears without damage. High-voltage circuit breakers often use high-pressure air bursts, special dielectric gases (like SF 6 under pressure), or immersion in mineral oil to extinguish the arc when high-voltage circuit breaks.
Arc flash hazard
Depending on the prospective short-circuit current available on the switchgear's line-up, hazards are given to maintenance and operation personnel due to the possibility of high intensity arc. The maximum arc temperature can exceed 10,000 kelvins, and radiating heat, widen the hot air, and evaporation of explosive metals and insulation can cause severe injury to unprotected workers. Linegear switchgear and source of this high-energy arc are generally present in electrical substations and power stations, industrial plants and large commercial buildings. In the United States, the National Fire Protection Association has issued NFPA 70E guidance standards to evaluate and calculate the arc flash hole, and provide the standard for the protective clothing required for electric workers exposed to such hazards in the workplace.
Danger of explosion
Even insufficient tension to break the air can be attributed to enough energy to ignite an atmosphere containing combustible gas or vapor, or suspended dust. For example, hydrogen gas, natural gas, or gasoline/gas mixed with air can be ignited by the spark generated by electrical equipment. Examples of industrial facilities with hazardous areas are petrochemical refineries, chemical plants, grain elevators, and coal mines.
Measures taken to prevent the explosion include:
- Intrinsic security with the use of equipment designed not to accumulate enough stored electrical energy to trigger an explosion
- Enhanced security, which applies to devices that use actions such as oil-filled enclosures to prevent sparks
- Explosion proof casing (flame retardant), designed so that the explosion inside the enclosure can not escape and ignite the surrounding explosive atmosphere (this designation does not imply that equipment can withstand internal or external explosions)
In recent years, the standard of explosion hazard protection has become more uniform between European and North American practices. The "zone" classification system is now used in the form of modifications in the U.S. National Electrical Code. and in the Canadian Electrical Code. Intrinsic security equipment is now approved for use in North American applications.
Poisonous gas
Electrical discharges, including partial discharges and corona, can produce small toxic gases, which in a confined space can be a serious health hazard. These gases include ozone and various nitrogen oxides.
Lightning
The largest scale spark is produced naturally by lightning. The negative lightning bolts carry 30 to 50 kiloamperes, transfer the charge of 5 coulombs, and dispose of 500 megajoules of energy (equivalent to 12 kg of LNG, or enough to light a 100 watt bulb for about 2 months). However, the positive lightning average (from the peak of the storm) can carry 300 to 500 kiloamperes, transfer the charge to 300 coulombs, has a potential difference of up to 1 gigavolt (one billion volts), and can eliminate 300 GJ of energy (72 tons of TNT; enough energy to power 100 watts of light up to 95 years). A negative lightning strike usually only lasts for tens of microseconds, but some strikes often occur. A positive lightning stroke is usually a single event. However, larger peak flows can flow for hundreds of milliseconds, making it much hotter and more dangerous than negative lightning.
The dangers of lightning obviously include a direct strike on people or property. However, lightning can also create hazardous voltage gradients on the earth, as well as electromagnetic pulses, and can charge expanded metal objects such as telephone cords, fences, and pipelines to hazardous voltages that can be carried for miles from strike locations.. Although many of these objects are usually not conductive, very high voltages can cause electrical disturbance of the insulator, causing them to act as conductors. This transferred potential is harmful to humans, livestock, and electronic equipment. Lightning strikes also start fires and explosions, resulting in casualties, injuries, and property damage. For example, every year in North America, thousands of forest fires start with lightning strikes.
Measures to control lightning can reduce harm; These include lightning rods, protective wire, and the binding of electrical and structural components of buildings to form cages continuously.
The high voltage lightning voltage in Jupiter's atmosphere is considered to be a powerful source of radio frequency emissions on the planet.
See also
- Voltage voltage transformer
- Charging station
- Electrical engineering
- Power transmission (including 'Health issues' section)
- High voltage electric current
- Low voltage
- Tesla coil
- Spark gap
References
External links
- NFPA 70E : Electrical Safety at Work, USA
- USA Department of Energy electrical security handbook
- Electrical Safety chapter of Lessons in Vol 1 DC series of books books and series.
Source of the article : Wikipedia
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