Arc flash

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An arc flash (also called flashover ), is the light and heat generated as part of the arc fault , a kind of electric explosion or discharge generated from low impedance connection by air to ground or other voltage phases in electrical system.

The flash arc obviously differs from the arc blast , which is the supersonic shock wave generated when the uncontrolled arc evaporates the metal conductor. Although both are part of the same arc errors, and are often referred to simply as arc flash, from a safety standpoint they are often treated separately. For example, personal protective equipment (APD) can be used to effectively protect workers from arc flash radiation, but the same APD may be ineffective against flying objects, molten metals, and violent concussions that arc booms can generate.. (For example, the protection of the category-4 battle arc, similar to a bomb suit, is unlikely to protect a person from a very large blast, even if it will prevent the worker from evaporating by a strong flash.) For this reason, others are usually taken in addition to wearing PPE, helping to prevent injury. However, the arc burst phenomenon is sometimes used to extinguish the arc by some types of self-chamber circuit breakers.

Video Arc flash

Definisi

Flash arc is light and heat generated from an electric arc supplied with sufficient electrical energy to cause damage, danger, fire, or substantial injury. The electric arc suffers from negative incremental resistance, which causes electrical resistance to decrease with increasing arc temperature. Therefore, when the arc develops and becomes hotter, the resistance decreases, draws more and more current (escape) until some part of the system melts, trips, or evaporates, providing enough distance to break the circuit and extinguish the arc. The electric arc, when well controlled and fed by limited energy, produces very bright light, and is used in arc lamps (closed, or with open electrodes), for welding, plasma cutting, and other industrial applications. The welding arc can easily turn the steel into a liquid with an average of only 24 volts DC. When an uncontrolled form of the arc at high voltages, and especially where large supply cables or high current conductors are used, flashing arcs can produce deafening sounds, concussive supersonic forces, super-heated fractions, temperatures far greater than the surface of the Sun , and intense, high-energy radiation capable of evaporating nearby materials.

The arc flash temperature can reach or exceed 35,000 ° F (19,400 ° C) in the arc terminal. The large energy released in the fault quickly vaporizes the involved metal conductor, melting the molten metal and extending the plasma outward with extraordinary strength. The typical arc flash incident can be unimportant but can easily be imagined to produce a more severe explosion (see calculation below). The outcome of violent events can cause damage to the equipment involved, fire, and injury not only to electrical workers but also to observers. During a rapid arc, the electrical energy evaporates the metal, which changes from solid to gas vapor, developing it with explosive force. For example, when copper evaporates suddenly expands with a factor of 67,000 volumes.

In addition to explosive explosions, called bursts of bows of such errors, destruction also arises from the intense heat of the jets produced by the arc. Metal arc bows produce large amounts of light energy from far infrared to ultraviolet. The surface of nearby objects, including people, absorbs this energy and is instantly heated until the temperature evaporates. These effects can be seen on adjacent walls and equipment - they are often ablated and eroded from luminous effects.

Maps Arc flash

Example

One of the most common examples of arc flash occurs when the incandescent light bulb is burned out. When the filament is broken, the bow is propped up throughout the filament, wrapping it in a plasma with a bright blue flash. Most household bulbs have a built-in fuse, to prevent a continuous arc from forming and blowing fuses on the circuit board. Most 480 V power services have sufficient capacity to cause a flash arc hazard. Medium voltage equipment (above 600 V) is a higher potential and therefore a higher risk for arc flash hazards. Higher voltage can cause splashing to jump, start arc flash without need of physical contact, and can retain arc in longer slit. Most power lines use voltages in excess of 1000 volts, and can be the danger of a light bulb against birds, squirrels, people, or equipment such as vehicles or stairs. Flashing arcs are often watched from lines or transformers just before power outages, creating flashes of light like lightning that can be seen for long distances.

High voltage powerline often operates in the range of tens to hundreds kilovolt. Treatment should usually be done to ensure that the lines are isolated with the appropriate "flashover rating" and sufficiently distance each other to prevent arc flash from developing spontaneously. If high voltage lines become too close, either to one another or to the ground, corona discharge can form between conductors. These are usually blue or reddish lights caused by ionizing the air, accompanied by a sizzling or frying sound. Release of the corona can easily cause a flash arc, by creating a conductive path between lines. This ionization can be increased during electrical storms, causing spontaneous flashes and causing power outages.

For example the energy released in the arc flash incident, in a single phase-to-phase error on a 480 V system with 20,000 amps of noise current, the power generated is 9.6 MW. If the error lasts for 10 cycles at 60 Hz, the resulting energy will be 1.6 megajoules. By comparison, TNT releases 2175 J/g or more when it is detonated (conventional value 4,184 J/g is used to be equivalent to TNT). Thus, this error energy is equivalent to 380 grams (about 0.8 pounds) of TNT. The character of the bow burst is very different from the chemical explosion (more heat and light, less mechanical shock), but the resulting damage is comparable. The super-fast heat steam produced by the arc can cause serious injury or damage, and intense UV, visible, and IR rays generated by the arc can be temporary and sometimes even permanently blind or cause eye damage to people.

There are four different types of arc type flash events to be assessed when designing a safety program:

• Open air
• Rejected
• Arc-in-a-box
• Track

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Precautions

Switch

One of the most common causes of arc-flash injuries occurs when powering an electrical circuit and, in particular, a disconnected circuit breaker. A broken circuit breaker often indicates an error occurred somewhere below the line from the panel. Errors should usually be isolated before powering on, or an arc flash can be easily generated. A small arc usually forms on the switch when the contact is first touched, and can provide a place for arc flash to be developed. If the voltage is high enough, and the cable leading to the fault is large enough to allow a large amount of current, the arc flash may form inside the panel when the breaker is switched on. In general, either an electric motor with short circuit or short power transformer is the cause, because it is able to attract the energy needed to maintain a dangerous arc-flash. Motors with two horsepower usually have a magnetic start, to isolate the operator from high-energy contacts and allow contactor discharge if the breaker travels.

Circuit breakers are often the main defenses against the current escape, especially if there is no secondary fuse, so if the arc flash develops in the breaker there may be no stopping the flash from uncontrollable. Once the flash arc starts in the breaker, it can quickly migrate from one circuit to the busbar panel itself, allowing very high energy to flow. Precautions should normally be used when changing circuit breakers, such as standing to the side when switching to keep the body out of the way, wearing protective clothing, or turning off the equipment, circuit and downline panel before switching. Very large switchgear are often able to handle very high energy and, thus, many places require the use of full protective equipment before turning it on.

In addition to heat, light power and creasing, the arc flash also produces plasma clouds and ionized particles. When inhaled, this ionized gas can cause severe burns to the airways and lungs. Plasma loads may also be attracted to metal objects worn by the people around them, such as earrings, belts, keys, body jewelry, or glasses, causing severe burns. When replacing the circuit, the technician must be careful to remove any metal from his body, hold his breath, and close his eyes. The arc flash is more likely to form in a closed switch slowly, allowing time to form arcs between contacts, so it is usually more desirable to "throw" the switch in fast, fast and firm movements to make good contacts.. High current switches often have spring and lever systems to help with this.

Live test

When testing on high-power energy circuits, technicians will observe precautions for maintenance and maintenance of testing equipment and to maintain the cleanliness of the area and free of debris. A technician will use protective equipment such as rubber gloves and other personal protective equipment, to avoid starting the arc and to protect personnel from the arc that may start during the test.

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Protect personnel

There are many methods to protect personnel from the dangers of flash arc. These may include personnel who use personal arc personal protective equipment (PPE) or modify the design and configuration of electrical equipment. The best way to eliminate the dangers of arc flash is to turn off the energy of the electrical equipment when interacting with it, but the electrical equipment that does not provide energy itself is a flash arc hazard. In this case, one of the latest solutions is to allow the operator to stand away from electrical equipment by operating the equipment remotely, this is called remote racking.

Flash arc protection equipment

With the recent increase in awareness about the dangers of arc flash, there are many companies offering personal protective equipment (PPE). The materials are tested for their arc rating. The arc rating is the maximum incident energy resistance indicated by the material before the breakopen (hole in the material) or needs to be skipped and causes a 50% chance of second degree burns. The arc rating is usually expressed in cal/cm ² (or small calories of heat energy per square centimeter). The test for determining arc rating is defined in ASTM F1506 Standard Performance Specifications for Resilient Textile Materials for Wearing Clothes to be Used by Electrical Workers Affected Short-Time Electric Arc and Related Heat Hazards .

The selection of the appropriate PPE, given the specific task to be performed, is usually handled in one of two possible ways. The first method is to consult the hazard category classification table, as found in NFPA 70E Table 130.7 (C) (15) (a) lists a number of typical electrical tasks by various stress levels and recommends the category of PPE to be used. For example, when working on 600 V switchgear and performing the removal of the bolt cover to expose naked, energy parts, the table recommends a 3rd System Protective Clothing Category. This Category 3 system corresponds to the APD ensemble which together offers protection up to 25 cal/cmÃ,² (105 J/cmÃ,² or 1.05 MJ/m²). The minimum value of the required APD for each category is the maximum energy available for that category. For example, Category 3 arc-flash hazard requires APD for not less than 25 cal/cmÃ,² (1.05 MJ/mÃ,²).

The second method for selecting PPE is to perform a flash arc hazard calculation to determine the available incident arc energy. IEEE 1584 provides guidance for performing this calculation considering that the maximum fault current, fault duration, and other common equipment information are known. Once the incident energy is calculated, the appropriate APE ensemble that offers greater protection of available energy can be selected.

PPE provides protection after an arc flash event occurs and should be seen as the last line of protection. Reducing the frequency and severity of the incident should be the first choice and this can be achieved through a complete arc flash hazard assessment and through the application of technologies such as high endurance grounding that has been shown to reduce the frequency and severity of the incident.

Reduce the hazard by design

Three key factors determine the intensity of arc flash on personnel. These factors are the amount of noise current available in a system, the time until the flash arc error is cleared, and the distance a person is from arc disturbance. A wide selection of design and equipment configurations can be made to influence these factors and in turn reduce the danger of arc flash.

Flash current

The current damage may be limited by using current limiting devices such as a limiting circuit breaker, earthing resistor or fuse. If the fault current is limited to 5 amperes or less, then many ground errors extinguish themselves and do not spread to phase-to-phase errors.

Archive time

Tensile time can be reduced by temporarily setting the upstream protection device to the lower setpoint during the maintenance period, or by using a selective zone-selective interconnection (ZSIP) protection. With selective-zone interlocking, a downstream detector that detects an error communicates with the upstream breaker to delay the tripping function for a moment. In this way the "selectivity" will be maintained, in other words the error in the circuit is cleared by the nearest breaker of the error, minimizing the effect on the whole system. Errors on the branch circuit will be detected by all the upstream breakers of the error (closer to the resource). The circuit breaker closest to the downstream interference will send a hold signal to prevent the upstream breaker from tripping instantly. Any interference will still activate the trip timer timer (s) from the upstream circuit breaker; this will allow the upstream circuit breaker to interrupt the error, if still required after the specified time has elapsed. The ZSIP system allows instantaneous shorter travel arrangements to be used, without loss of selectivity. Faster travel time reduces total energy in arc discharges discharge.

The arcing time can be significantly reduced with the protection based on flash-arc light detection. Optical detection is often combined with more current information. Light and current-based protection can be set with special protective flash-arc relays, or by using a normal protective relay equipped with an arc-flash add-on option.

One of the most efficient ways to reduce arc time is to use an arc eliminator that will extinguish the arc in a few milliseconds. The arc removers operate within 1-4 ms and create 3-phase short circuits on other parts of the system, usually upstream at higher voltages. This device contains a quick contact pin which, after being powered by an external relay, creates a physical contact with the energized bus which then creates a short circuit. The arc removal will protect the human if they stand in front of the event of arc flash and the relay detects the arc flash by transferring the arc flash to another location, although redirection can cause system failure at the location of the short circuit it is diverted to. This device must be replaced after the operation.

Another way to reduce arc flash is to use a triggered current trigger or current limiter used to insert a low-current continuous current divider fuse that melts and interjects the flash arc within 4 ms. The advantage of this device is that it removes the arc flash at its source and does not redirect it to other parts of the system. The current delimiter triggered will always "Stream Limit" which means it will interrupt the circuit before the first peak current occurs. These devices are electronically controlled and felt and provide feedback to users about their operational status. They can also be turned on and OFF as desired. This device must be replaced after the operation.

Distance

The radiation energy released by the electric arc is capable of permanently harming or killing humans at a distance of up to 20 feet (6.1 m). The distance from the source of the flash arc in which the unprotected person has a 50% chance of receiving a second degree burn is referred to as the "flash protection limit". The incident energy of 1.2 cal/cm ^ 2 on bare skin was chosen in solving the equations for the limit of the flash arc in IEEE 1584. IEEE 1584 arc flash boundary equations can also be used to calculate flash arc limits with other boundary energies. from 1.2 cal/cm ^ 2 as onset to level 2 burns energy. Those who do flash hazard analysis should consider this limit, and then must determine what APD should be used within the flash protection limits. The remote operator or robot can be used to perform activities that have a high risk for the occurrence of arc flash, such as inserting the drawing circuit breaker on the live electric bus. Remote racking systems are available that keep operators outside of the arc flash danger zones.

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Research

Both the Institute of Electrical and Electronics Engineers (IEEE) and the National Fire Protection Association (NFPA) have joined in an initiative to fund and support research and testing to improve understanding of arc flash. The results of this collaborative project will provide information that will be used to improve electrical safety standards, predict hazards associated with arc errors and the accompanying bow bursts, and provide practical protection for employees at work.

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Standard

• OSHA Standards 29 CFR, Sections 1910 and 1926. Occupational Safety and Health Standards. Section 1910, sub section S (electrically) Ã,§Ã,§ 1910.332 to 1910.335 contains general applicable terms for safety related work practices. On April 11, 2014, OSHA adopted revised standards for electric power, transmission, and distribution work in section 1910, 1910.269 and part 1926, sub section V, which contains requirements for arc flash protection and guidelines for assessing flash-arc hazards , making a reasonable estimate of the incident heat energy of the electric arc, and selecting the appropriate protective device (79 FR 20316 et seq., 11 April 2014). All OSHA standard references are NFPA 70E.
• The National Fire Protection Association (NFPA) Standard 70 - 2014 "The National Electrical Code" (NEC) contains requirements for warning labels. See NEC Article 110.16 & amp; NEC Article 240.87
• NFPA 70E 2012 provides guidance for applying the appropriate work practices necessary to protect workers from injury while working or near open electrical conductors or circuitry that can become energized.
• The Standard Canadian Basic Association CSA Z462 Arc Flash Standard is a Canadian version of NFPA70E. Released in 2008.
• The Underwriters Laboratories of Canada's Standard at Electric Utility Workplace Electrical Safety for Generation, Transmission and Distribution CAN/ULC S801
• Electronic Institute and Electrical Engineer IEEE 1584 - 2002 Guidelines for Conducting Arc-Flash Danger Calculations.

Flash Arc's danger software allows businesses to comply with government regulations while providing their workforces with an optimally safe environment. Many software companies now offer arc flash danger solutions. Some power service companies calculate secure flash limits.

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References

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External links

• Information, Statistics, and Video Resources in Arc Flash
• Quick Facts about Arc Flash.
• Flash Arc Resource Center
• Flash Arc Awareness Video is available on YouTube or from NIOSH
• Free Online Arc Arc Calculator and Labels Maker.
• How to apply NFPA 70E to reduce Flash Arc dangers.
• my online Arc Arc calculator calculator
• Free Arc Arc Calculator
• Flash Arc Guide

Source of the article : Wikipedia