From Wikipedia, the free encyclopedia - View original article
|This article needs additional citations for verification. (December 2009)|
An arc flash (also called a flashover), which is distinctly different from the arc blast, is part of an arc fault, a type of electrical explosion that results from a low-impedance connection to ground or another voltage phase in an electrical system.
An arc flash is the light and heat produced from an electric arc supplied with sufficient electrical energy to cause substantial damage, harm, fire, or injury. Electrical arcs experience negative resistance, which causes the electrical resistance to decrease as the arc temperature increases. Therefore, as the arc develops and gets hotter the resistance drops, drawing more and more current (runaway) until some part of the system melts, trips, or evaporates, providing enough distance to break the circuit and extinguish the arc. Electrical arcs, when well controlled and fed by limited energy, produce very bright light, and are used in arc lamps (enclosed, or with open electrodes), for welding, plasma cutting, and other industrial applications. Welding arcs can easily turn steel into a liquid with an average of only 24 DC volts. When an uncontrolled arc forms at high voltages, arc flashes can produce deafening noises, supersonic concussive-forces, super-heated shrapnel, temperatures far greater than the Sun's, and intense, high-energy radiation capable of vaporizing nearby materials.
Arc flash temperatures can reach or exceed 35,000 °F (19,400 °C) at the arc terminals. The massive energy released in the fault rapidly vaporizes the metal conductors involved, blasting molten metal and expanding plasma outward with extraordinary force. A typical arc flash incident can be inconsequential but could conceivably easily produce a more severe explosion (see calculation below). The result of the violent event can cause destruction of equipment involved, fire, and injury not only to an electrical worker but also to bystanders. During the arc flash, electrical energy vaporizes the metal, which changes from solid state to gas vapor, expanding it with explosive force. For example, when copper vaporizes it suddenly expands by a factor of 67,000 times in volume.
In addition to the explosive blast, called the arc blast of such a fault, destruction also arises from the intense radiant heat produced by the arc. The metal plasma arc produces tremendous amounts of light energy from far infrared to ultraviolet. Surfaces of nearby objects, including people absorb this energy and are instantly heated to vaporizing temperatures. The effects of this can be seen on adjacent walls and equipment - they are often ablated and eroded from the radiant effects.
One of the most common examples of an arc flash occurs when an incandescent light bulb burns out. When the filament breaks, an arc is sustained across the filament, enveloping it in plasma with a bright, blue flash. Most household lightbulbs have a built-in fuse, to prevent a sustained arc-flash from forming and blowing fuses in the circuit panel. In general, arc flash incidents which ignite clothing are highly improbable on systems operating at less than 208 AC volts, phase to phase (120 V to ground), when fed by less than a 125 kVA transformer, as 120 volts does not provide sufficient potential to cause an arc flash hazard. Most 480 V electrical services have sufficient capacity to cause an arc flash hazard. Medium-voltage equipment (above 600 V) is higher potential and therefore a higher risk for an arc flash hazard. Higher voltages can cause a spark to jump, initiating an arc flash without the need for physical contact, and can sustain an arc across longer gaps. Most power-lines use voltages exceeding 1000 volts, and can be an arc-flash hazard to birds, squirrels, people, or equipment such as vehicles or ladders. Arc flashes are often witnessed from lines or transformers just before a power outage, creating bright flashes like lightning that can be seen for long distances.
High-tension power-lines often operate in the range of tens to hundreds of kilovolts. Care must usually be taken to ensure that the lines are insulated with a proper "flashover rating" and sufficiently spaced from each other, or an arc flash can spontaneously develop. If the high-tension lines become too close, either to each other or ground, a corona discharge may form between the conductors. This is typically a blue or reddish light caused by ionization of the air, accompanied by a hissing or frying sound. The corona discharge can easily lead to an arc flash, by creating a conductive pathway between the lines. This ionization can be enhanced during electrical storms, causing spontaneous arc-flashes and leading to power outages.
One of the most common causes of arc flash injuries happens when switching-on electrical circuits and, especially, tripped circuit-breakers. A tripped circuit-breaker often indicates a fault has occurred somewhere down the line from the panel. The fault must usually be isolated before switching the power on, or an arc flash can easily be generated. Small arcs usually form in switches when the contacts first touch, and can provide a place for an arc flash to develop. If the voltage is high enough, and the wires leading to the fault are large enough to allow a substantial amount of current, an arc flash can form within the panel when the switch is turned on. Generally, either an electric motor with shorted windings or a shorted power-transformer are the culprits, being capable of drawing the energy needed to sustain a dangerous arc-flash. Circuit breakers are often the primary defense against current runaway, especially if there are no secondary fuses, so if an arc flash develops in a breaker there may be nothing to stop a flash from going out of control. Once an arc flash begins in a breaker, it can quickly migrate from a single circuit to the phases of the panel itself, allowing very high energies to flow. Precautions must usually be used when switching circuit breakers, such as standing off to the side while switching to keep the body out of the way, wearing protective clothing, or turning-off equipment, circuits and panels downline prior to switching. Very large switchgear is often able to handle very high energies, and, thus, many places require the use of full protective equipment before turning it on.
As an example of the energy released in an arc flash incident, a single phase-to-phase fault on a 480 V system with 20,000 amps of fault current. The resulting power is 9.6 MW. If the fault lasts for 10 cycles at 60 Hz, the resulting energy would be 1600 kilojoules. For comparison, TNT releases 2175 J/g or more when detonated (a conventional value of 4,184 J/g is used for TNT equivalent). Thus, this fault energy is equivalent to 380 grams (approximately 0.8 pounds) of TNT. The character of an arc flash blast is quite different from a chemical explosion (more heat and light, less mechanical shock), but the resulting devastation is comparable. The rapidly expanding superheated vapor produced by the arc can cause serious injury or damage, and the intense UV, visible, and IR light produced by the arc can temporarily and sometimes even permanently blind or cause eye damage to people.
There are four different arc flash type events to be assessed when designing safety programs:
There are many methods of protecting personnel from arc flash hazards. This can include personnel wearing arc flash personal protective equipment (PPE) or modifying the design and configuration of electrical equipment. The best way to remove the hazards of an arc flash is to de-energize electrical equipment when interacting with it, however de-energizing electrical equipment is in and of itself an arc flash hazard. In this case, one of the newest solutions is to allow the operator to stand far back from the electrical equipment by operating equipment remotely, this is called remote racking.
With recent increased awareness of the dangers of arc flash, there have been many companies that offer arc flash personal protective equipment (PPE). The materials are tested for their arc rating. The arc rating is the maximum incident energy resistance demonstrated by a material prior to breakopen (a hole in the material) or necessary to pass through and cause with 50% probability a second or third degree burn. Arc rating is normally expressed in cal/cm² (or small calories of heat energy per square centimeter). The tests for determining arc rating are defined in ASTM F1506 Standard Performance Specification for Flame Resistant Textile Materials for Wearing Apparel for Use by Electrical Workers Exposed to Momentary Electric Arc and Related Thermal Hazards. Among the best fabrics for protection against electric arc flash are the Modacrylic-cotton blends.
Selection of appropriate PPE, given a certain task to be performed, is normally handled in one of two possible ways. The first method is to consult a hazard category classification table, like that found in NFPA 70E. Table 130.7(C)(15)(a) lists a number of typical electrical tasks by various voltage levels and recommends the category of PPE that should be worn. For example when working on 600 V switchgear and performing a removal of bolted covers to expose bare, energized parts, the table recommends a Category 3 Protective Clothing System. This Category 3 system corresponds to an ensemble of PPE that together offers protection up to 25 cal/cm² (105 J/cm² or 1.05 MJ/m²). The minimum rating of PPE necessary for any category is the maximum available energy for that category. For example, a Category 3 arc-flash hazard requires PPE rated for no less than 25 cal/cm² (1.05 MJ/m²).
The second method of selecting PPE is to perform an arc flash hazard calculation to determine the available incident arc energy. IEEE 1584 provides a guide to perform these calculations given that the maximum fault current, duration of faults, and other general equipment information is known. Once the incident energy is calculated the appropriate ensemble of PPE that offers protection greater than the energy available can be selected.
PPE provides protection after an arc flash incident has occurred and should be viewed as the last line of protection. Reducing the frequency and severity of incidents should be the first option and this can be achieved through a complete arc flash hazard assessment and through the application of technology such as high-resistance grounding which has been proven to reduce the frequency and severity of incidents.
Three key factors determine the intensity of an arc flash on personnel. These factors are the quantity of fault current available in a system, the time until an arc flash fault is cleared, and the distance an individual is from a fault arc. Various design and equipment configuration choices can be made to affect these factors and in turn reduce the arc flash hazard.
Fault current can be limited by using current limiting devices such as grounding resistors or fuses. If the fault current is limited to 5 amperes or less, then many ground faults self-extinguish and do not propagate into phase-to-phase faults.
Arcing time can be reduced by temporarily setting upstream protective devices to lower setpoints during maintenance periods, or by employing zone-selective interlocking protection (ZSIP). With zone-selective interlocking, a downstream breaker that detects a fault communicates with an upstream breaker to delay its instantaneous tripping function. In this way "selectivity" will be preserved, in other words faults in the circuit are cleared by the breaker nearest to the fault, minimizing the effect on the entire system. A fault on a branch circuit will be detected by all breakers upstream of the fault (closer to the source of power). The circuit breaker closest to the downstream fault will send a restraining signal to prevent upstream breakers from tripping instantaneously. The presence of the fault will nevertheless activate the preset trip delay timer(s) of the upstream circuit breaker(s); this will allow an upstream circuit breaker to interrupt the fault, if still necessary after the preset time has elapsed. The ZSIP system allows faster instantaneous trip settings to be used, without loss of selectivity. The faster trip times reduce the total energy in an arc fault discharge.
Arcing time can significantly be reduced by protection based on detection of arc-flash light. Optical detection is often combined with overcurrent information. Light and current based protection can be set up with dedicated arc-flash protective relays, or by using normal protective relays equipped with an add-on arc-flash option.
The most efficient means to reduce arcing time is to use an arc eliminator[further explanation needed] that will extinguish the arc within a few milliseconds.
The distance from an arc flash source within which an unprotected person has a 50% chance of receiving a second degree burn is referred to as the "flash protection boundary". Those conducting flash hazard analyses must consider this boundary, and then must determine what PPE should be worn within the flash protection boundary. Remote operators or robots can be used to perform activities that have a high risk for arc flash incidents, such as inserting draw-out circuit breakers on a live electrical bus. Remote racking systems are available which keep the operator outside the arc flash hazard zone.
Both the Institute of Electrical and Electronics Engineers (IEEE) and the National Fire Protection Association (NFPA) have joined forces in an initiative to fund and support research and testing to increase the understanding of arc flash. The results of this collaborative project will provide information that will be used to improve electrical safety standards, predict the hazards associated with arcing faults and accompanying arc blasts, and provide practical safeguards for employees in the workplace.
Arc flash hazard software exists that allows businesses to comply with the myriad government regulations while providing their workforce with an optimally safe environment. Many software companies now offer arc flash hazard solutions. Few power services companies calculate safe flash boundaries.