An arc flash is the light and heat produced as part of an arc fault (sometimes referred to as an electrical flashover), a type of electrical explosion or discharge that results from a connection through air to ground or another voltage phase in an electrical system.
Arc flash is different from the arc blast, which is the supersonic shockwave produced when the conductors and surrounding air are heated by the arc, becoming a rapidly expanding plasma. Both are part of the same arc fault, and are often referred to as simply an arc flash, but from a safety standpoint they are often treated separately. For example, personal protective equipment (PPE) can be used to effectively shield a worker from the radiation of an arc flash, but that same PPE may likely be ineffective against the flying objects, molten metal, and violent concussion that the arc blast can produce. (For example, category-4 arc-flash protection, similar to a bomb suit, is unlikely to protect a person from the concussion of a very large blast, although it may prevent the worker from being fatally burned by the intense light of the flash.) For this reason, other safety precautions are usually taken in addition to wearing PPE, helping to prevent injury.Safe Work Practices for the Electrician by Ray A. Jones, Jane G. JonesJones and Bartlett Publishing 2009 Page 40 However, the phenomenon of the arc blast is sometimes used to extinguish the electric arc by some types of self-blast–chamber circuit breakers.
Definition
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 incremental 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, and especially where large supply-wires or high-current conductors are used, arc flashes can produce deafening noises, supersonic concussive forces, super-heated shrapnel, temperatures far greater than the Sun's surface, and intense, high-energy radiation capable of vaporizing nearby materials.
Arc flash temperatures can reach or exceed at the arc terminals.KM Kowalski-Trakofler, EA Barrett, CW Urban, GT Homce. "Arc Flash Awareness: Information and Discussion Topics for Electrical Workers ". DHHS (NIOSH) Publication No. 2007-116D. Accessed January 10, 2013. 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 in volume.Electrical Safety in the Workplace By Ray A. Jones, Jane G. JonesNational Fire Protection Agency 2000 Page 32
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.
Examples Most 400 V and above electrical services have sufficient capacity to cause an arc flash hazard. Medium-voltage equipment (above 1000 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 powerlines 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, as bright flashes like lightning that can be seen from long distances, are often witnessed from lines or transformers just before a power outage.Electrical Injuries: Engineering, Medical, and Legal Aspects By Robert E. Nabours, Raymond M. Fish, Paul F. HillLawyers & Judges 2004 Page 96
High-voltage powerlines often operate in the range of tens to hundreds of kilovolts, which can result in very long arc-flashes, often referred to as a flashover. Care must usually be taken to ensure that the lines are insulated with a proper "flashover rating" and sufficiently spaced from each other to prevent an arc flash from spontaneously developing. If the high-voltage 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.Electric power generation: Transmission and distribution By S. N. SinghPHI Limited, 2008, pp. 235–236, 260–261
As an example of the energy released in an arc flash incident, in 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 1.6 megajoules. For comparison, TNT releases 2,175 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:
Open air Ejected Equipment focused (arc-in-a-box) Tracking
Precautions Switching 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 breaker is turned on. Generally, either an electric motor with shorted windings or a shorted power-transformer is the culprit, being capable of drawing the energy needed to sustain a dangerous arc-flash. Motors over two horsepower usually have magnetic starters, to both isolate the operator from the high-energy contacts and to allow disengagement of the contactor if the breaker trips.
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 busbars 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 switching one on.ARC Flash Hazard Analysis and Mitigation by J. C. DasIEEE Press 2012
In addition to the heat, light and concussive forces, an arc flash also produces a cloud of plasma and ionized particles. When inhaled, this ionized gas can cause severe burns to the airways and lungs. The charged plasma may also be attracted to metallic objects worn by people in the vicinity, such as earrings, belt buckles, keys, body jewelry, or the frames of glasses, causing severe localized burns. When switching circuits, a technician should take care to remove any metals from their body, hold their breath, and close their eyes. An arc flash is more likely to form in a switch that is closed slowly, by allowing time for an arc to form between the contacts, so it is usually more desirable to "throw" switches with a fast motion, quickly and firmly making good contact. High-current switches often have a system of springs and levers or even pneumatic assists to assist with this.
Live testing When testing in energized high-power circuits, technicians will observe precautions for care and maintenance of testing equipment and to keep the area clean and free of debris. A technician would use protective equipment such as rubber gloves and other personal protective equipment, to avoid initiating an arc and to protect personnel from any arc that may start while testing.Electrical Safety Handbook 3E By John Cadick, Mary Capelli-Schellpfeffer, Dennis NeitzelMcGraw-Hill 2006High Voltage Engineering and Testing By Hugh McLaren RyanInstitute of Electrical Engineers 2001
Protecting personnel
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 can be done with equipment that has remotely operated switches or with remote racking.
Arc flash protection equipment
With recent increased awareness of the dangers of arc flash, there have been many companies that offer arc flash personal protective equipment (PPE), such as suits, overalls, helmets, boots, and gloves.