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 or discharge that results from a low-impedance connection through air to ground or another voltage phase in an electrical system. A controlled arc-flash, produced in a flashtube. Even though the energy level used is fairly low (85 joules), the low-impedance, low-inductance
circuit produces a flash of 24,000,000 watts. With an arc temperature of 17,000 K (30,100 F), the radiation output is centered at 170 nanometers, in the far UV. The intense burst of radiation easily penetrates the shade #10 welding filter which shields the camera. 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-amperage 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 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. Examples 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. 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 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 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 powerlines 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 to prevent an arc flash from spontaneously developing. 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 arcflashes and leading to power outages. 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 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: Open Air Arc Flashes Ejected Arc Flashes Equipment Focused Arc Flashes (Arc-in-abox) Tracking Arc Flashes
Precautions 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 powertransformer are the culprits, 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. A 480 volt switchgear and distribution panel, requiring category-4 arc-flash protection.
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 on one. 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 ear rings, belt buckles, keys, body jewelry, or the frames of glasses, causing severe, localized burns. Care should usually be taken when switching circuits to remove any metals from a person's body, to 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-amperage switches often have a system of springs and levers to assist with this.