Definition and Characteristics of Arc Flash
– Arc flash is the light and heat produced from an electric arc supplied with sufficient electrical energy to cause damage, harm, fire, or injury.
– Electrical arcs experience negative incremental resistance, causing the resistance to decrease as the arc temperature increases.
– Well-controlled electrical arcs produce bright light and are used in arc lamps, welding, plasma cutting, and other industrial applications.
– Uncontrolled arc flashes at high voltages can produce deafening noises, supersonic concussive forces, super-heated shrapnel, and intense radiation.
– Arc flash temperatures can reach or exceed 35,000°F (19,400°C) at the arc terminals.
Examples and Energy Release
– Examples of arc flash incidents include a burned-out incandescent light bulb sustaining an arc across the filament and high-voltage powerlines posing a hazard to birds, squirrels, people, or equipment.
– In a single phase-to-phase fault on a 480V system with 20,000 amps of fault current, the resulting power is 9.6 MW.
– The energy released in an arc flash incident can be equivalent to 380 grams (approximately 0.8 pounds) of TNT.
– The rapidly expanding superheated vapor produced by the arc can cause serious injury or damage.
– The intense UV, visible, and IR light produced by the arc can temporarily or permanently blind or cause eye damage.
Precautions and Causes of Arc Flash
– Arc-flash injuries commonly occur when switching on electrical circuits or tripped circuit-breakers.
– Precautions must be taken to prevent arc flashes during switching operations.
– Causes of arc flash include shorted power transformers, motors with magnetic starters, high-voltage switchgear and distribution panels, and circuit breakers.
Arc Flash Protection Equipment and Reducing Hazards by Design
– Arc flash personal protective equipment (PPE) includes suits, overalls, helmets, boots, and gloves.
– Protective equipment effectiveness is measured by its arc rating, expressed in cal/cm.
– Fault current can be limited using current limiting devices, and arcing time can be reduced by temporarily setting upstream protective devices to lower setpoints.
– Zone-selective interlocking protection (ZSIP) and protection based on detection of arc-flash light are methods to reduce arc flash hazards.
– Design and equipment configuration choices can also affect fault current, arcing time, and distance from a fault arc.
Standards, Regulations, and Notable Incidents
– OSHA standards, NFPA 70E, CSA Z462 Arc Flash Standard, and NEC provide guidance and requirements for arc flash protection.
– The Institute of Electrical and Electronics Engineers (IEEE) and the National Fire Protection Association (NFPA) collaborate to fund and support arc flash research.
– Notable incidents include an arc flash in Astoria, Queens, which caused a temporary change in the color of the sky and gained media attention. The incident was explained as an arc flash by Con Ed, a power company. Source: https://en.wikipedia.org/wiki/Arc_flash
An arc flash is the light and heat produced as part of an arc fault, 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 distinctly different from the arc blast, which is the supersonic shockwave produced when the uncontrolled arc vaporizes the metal conductors. 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 vaporized 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. However, the phenomenon of the arc blast is sometimes used to extinguish the electric arc by some types of self-blast–chamber circuit breakers.
