Arc flash
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An arc flash (or arc blast) event is a type of electrical explosion that results from a low impedance connection to ground or another voltage phase in an electrical system.
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[edit] Definition
An arc flash is a voltage breakdown of the resistance of air resulting in an arc which can occur where there is sufficient voltage in an electrical system and a path to ground or lower voltage. An arc flash where there is a high level of current - in the range 1000 amps or more can cause substantial damage, fire or injury. The massive energy released in the fault instantly vaporizes the metal conductors involved, blasting molten metal and expanding plasma outward with extreme 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 the worker but also to nearby people.
In addition to the explosive 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 people and objects 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.
[edit] Examples of Arc Flash
In general, arc flash incidents are highly improbable on systems operating at less than 240 volts phase to phase (120V to ground) when fed by less than a 125 KVA transformer. 120 volts does not provide sufficient energy to cause an arc flash hazard. Most 480V electrical services have sufficient capacity to cause an arc flash hazard. Medium-voltage equipment (above 600V) is higher energy and therefore a higher arc flash hazard.
As an example of the energy released in an arc flash incident, consider a single phase-to-phase fault on a 480V system with 20,000 amps of fault current. The resulting power is 9.6MW. If the fault lasts for 10 cycles at 60 Hz, the resulting energy would be 1.6 megajoules. For comparison, TNT releases 2175J/g when detonated. Thus, this fault energy is equivalent to 736 grams of TNT, or approximately 1.5 pounds. 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 and IR light produced by the arc can temporarily and sometimes even permanently blind or cause eye damage to people.
[edit] 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.
[edit] Arc Flash PPE
With recent increased awareness of the dangers of Arc Flash, there has been many companies that offer Arc Flash PPE. The fabrics or materials are tested for their Arc Rating. The Arc Rating is the maximum incident energy resistance demonstrated by a material prior to breakopen or at the onset of a second-degree skin burn [1]. Arc Rating is normally expressed in cal/cm2 (or calories of heat energy per square centimeter). The tests for determining Arc Rating is 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.
Selection of appropriate PPE, given a certain task to be performed, is normally handled by 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)(9)(a) lists a number of typical electrical tasks are various voltage levels and recommends the category of PPE that should be worn. For example when working on 600V switchgear and preforming a removal of bolted covers to expose bare, energized parts, the table recommends Category 3 Protective Clothing System. This Category 3 system corresponds to an ensemble of PPE that together offers protection up to 25 cal/cm2.
The second method to select PPE to perform an Arc Flash Hazard Calculation to determine the available incident arc energy. IEEE 1584 provides a guide to perform these calculations given the bolted fault current, duration of a 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
[edit] Reducing Hazard by Design
Three key factors determine the intensity of an Arc Flash event on personnel. These factors are the quantity of fault current available in a system, the time fault until an arc flash is cleared, and the distance an individual is from an arc. Various design and equipment configuration choices can be made to affect these factors and in turn reduce the Arc Flash hazard.
Fault Current
- Fault current can be limited by using current limiting fuses.
Arcing Time
- Arcing time can be reduced by temporarily setting upstream protective devices to lower setpoints during maintenances periods.
Distance
- Remote operators or robots can be used to perform activities that are high risk for Arc Flash incidents like racking breakers on a live electrical bus.
[edit] Research
Both the Institute of Electrical and Electronics Engineers (IEEE) and the National Fire Protection Association (NFPA) have joined forces on an initiative to fund and support research and testing to increase the understanding of Arc Flash [2]. The results of this collaborative project will provide information that will be used to improve electrical safety standards, predict the hazards associated with arching faults and accompanying arc blasts, and provide practical safeguards for employees in the workplace.
[edit] Standards
Four principal standards govern safe work practice when arc flash hazards are present:
- OSHA Standards 29-CFR, Part 1910. Occupational Safety and Health Standards. 1910 sub part S (electrical) Standard number 1910.333 specifically addresses Standards for Work Practices and references NFPA 70E.
- The National Fire Protection Association (NFPA) Standard 70 - 2002 “The National Electrical Code” (NEC) contains requirements for warning labels.
- NFPA 70E 2000 provides guidance on implementing appropriate work practices that are required to safeguard workers from injury while working on or near exposed electrical conductors or circuit parts that could become energized.
- The Canadian Standards Association's upcoming CSA Z462 Arc Flash Standard is under development and will become Canada's version of NFPA70E when it is released in 2008 [3]
- The Institute of Electronics and Electrical Engineers IEEE 1584 – 2002 Guide to Performing Arc-Flash Hazard Calculations. [4]
Arc flash hazard software exists that allows businesses to comply with the myriad of 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.
[edit] References
- ^ NFPA 70E - Electrical Safety in the workplace
- ^ IEEE/NFPA Collaborative Research Project
- ^ CSA Electrical Safety Conference
- ^ IEEE 1584 Working Group website
