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What_is_Arc_Flash

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					What is Arc Flash? Arc flash is a short circuit through air that flashes over from one exposed live conductor to another conductor or to ground. Arc flash incidents are common and costly, and the frequency of reported accidents is increasing. An arc flash (or arc blast) is 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 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 with 1000 amps or more can cause substantial damage, fire or injury. The massive energy released in the fault rapidly vaporizes the metal conductors involved, blasting molten metal and expanding plasma outward with extreme force. 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. 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. 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. Reducing Hazard by Design 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 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 devices such as grounding resistors or fuses. If the fault current is limited to 5 amps or less, then many ground faults self-extinguish and do not propagate into phase-to-phase faults.

Arcing Time Arcing time can be reduced by temporarily setting upstream protective devices to lower set points during maintenance periods or by employing zone interlocking (ZSIP). Arcing time can significantly be reduced by protection based on detection of arc-flash light. Optical detection is often combined with over current information. Light and current based protection can be set up with dedicated arc-flash protection relays or by using normal protection relays equipped with arc-flash option. The most efficient means to reduce arcing time is to use arc eliminator that will extinguish the arc within a few milliseconds. 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. 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. What Causes Arc Flash? - Arc flashes can be caused in a variety of ways: o Just coming close to a high-amp source with a conductive object can cause the electricity to flash over. o Dropping a tool or otherwise creating a spark can ignite an arc flash. o Equipment failure due to use of substandard parts, improper installation, or even normal wear and tear. o Breaks or gaps in insulation. o Dust, corrosion or other impurities on the surface of the conductor. How Common Is Arc Flash? - In the past, if someone suffered burns in an electrical accident, people thought the burns were caused by the electrical shock passing through the body. Electrical shocks can cause burns. But what research has shown is that most burns from electrical accidents actually come from arc flash. Safety Facts Fact: Almost 8000 electrical contact accidents occur in the U.S. each year, never mind in Canada. Fact: At least One worker dies each day from electrical contact. Fact: Fatalities from electrical accidents with a potential arc flash component have been trending downward since recent mandatory safe work practices have become “law.” The majority of this work and effort surrounds dealing with work on or near live electrical equipment. Placing electrical equipment in a "safe working condition" highly reduces or even eliminates the likelihood that an arc flash burn incident will or can occur and is required by federal law. Arcing faults are not selective. They can injure or kill anyone within their vicinity, not just the person working on the equipment. Oftentimes this includes electrical inspectors.

Equipment must be de-energized for inspection (if the doors are open or the covers are off). If it is more dangerous to turn it off, wear adequate PPE for the distance from exposed, energized parts. Safe work practices are the key to the reduction in electrically related arc flash burn injuries; however, safe work practices work only when they are implemented. Implementing the practices and procedures available today will enhance worker safety and is critical to reduce the number of electrically related accidents that occur. Training Do you and your employees know the risks? Knowing the level of potential hazard is critical to taking the proper level of precaution. Fact: Everyone at risk of arc flash needs a basic understanding of: Qualified persons must also be familiar with:
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Special precautionary techniques based on the tasks being undertaken; Personal protective equipment and clothing (PPE and PPC); Insulating and shielding materials; Grounding equipment; Insulated mats, tools and test equipment; Skills and techniques for distinguishing exposed energized parts from other parts of electrical equipment; Skills and techniques for determining the nominal voltage of exposed energized parts; Safe approach distances and the corresponding voltages to which he will be exposed; The decision-making process for determining the degree and extent of the hazard, the job planning required to avoid the hazard and the PPE necessary to perform the task; The duties of a supervisor; Lockout/tagout (LOTO) and test-before-touch procedures; Procedures for ensuring permits are completed and instructions followed; Procedures for enforcing company policy and safe work procedures; and Any company-specific requirements that may apply


				
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posted:6/9/2009
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