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Unformatted text preview: dous environments. Overcurrent protection devices can heat up and occasionally arc or spark, which could cause a fire or an explosion in certain areas. Hazardous environments are places that contain flammable or explosive materials such as flammable gases or vapors (Class I Hazardous Environments), finely pulverized flammable dusts (Class II Hazardous Environments), or fibers or metal filings that can catch fire easily (Class III Hazardous Environments). Hazardous environments may be found in aircraft hangars, gas stations, storage plants for flammable liquids, grain silos, and mills where cotton fibers may be suspended in the air. Special electrical systems are required in hazardous environments. If an overcurrent protection device opens a circuit, there may be a problem along the circuit. (In the case of circuit breakers, frequent tripping may also indicate that the breaker is defective.) When a circuit breaker trips or a fuse blows, the cause must be found. A circuit breaker is one kind of overcurrent protection device. It is a type of automatic switch located in a circuit. A circuit breaker trips when too much current passes through it. A circuit breaker should not be used regularly to turn power on or off in a circuit, unless the breaker is designed for this purpose and marked "SWD" (stands for "switching device"). A fuse is another type of overcurrent protection device. A fuse contains a metal conductor that has a relatively low melting point. When too much current passes through the metal in the fuse, it heats up within a fraction of a second and melts, opening the circuit. After an overload is found and corrected, a blown fuse must be replaced with a new one of appropriate amperage. Find the cause of an overload. Only circuit breakers marked "SWD" should be used as switches. Section 7 Page 53 S AF ET Y M O D EL S TAG E 3--C O N T rOLLING HAZArDS: SAFE WOrK ENVIrON M EN T When You Must Work on or Near Live Circuits
Working on live circuits means actually touching energized parts. Working near live circuits means working close enough to energized parts to put you at risk even though you may be working on de-energized parts. Common tasks where you need to work on or near live circuits include: taking voltage and current measurements, opening and closing disconnects and circuit breakers, racking circuit breakers on and off the bus, removing panels and dead fronts, and opening electric equipment doors for inspection. There should be standard written procedures and training for these common tasks. For instance, when opening and closing disconnects, use the left-hand rule when possible (stand to the right side of equipment with a disconnect on the right, and operate the disconnect with your left hand). For other situations where you might need to work on or near live circuits, your employer should institute a written live-work permit system, which must be authorized by a qualified supervisor. Live-work permit system A live-work permit should, at least, contain this information: a description of the circuit and equipment to be worked on and the location, explanation why the work must be done "live", date and time covered by the permit, a description of the safe work practices to be used, results of shock hazard analysis and determination of shock protection boundaries, results of flash hazard analysis and determination of the flash protection boundary, PPE needed to safely perform the job, who will do the work and how unqualified persons will be kept away, and evidence of completion of job briefing, including discussion of job-specific hazards. Page 54 Section 7 Energized-work approval signatures (authorizing or approving management, safety officer, owner, etc.). To work on or near live parts, you must do the following: Have a written live-work permit for the work to be done. Wear the right PPE to protect against electric shock and arc flash. Never wear clothing made from synthetic materials, such as acetate, nylon, polyester, polypropylene, or rayon alone or combined with cotton. Such clothing is dangerous because it can burn and melt into your skin. The PPE that's needed depends on the type of electric work being done. The minimum PPE required would be an untreated natural fiber long-sleeve shirt and long pants plus safety glasses with side shields. Depending on the voltage and the electric task to be done, different types of PPE are required. Fire-resistant protective clothing can include multi-layer flash suit jacket and pants, wraparound face shield, double-layer switching hood, voltage-rated gloves with leather protectors, electrically rated hard hats, and so forth. [(See Table 130.7(C)(9)(a) Hazard/Risk Category Classifications and Table 130.7(C)(10)) (NFPA 70E, 2004 Edition). Use the proper type of protective equipment, such as insulated tools and/or handling equipment that is rated for the voltage. These can include insulated fuse or fuse holding equipment, noncon...
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This document was uploaded on 03/14/2014 for the course ECE 482 at University of Tennessee.
- Spring '09