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“Two employees of an electrical contractor were removing a 225-ampere circuit breaker from a 1,600-ampere, 277/480-volt, Square D switchboard. The employees had not deenergized the switchboard before starting the job, nor were they wearing personal protective equipment.
“Employee #1 was performing the task, kneeling in front of the switchboard. Employee #2 was kneeling to his left. An electrical fault occurred, and the ensuing electric arc burned the two employees and ignited their clothing. Employee #1 received burns to his chest, face, and arms. He died from his injuries. Employee #2 received burns to his right side, and he burned his hands from putting out the fire on himself and his co-worker.”
A few simple precautions could easily prevent injuries and fatalities. But what will it take for action? This article will review the intensity of an electrical arc, the trauma it causes and precautionary examples.
Many arcs are generated when equipment fails or a current overload occurs. Even more are caused by accidental contact. This can happen during a wide variety of operations. Typical equipment involved in an arc are switchgear (low and high voltage), disconnects, circuit breakers, panelboards, motor control centers (MCCs), transformers and metering devices.
Temperatures of up to 35,000°F have been recorded in an electrical arc. Metals are vaporized. The change in air temperature and the reaction of the metal cause arc blasts.
The heat burns through skin causing first-, second- and third-degree burns. If fortunate enough to be protected or far enough away, first-degree burns will leave the victim with red, sore skin.
The unfortunate, closer or less protected individual will receive higher degree burns that blister, blacken and destroy underlying tissue. As a first aid responder, a review of electric arc burns was the first time I heard the term “fourth degree” used. This describes the burning of muscle. In addition to burns, the pressure from the wave of air cause by the heat damages hearing, causes concussions and shoots metal from equipment like bullets from a gun.
The intensity of an arc is measured by the heat energy generated over a square area or calories per centimeter squared, “cals” for short. The amount of cals depends on amperage, voltage, arc gap, closure time, distance away from the arc, three-phase versus single-phase and whether it occurs in a confined space. It only takes 1 to 2 calories of heat to cause second-degree burns on human skin. A typical arc will release about 5 to 30 cals, but up to 60 cals have been recorded in an arc.
The first step in avoiding electrical arc accidents is prevention. Employers must have an electrical safety program. It should ensure a plan is in place for every job based on the hazards present. All employees must be trained. The plan should be to deenergize before performing work. Energized work is only allowed when it is not feasible to deenergize or a greater hazard exists when the system is deenergized. An example might be the interruption of life-support systems.
When work is performed on energized circuits, a permit program must be implemented. It should include the determination of a shock and flash protection boundary, how the area will be protected from unqualified workers, job briefing notes and what personal protective equipment (PPE) should be worn.
The shock and flash boundaries can be determined using formulas and tables contained in the National Fire Protection Association NFPA 70E Standards for Electrical Safety in the Workplace. The standard also assigns a hazard category within the flash protection boundary, which determines the level of PPE and equipment to be used to perform work.
At the lowest hazard/risk category 0, the minimum PPE includes an untreated natural fiber long-sleeve shirt, long pants and safety glasses with side shields. Synthetic materials must never be worn. This includes clothing made of acetate, nylon, polyester or rayon. Synthetic material will melt into your skin, increasing the severity of the burns.
Employers should become familiar with NFPA 70E. For assistance in understanding and complying with the standard, the National Electrical Contractors Association (NECA) has several aids. The NECA Protective Clothing Guide aids in the selection of PPE and requires tools.
The NECA Safety Expert System contains a module on NFPA 70E, which offers model documents such as an Electrical Safety Program, Live Work Permit and toolbox talks. Contact NECA at 301.657.3110 for more information. EC
O’CONNOR is with Intec, a safety consulting, training and publishing firm that offers on-site assistance and produces manuals, training videos and software for contractors. Based in Waverly, Pa., he can be reached at 607.624.7159 or [email protected].