Historically, shock and electrocution have been seen as the primary electrical hazards to people, along with fires of electrical origin, but today, awareness is growing of two other electrical hazards: arc-flash and arc-blast. This growing awareness is largely due to NFPA 70E-2004, Standard for Electrical Safety in the Workplace.

Electric Shock: Electric shock is the leading of injuries and deaths due to electricity. Approximately 30,000 nonfatal electrical shock accidents occur each year. About 1,000 electrocutions (deaths from electric shock) occur each year, more than half of them while working on conductors and equipment energized at less than 600 volts. Electric shocks happen when people contact live parts or in some other way become part of an electric circuit. Even the current needed to light an old-fashioned Christmas tree bulb (7 1/2 watts, 120 volts) is enough to kill a person if it passes across the chest and through the heart.

Arc-Flash: When electric current passes through the air, temperatures can get hot enough to ignite clothing, melt plastic and burn people’s skin up to 10 feet away. Each year more than 2,000 people are admitted to hospitals with arc-flash burns. In addition to burn danger, the very bright light associated with an arc-flash events can cause temporary (and sometimes permanent) blindness.

Arc-flashes are usually short events lasting less than a half-second. They can be caused by such things as dropping a tool or metal part across energized busbars, operating damaged electrical equipment or voltage testing with a meter that isn’t properly rated.

Arc-Blast: At higher energy levels, explosion hazards—high pressure, loud noise and flying shrapnel—are added to the thermal and light dangers of arc-flashes. Extreme temperatures generated by the arc cause explosive expansion of both the surrounding air and of metal in the arc path (copper expands by a factor of 67,000 when it vaporizes). This rapid expansion creates a pressure wave that can explode equipment enclosures, knock workers off ladders, spray molten metal, rupture eardrums and destroy personal protective equipment (PPE).

Paradoxically, workers exposed to arc-blasts sometimes suffer fewer burns than those exposed to arc-flashes. The blast energy can blow workers away from the arc, thus reducing their exposure to heat but increasing the risk of serious falls or impact injuries.

Conclusion

The most effective protection against all three classes of electrical hazards is turning off the power before working on electrical equipment and systems. NFPA 70E calls this “establishing an electrically safe work condition,” and we’ll explain that important subject in a future safety article here on www.ecmag.com.

But sometimes it’s necessary to do repair and maintenance work on or around live equipment. When that’s the case, it’s critical to wear and use the proper protective gear defined in NFPA 70E. NECA publishes a Personal Protective Equipment Selector in both CD and manual form, to help electrical professionals understand and comply with the safety requirements of NFPA 70E. Visit www.necanet.org/store and click on “Safety.”  

STAUFFER is executive director of standards and safety at the National Electrical Contractors Association, headquartered in Bethesda, MD. He is a member of the National Electrical Code committee and the NFPA 70E committee, and has written several books.