Arc flash may be the greatest—yet least understood—danger in today’s power plant. Its danger is not just the risk of electrocution facing technicians when maintaining switchgear but also the equally deadly concussive blast effects and exceptionally high temperatures that instantly occur when a short circuit occurs. In some instances, all three effects occur in chorus, further dropping the survival rate. (See “Arc flash protection should be job No. 1”in POWER, February 2007 for a special report on arc flash causes and technologies available to improve worker safety.)
The typical arc flash occurs in fractions of a second and without warning when electrical current, designed to move along a particular path, short-circuits either through the air to the wrong conductor or to a ground path. The arc flash that results has been compared to looking directly at the noonday sun, and the temperature of the arc can be as high as that near the surface of the sun—10,000F. Secondary effects include explosion, with perhaps shrapnel damage, and an intense sound pressure wave. Because power plants have installed high-voltage switchgear in enclosed rooms in recent years, the blast, flash, and temperature effects are further magnified.
In older plants where switchgear is sitting in open bays and standing on the turbine room floor gratings, the blast cone can reach out and touch plant staff several floors away (Figure). The cost of arc flash events every year is many millions of dollars in equipment damage, lost production, and lawsuits. But the human toll is without price. A 10-year study by lectricité de Francefound that arcing caused 77% of all recorded electrical injuries. Similarly, one corporation noted that up to 80% of its electrical injuries involve thermal burns due to arcing faults. The U.S. National Institute of Occupational Safety and Health (NIOSH) tabulated 44,363 electrical-related lost-workday cases from 1992 to 2001.Arc flash burns accounted for 17,101 (or about 39%) of those cases.
Another misconception is that only high-voltage switchgear requires arc flash protection. Not true. I can tell you from personal experience that the heat and blast effects from a standard 480-volt motor control center can cause injuries. According to National Fire Protection Association (NFPA)70E, the following flash protection boundaries (Figure) define the safe working distances from an energized component:
■ Up to 750 V: 3 feet
■ 750 V to 2 kV: 4 feet
■ 2 kV to 15 kV: 16 feet
■ 15 kV to 36 kV: 19 feet
■ Over 26 kV: must be calculated
An often overlooked risk group is thermographers, who must understand these safety standards and be properly outfitted with protective gear when taking readings. They also should be accompanied by a qualified electrician. Gone are the days when you stuck your hands into any electrical switchgear, regardless of voltage, protected by only a hardhat, safety glasses, and a pair of rubber gloves. Remember, the human heart will begin defibrillating, and soon stop for good, when hit with a relatively miniscule 80 milliamps of current. New regulations are making arc flash protection from potential electric shock, flash, and blast effects mandatory for plant owners.
NFPA 70E now requires protection from arc flash events and also requires a flash hazard analysis. The Occupational Safety and Health Administration (OSHA) has followed suit and has updated its electrical safety procedures to account for arc flash for the first time. The OSHA change was effective August 13, 2007. The OSHA regulations also recognize NFPA 70E and the National Electric Code (NEC) as industry standards that should be followed. Also, check with your insurance carrier, because it’s typically their practice to require conformance with standard industry specifications. IEEE 1584 provides a guide for performing the arc flash hazard calculations.
There is one area in the calculations that many utilities have found to be problematic—how to accurately determine electrical protective device characteristics or relaying in the plant. The response time assumed has a stunning effect on the calculated arc magnitude and duration. Although it’s not required by the regulations, a sensitivity analysis using relay actuation time is easy to perform—many plants have decided to assume instantaneous relay actuation times for the most conservative calculation.
OSHA has cited plant owners after arc flash events under the General Duty Clause, which states: “each employer shall furnish to each of its employees a place of employment that is free from recognized hazards that are likely to cause death or serious physical harm.” The crackdown follows the release of industry statistics showing that the 80% of accidents and fatalities related to electrical work by “qualified workers” are caused by arc flash. Other facilities have been cited under 29 CFR 1910.335 (a)(1)(i), which requires the use of protective equipment when working where a potential electrical hazard exists and/or 29 CFR 1910.132 (d)(1), which requires an employer assessment of all workplace hazards and the use of personal protective equipment. I suspect many readers also understand that NFPA 70E specifically excludes utilities from the code’s coverage. However, just because a code or regulation has a “loophole,” plant owners shouldn’t hide behind it when worker safety is involved.
Chapter one of NFPA 70E states that employers are responsible for providing workplace practices and procedures and employees are responsible for following those procedures.If your facility does not have an electrical safety program that addresses the extreme dangers of arc flash, suitable protective equipment for workers, and a training program for the equipment and arc flash protection, then you are gambling with your life and the lives of your coworkers.