Assessing the Arc Flash Risk

Every year, arc flash kills between 200 and 300 people. Between five and 10 times a day, somewhere in the United States an arc flash explosion in electric equipment sends burn victims to a special burn center. This number does not include cases sent to ordinary hospitals and clinics, nor unreported case and "near misses.

09/01/2004


Every year, arc flash kills between 200 and 300 people. Between five and 10 times a day, somewhere in the United States an arc flash explosion in electric equipment sends burn victims to a special burn center. This number does not include cases sent to ordinary hospitals and clinics, nor unreported case and "near misses."

"An arc flash is an explosion with all the implications of that word," says George Gregory, manager, Industry Standards, Square D/Schneider Electric, Cedar Rapids, Iowa. "It involves a fireball with temperatures at the center of the arc plasma that are over 5,000°F—hot enough to vaporize metal. The arc also produces a pressure blast that propels hot gases and molten metal outward. It presents a brown cloud of toxic materials. The ionized gases are conductors that will cause electrical shock or current paths through the body."

Is Your Facility at Risk?

Facility engineers and consultants alike must take arc flash seriously. Assuming that the danger does not exist in a given facility could prove fatal. It is important to perform ongoing arc-flash hazard analyses to quantify the risks and take steps to prevent them. The reasons for this risk analysis are to:

  • determine the incident energy for each electrical distribution path to each piece of electrical equipment;

  • determine the level of personal protective equipment (PPE) required when working on energized circuitry:

  • determine labeling requirements:

  • comply with OSHA regulations through standards determined by NFPA and IEEE:

  • minimize a facilities vulnerability to insurance and legal issues;

  • and minimize the risk of personal injury or death.

Arc-flash hazard exists wherever electrical circuits have enough available energy to sustain an electrical plasma arc. The current required to clear a protection device is determined by bolted fault current. However, a bolted fault is always a worst-case scenario in terms of a dead short.

Arc flash presents a different problem. A worker drops a tool, insulation on aging conductors deteriorates, or animals create a momentary fault. Regardless of the incident, when a flash-over occurs, a plasma is created that conducts electricity. But it conducts with higher impedance than a dead short. Because the impedance through the plasma arc is higher, the current is lower. A problem exists when this current is insufficient to clear the fault at the protection device—regardless of whether it is a circuit breaker, fuse or tie breaker. Upstream protection devices have even higher ratings, allowing the fault to continue.

When the fault finally clears, the damage is done. The idea behind doing the analyses and taking corrective measures is to limit the amount of available fault current, and therefore the incident energy, at each piece of equipment and provide adequate PPE.

Documentation is Key

Before you even consider an arc flash analysis, electrical documentation must be accurate. The facility one-line diagram is the culmination of all electrical documentation. If the one-line diagram is not accurate, further efforts toward analyzing arc flash are futile.

Once electrical documentation is in order, you can proceed to analyze the facility. According to John Lane, principal electrical engineer, AVO Training Institute, Dallas, Texas, the steps include:

  • Identification of locations/equipment

  • Collecting data

  • Short circuit analysis

  • Protective device coordination

  • Actual arc flash analysis

  • Arcing current

  • Arcing time

  • Determine incident energy

  • Determine personal protective equipment (PPE) category

  • Documentation

Lane stresses the necessity of a one-line diagram that matches the plant's electrical system as a starting point in assessing arc flash hazards. "Every electrical distribution path in every plant is different," he explains. "Each component in each of these paths is a variable that must be considered when evaluating potential arc flash hazards. An arc flash hazard analysis is more than recommended—it is urged."

Notice that arc flash analysis is listed within the steps to perform an arc flash analysis. This is not doublespeak; it is presented to emphasize the necessary involvement before and after making any calculations that quantify risks.

The physics behind arc flash is interactive. Available fault current is directly proportional to flash protection, limited approach, restricted approach and prohibited approach boundaries. Available energy depends on current and time. Electrical system impedance affects the magnitude of fault current directly. Fault current clearing time depends on the protective system. And an accurate one-line diagram of a facility's electrical system is crucial to determining available fault current.

Energized Equipment

Common sense suggests strongly that the power must be off if equipment is to be serviced. OSHA prevails where common sense fails. OSHA requires in its 29 CFR 1910.333(a)(1) that "live parts be de-energized unless the employer can demonstrate that the de-energizing introduces additional or increased hazards."

The 2004 release of NFPA-70E includes language that details requirements of an energized electrical work permit, which calls for written authorization for work on or near a circuit that is energized at 50 volts or greater. There must be written documentation to prove that it is infeasible to work on equipment or circuits de-energized as required by the OSHA regulations. The permit intends to verify that electrical workers are fully aware of the hazards.

Continuous compliance

Once your facility is compliant, you can relax, right? Wrong! Just because you now have an accurate one-line diagram of your facility does not mean that it will stay accurate. For example, changes that will be made to three older production lines during summer shutdown changes the drawing. But the point is, it changes the energy paths to the equipment on the updated lines. Changing the energy paths directly affects the timing of fault-clearing devices upon which your arc flash analysis calculations were based. Change the system and you must change your documentation, and you must recalculate all affected energy paths.



For Further Information

For more information on arc flash, NFPA-70E, and electrical documentation accuracy, go to

Don't miss the latest webcast in Plant Engineering magazine's arc flash series. "Protecting People and Equipment from Arc Flash" addresses electrical documentation accuracy, protection devices available to increase worker and equipment safety, and the knowledge to make all these pieces fit together. Go to



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