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Basics of Arc Flash

Arc flash is a term used to describe an electrical explosion caused by an arcing fault. The electrical explosion results in a thermal, pressure and sound wave that can cause extensive equipment damage, severe injury and death.

A recent claim involved an arc flash incident on a high school campus. The arc flash occurred at a 100-amp, 480 volt fused disconnect switch during troubleshooting activities. Personnel were injured, the campus lost power for the day and equipment repair costs totaled approximately $140,000. If appropriate safety and maintenance practices had been followed, this incident may have been avoided.

What is an Arcing Fault?

An arcing fault is caused by the breakdown of insulating material (usually air in low voltage systems) between energized components and the ground. Arc fault current is usually low. Therefore, protective devices, such as fuses and circuit breakers, take a protracted period of time to open to stop the fault. The arcing fault continues until the arc flash occurs.

Electrocution vs. Arc Flash

Arc flash is not the same as electrocution. Electrocution occurs when a person comes in contact with an energized component. Electrocution can cause significant injuries and death, but typically will not cause extensive equipment damage. Since there is a significant amount of awareness concerning electrocution, people are careful to follow all safety procedures and use safety equipment when around energized equipment. Arc flash is a recently recognized electrical hazard. The National Fire Protection Association (NFPA) 70E, fifth edition, was one of the first standards to specifically address arc flash hazards. It was published in 1995. Industry awareness of arc flash hazards, arc flash mitigation techniques and safety standards are continuing to evolve.

Arc Flash Factors

It is important to recognize that arc flash can happen on low voltage and high voltage systems. The following factors determine the extent of the hazard:

  • Amount of energy the power system can deliver
  • Time to sense a fault and open a protective device
  • Distance a person is from the flash

These factors should be evaluated in an arc flash study. The results of an arc flash study aid in the establishment of flash protection boundaries, selection of personal protective equipment (PPE) and selection of arc flash hazard labels.

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