Arc Flash Boundary - Hazard Mitigation

By R.W. Hurst, The Electricity Forum

Arc Flash Boundary
Arc Flash Boundary

The Arc Flash Boundary has been developed by NFPA70e to minimize the risk of injury to electrical workers, working in proximity to energized electrical equipment. The Arc Flash Boundary determines the distance from the equipment at which the Incident Energy of an arc flash would be 1.2 cal/cm². The electrical worker has to be able to determine his relative risk, according to the distance from the equipment. The Boundary helps him to determine a safe distance.

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In an arc flash explosion, the most common and most severe injuries are usually burns. These injuries are the result of the extreme heat given off by the arc flash. In an explosion, radiant heat spreads out in all directions, weakening as the energy dissipates farther and father from the source. The result of this is that NFPA 70E shows an arc flash boundary distance intended to limit these burns.

Arc Flash Boundary is the distance at which an electrical arc can flash outward, which may endanger electrical workers near electrical equipment.

There are four main Arc Flash Boundaries:

· Arc Flash Protection Boundary (outer boundary)

· Arc Flash Limited Approach Boundary

· Arc Flash Restricted Approach Boundary

· Arc Flash Prohibited Approach Boundary (inner boundary)


Arc Flash Protection Boundary (outer boundary)

The Flash Protection Boundary is the calculated safe working distance from electrical equipment which would not expose the employee to the hazards associated with an electrical explosion. If an explosion occurs within this boundary, it is considered that an electrical worker would be exposed to a curable second degree burn (1.2 calories/cm2 ).


Limited Approach Boundary

The Limited Approach Boundary is defined as an approach limit at a distance from an exposed live part where a hazard exists. Where there is a need for an unqualified person(s) to cross the Limited Approach Boundary, NFPA 70e dictates that a qualified person shall advise him or her of the possible hazards and continuously escort the unqualified person(s) while inside the limited Approach Boundary.


Restricted Approach Boundary

The Restricted Approach Boundary is defined as approach limit at a distance from an exposed live part which there is an increased risk of shock.


Prohibited Approach Boundary (inner boundary)

The Prohibited Approach Boundary is defined as a distance from an exposed part which is considered the same as making contact with the live part. This distance is not common between equipment. Some equipment will have a greater flash protection boundary while other equipment will have a lesser boundary.

In terms of electrical safety, any electrical worker crossing the Prohibited Approach Boundary must be protected as if they were protected for making direct contact with the exposed live equipment.

Establishing these Arc Flash boundaries is considered the best way to protect workers from the dangers of electrocution and injuries. Under no circumstances, should an electrical worker (qualified and/or unqualified) enter the prohibited approach boundary, unless:

  • A responsible and trained person has carried out a full risk assessment.
  • The electrical work has been documented through an energized work permit
  • The qualified electrical worker has been trained and is authorized to conduct work on live electrical equipment
  • The electrical worker has been properly trained and equipped with appropriate PPE.
  • It is ensured that electrical workers use the correct equipment and follow precise procedures when in the proximity of live electrical equipment.

The protection boundary and the rules governing access within it take precedence over the shock hazard boundaries. So, for example, if the flash protection boundary is greater than the limited approach boundary then no unqualified person can be permitted in the limited approach area and even qualified workers must wear appropriate arc-resistant PPE.