Arc flash describes a dangerous event which occurs when electric current leaves its intended path and arcs through the air until making contact with another conductor or the ground. The results of an arc flash are explosive and violent, and can leave anyone in its path with serious injuries or death.
To lower the risk of arc flash injuries, the National Fire Protection Association® developed a set of arc flash boundary limits to guide decisions on both equipment operation and personal protective equipment. The NFPA 703® recognizes three arc flash boundary levels: the limited approach boundary, the restricted area, and the prohibited area boundary.
What is the Limited Approach Boundary?
NFPA 70e® defines the limited approach boundary (also known as the flash protection boundary) as the farthest point at which a shock hazard exists.
Should an arc flash occur, people in the limited approach boundary would be exposed to curable second degree burns of 1.2 calories/cm2.
The exact dimensions of the limited approach boundary are determined by the equipment’s incident energy risk assessment. Qualified workers wearing appropriate personal protective equipment (PPE) can enter the limited approach boundary. An unqualified person wearing appropriate PPE can also enter the boundary, but only if escorted at all times by a qualified individual, and then only if informed of arc flash hazards.
Arc Flash Restricted Approach Boundary
The closer an employee is to exposed electrical components the greater the risk of serious arc flash injuries. Only qualified individuals with energized electrical work permits can work within the restricted approach boundary.
Prohibited Approach Boundary
The NFPA 70e® considers the inner arc flash boundary--the prohibited approach boundary--as the distance from an exposed energized component where the effects of an electric arc would be the same as actually touching the live part. No-one should be within this boundary while equipment is energized.
Who is Considered a Qualified Worker?
To be qualified to work within arc flash boundaries an employee must have proper electrical safety training, be able to demonstrate the skills needed for safely operating equipment, and an understanding of the hazards involved. Additionally, an employee who has received on-the-job training and has the ability to perform such duties safely may work within the limited approach boundary under the direct supervision of qualified personnel.
Determining Arc Flash Approach Boundaries
Approach boundaries must be calculated for equipment representing an arc flash hazard. Several ways to do this exist, including:
- Referring to NFPA 703® - 200 table 303.9.1 or table 130.7(C)(9)(a).
- Using software designed to calculate arc flash boundaries and create one-line diagrams and approach boundaries required by NFPA 70e.
- Using approach boundary formulas available in IEEE Standard 1584 or NFPA 70e®. Note these formulas are complex, require detailed information, and should only be performed by qualified electrical engineers. Human error is a potential factor when using such formulas.
- Using the IEEE spreadsheet-based calculator. As with approach boundary formulas, use of the IEEE spreadsheet should be the responsibility of an electrical engineer.