Arc flash boundary

Responding is Wesley J. Maertz, CSP, technical safety specialist, Grainger, Lake Forest, IL.

Paragraph 130.5(C) of the 2015 edition of NFPA 70E highlights two methods to follow to determine what arc flash PPE is necessary for a job task: Incident Energy Analysis Method or Arc Flash PPE Category Method. One or the other method must be selected for each piece of equipment, but not both. Both methods determine the level of hazard related to the arc flash based on a few variables: the available bolted fault current (the maximum available fault current at some point in the electrical system) supplied from the source, the clearing time (time at which a breaker opens or fuse blows) and impedance between the point of the flash and the source.

The Arc Flash PPE Category Method is based on the collective experience of a task group and estimated exposure levels. It is a fast and convenient way to determine appropriate PPE by using tables of commonly performed tasks to quickly assess PPE needs. However, users must have a sound understanding of all of the variables involved with each job task. The tables contain use parameters that specify a range of available fault current and clearing times for the upstream over-current protective device. This method may not be safely used beyond the parameters listed. This can commonly lead to misuse of this method, which can lead to overprotection or underprotection for the worker. If the task being assessed does not fit the task described or use parameters, then the Incident Energy Analysis method must be used.

The Incident Energy Analysis method is much more time-consuming and costly, but provides a complete evaluation of the power system with the actual incident energies for each piece of equipment clearly defined.

Some important questions to ask yourself when considering what method to use:

• Do I really know the bolted fault current supplied for the equipment?
• Do I know the clearing time of the equipment?
• Do I know the impedence of the equipment?

If you can answer yes with 100 percent confidence to all three questions, then the Arc Flash PPE Category Method may be an option. If not, the Incident Energy Analysis Method would be the preferred option.

Editor's note: This article represents the independent views of the author and should not be construed as a National Safety Council endorsement.