NFPA 70E requires the employer to provide and employees to wear appropriate flame resistant (FR) clothing and other PPE when they’re within the arc-flash boundary. Which FR clothing to use depends on the level of incident energy the individual will be exposed to in the event of an arc-flash. (Note: There are corresponding assessment forms in the Tools section of SSOL for this story.)
The level of incident energy is determined by the distance of the person from the arc-flash (incident energy increases rapidly as the individual moves closer to the arc-flash). Generally, 18 inches is assumed to be the distance between a worker’s face/chest and the arc-flash.
Using the information collected to establish the arc-flash boundary, the engineer can calculate the incident energy in cal/cm2 at 18 inches. FR clothing is rated in cal/cm2. This enables you to select appropriate clothing to protect against the incident energy of exposure.
It’s not uncommon for calculated results at 18 inches to show an arc-flash incident energy of less than 1.2 cal/cm2. In these circumstances, no FR clothing is required for the face/chest area; all that’s needed is clothing that won’t melt, such as cotton. However, additional PPE may be required for parts of the body that are closer than the18-inch basis.
It’s also not uncommon to find at least one location in a facility where the calculated incident energy at 18 inches exceeds 40 cal/cm2, the highest level that 70E recognizes as being practical to protect (some clothing manufacturers offer clothing with higher ratings). In these circumstances, the employer should either make changes to the electrical system to reduce the incident energy or prohibit work within the arc-flash boundary.
PPE for the Default Boundary
Small facilities that choose to use the 4-foot default boundary instead of using the formula won’t have the incident energy results needed to select the proper level of PPE for the arc-flash hazard.
For these facilities, 70E provides two tables to use in selecting PPE. The first table matches a “Hazard/Risk Category” to a specific task by voltage level and type of equipment. The second table describes the FR clothing and corresponding incident energy for each of five Hazard/Risk Categories.
Limitations of the NFPA 70E Tables
Using the 70E tables to select PPE has its limitations. The first table that matches the category to the task is limited to electrical systems that don’t exceed specified levels of available short circuit current and fault clearing times as described in the table footnotes.
Also, 70E states that for tasks not included in the table and for electrical systems that exceed the footnote limitations, the tables cannot be used and the incident energy must be calculated for PPE selection.
Using the tables when the electrical system exceeds the levels described may expose individuals to hazardous energies beyond the protection of their FR clothing, potentially resulting in serious injury or death. On the other hand, when the footnotes are met, the level of protection can be overly conservative, which may increase hazards to the individual by limiting vision, mobility and dexterity.
In other words, it’s always better to select the proper PPE based on the calculated incident energy of exposure. Selecting PPE based on incident energy may also result in substantial savings over the cost of selecting PPE based on the tables.[fbcomments]