Periodically, someone asks me a question that requires further research to answer. Such was the case a few months ago when I was teaching a two-day class on 2024 NFPA 70E, Standard for Electrical Safety in the Workplace. I could not answer the question about Note D at the bottom of Table 130.4(E)(a), so I did some digging and called two NFPA 70E members. 

I came back to class the next day with an answer, although it was not as complete as I felt the subject deserved. 

The table provides electrical shock protection approach boundaries to expose energized electrical conductors or circuit parts for AC systems. The same text was added in new Note B to the bottom of Table 130.4(E)(b) for DC systems. 

The notes in these two tables state the following: “The restricted approach boundary in Column 4 (for restricted approach boundaries) is based on an elevation not exceeding 900 m (3000 ft). For higher elevations, adjustment of the restricted approach boundary shall be considered.“

Limited approach boundary

The limited approach boundary and restricted approach boundary are covered in Article 130 in NFPA 70E. The limited approach boundary is defined as the “distance from an exposed energized electrical conductor or circuit part within which an electric shock hazard exists.“ 

This is the approach limit for unqualified employees and is intended to eliminate the risk of contact with an exposed energized electrical conductor or circuit part. The limited approach boundary is one of three that must be determined using the risk assessments NFPA 70E requires.

Restricted approach boundary

The restricted approach boundary is defined as “a distance from an exposed energized electrical conductor or circuit part within which there is an increased likelihood of electric shock, due to electrical arc-over combined with inadvertent movement.“ This is the approach limit for qualified employees. 

If a qualified employee crosses the restricted approach boundary, they must be protected from contact with energized or exposed conductors or circuit parts by wearing shock protection equipment. 

Arc flash boundary

The arc flash boundary is the distance “from an arc source at which incident energy equals 1.2 calories per cm².“ This boundary is determined by a calculation or the PPE category tables. It is an area where a person is likely to be exposed to a second-degree burn or worse and extends to where the potential for injury doesn’t likely include a burn. 

Arc flash burns could still occur outside this boundary but are not likely to be second-­degree. The 1.2 calories per cm² is a far enough distance to not require arc flash protection. It exists when electrical equipment is in other than a normal operating state.

Elevation considerations

The notes in tables 130.4(E)(a) and (b) state that a restricted approach boundary is based on an elevation location that does not exceed 3,000 feet. At higher elevations, air is not as dense and is less of an insulator. The opposite would be air as a conductor, which would be catastrophic to an electrical installation. Air density affects the electrical insulation properties negatively; therefore, the higher the altitude, the lower the dielectric properties of air. 

When air is less dense and has lower dielectric properties, energized bare connections might require a greater separation distance between phases. In addition, acceptable connections at 3,000 feet or lower may require greater separation distances. The air’s lower dielectric properties may also increase the probability of an arc flash. Conversely, there is also less oxygen above 3,000 feet, so an arc flash may not have the same intensity in higher elevations. 

Another effect is a decrease in heat transfer and, therefore, a worsening of conditions for components where significant heat could build up. This may affect air-cooled equipment, such as motors and other similar electrical equipment where heat buildup and lack of cooling may reduce the equipment’s longevity. Electrical equipment that is air-cooled must be able to dissipate the heat so the temperature limit is not exceeded. 

These issues must be further studied and addressed in the NEC and NFPA 70E, since the density and conductivity of air affects most electrical equipment. 

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