12 2024 4hr Warning Labels & Boundaries 4 of 6

Main Ideas

Electrical Boundaries

Three boundaries serve different purposes: two are shock-protection boundaries, and one is an arc-flash boundary. All should be understood and considered; the video cautions against choosing only the largest distance and ignoring the others.

Who may enter depends on the boundary:

• A properly protected unqualified person may enter the arc-flash boundary.
• Within the limited approach boundary—given in the video as typically 3.5 feet (42 inches)—an unqualified person may be present only when escorted and supervised by a qualified person.
• At the closer boundary, where someone could reach exposed energized parts, only a qualified person may enter. An unqualified trainee may observe and learn under supervision but must keep their hands away from the equipment.

Hazards Around Equipment

Electrical hazards can extend in unexpected directions. Boundaries should be treated as a radius around equipment, not simply as a zone in front of it.

A West Tex demonstration shows an arc leaving a 30-amp disconnect, traveling along a bus bar, and puncturing a fabric wall behind the equipment. The example illustrates why people and barriers should account for hazards around the equipment, including shock hazards.

Incident Energy and Arc-Flash Boundaries

Incident energy is the thermal energy impressed on a surface at a specified distance from an arc source. Expressed in calories per square centimeter, it indicates the severity of a potential arc-flash exposure under specified conditions.

An incident-energy calculation is only one component of an arc-flash risk assessment. Having labels does not, by itself, mean that a full risk assessment has been completed. Before relying on a calculation, check that its specified conditions have not changed.

The arc-flash boundary is the distance at which incident energy reaches 1.2 cal/cm². The video explains that this threshold is based on the Stoll skin-burn injury model: an exposure at that level for one second is likely to cause the onset of a second-degree burn on unprotected skin.

Safety Considerations

A boundary is not a guarantee of safety. PPE and compliance with codes or standards do not make a person invulnerable, and stated limits do not promise zero injury.

When controlling access, consider both shock boundaries and the arc-flash boundary. If the arc-flash boundary is greater than or close to the limited approach boundary, the video recommends extending the marked-off area roughly another one to two feet beyond the relevant boundary. Block off the area around the equipment, not just the front.

Speakers and Sources

• Two unnamed presenters discuss the boundaries and ask and answer questions.
• West Tex is identified as the source of the embedded arc demonstration video.
• NFPA 70E, Article 100 is referenced for definitions and boundary concepts.
• The Stoll skin-burn injury model is cited to explain the 1.2 cal/cm² threshold.