The 2015 edition of NFPA 70E defines three protective boundaries around electrical equipment. One, called the Arc Flash Boundary, is based on the expected Incident Energy from an arc flash, and is intended to protect workers from burns if such an accident occurs. The other two are the Limited Approach Boundary and the Restricted Approach Boundary, which are based on the voltage of the equipment, and are intended to protect workers from electric shock.
Protection from Arc Flash
In an arc flash, the most common and most severe injuries tend to be burns. As a result, NFPA 70E includes a "stay-back" distance intended to limit these injuries. Where an arc flash hazard is present, the Arc Flash Boundary marks the distance from the equipment at which the Incident Energy of an arc flash would be 1.2 cal/cm².
At this energy level, unprotected workers would be expected to receive second-degree burns as a result of an arc flash. Second-degree burns are certainly not desirable, but they are still treatable. Third-degree, untreatable burns would be expected only for workers who are closer to the equipment, inside the Arc Flash Boundary. Anyone needing to perform work inside this boundary needs personal protective equipment (PPE) to protect them from burns.
Protection from Electric Shock
Most equipment that poses an arc flash hazard also presents a risk of electric shock. However, the two hazards need to be addressed separately. The two shock protection boundaries are intended to protect workers from this second hazard.
The Limited Approach Boundary is the distance that unqualified persons (that is, people without any special training or equipment) should stay away. Qualified persons, who have been trained and equipped for the task at hand, may cross this boundary if needed. Qualified persons may also escort unqualified persons through the area, if appropriate protective equipment is provided for the visitor as well.
The Restricted Approach Boundary is closer to live parts, and may only be crossed by qualified persons. Crossing this boundary to perform work while the equipment is still powered also requires an energized electrical work permit, which includes a specific plan of action, a list of protective steps to be taken, and supervisory approval.
In older editions of NFPA 70E, there was another boundary even closer to the equipment, called the Prohibited Approach Boundary. This was removed in the 2015 edition of the standard; in practice, it didn’t do anything that the "Restricted" boundary hadn’t already covered.
Calculating Protection Boundaries
The two shock protection boundaries are exclusively based on the voltage of the equipment, and can be found in a set of tables in NFPA 70E. AC systems are covered in Table 130.4(D)(a), and DC systems are covered in Table 130.4(D)(b).
Calculating the Arc Flash Boundary is more complex, partly because the phenomenon of arc flash is still being researched. There are three major methods for determining this boundary:
- NFPA 70E provides a simplified formula for calculating incident energy, based on the theoretical work of Dr. Ralph Lee.
- IEEE 1584 provides an alternative, empirically-based formula, which was developed through laboratory testing. This formula is widely held to provide more accurate results than the theoretically-based "Lee Method."
- For certain common equipment, NFPA 70E includes tables that assign Arc Flash PPE Categories and Arc Flash Boundaries. Table 130.7(C)(15)(A)(b) covers AC systems, and Table 130.7(C)(15)(B) covers DC systems.
Arc Flash software, such as Arc Advisor or EasyPower, can calculate Arc Flash Boundaries, as well as performing many of the other tasks associated with electrical system analysis. For some facilities, it's better to contract an electrical engineering company to provide the necessary expertise. Whatever approach you take in your facility, be sure to post these boundaries and train your workers to understand and respect them.
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