should not be crossed by any one, including a qualified person, without wearing the required PPE, (Table 1.8). The PPE outfits are designed to minimize the risk of sustaining energy greater than 1.2/cal cm2. That is, the threshold of second-degree burns can still occur even with appropriate PPE, and these burns are considered curable.
Unqualified persons, that is, those not specifically trained to carry out the required tasks, are safe when they stay away from the energized part of a certain distance, which is the limited approach boundary. They should not cross the limited approach boundary and arc flash boundary unless escorted by a qualified person.
Crossing the restricted approach boundary means that special shock prevention techniques and equipment are required, and an unqualified person is not allowed to cross this boundary. Finally, the prohibited approach boundary establishes the space that can be crossed only, as if a live contact with exposed energized conductors or circuits was planned.
Figure 1.9. Hazard boundaries around an arcing source, adapted from NFPA 70E. Source: Reference [17].
TABLE 1.9. Limited Approach and Restricted Approach Boundaries
Source: NFPA 70E [17]. © 2012 National Fire Protection Association. (All details in the NFPA table have not been reproduced.)
| System Voltage Range | Limited Approach Boundary | ||
| Phase-to-Phase | Exposed Movable Conductor (i.e., Overhead Line Conductors Supported on Poles) | Exposed Fixed Circuit Part | Restricted Approach Boundary, Included Inadvertent Movement Adder |
| Less than 50 V | Not specified | Not specified | Not specified |
| 50–300 V | 3.0 m | 1.0 m | Avoid contact |
| 301–750 V | 3.0 m | 1.0 m | 0.3 m |
| 751 V–15 kV | 3.0 m | 1.5 m | 0.7 m |
| 15.1–36 kV | 3.0 m | 1.8 m | 0.8 m |
| 36.1–46 kV | 3.0 m | 2.5 m | 0.8 m |
| 46.1–72.5 kV | 3.0 m | 2.5 m | 1.0 m |
| 72.6–121 kV | 3.3 m | 2.5 m | 1.02 m |
| 138–145 kV | 3.4 m | 3.0 m | 1.15 m |
The limited approach, restricted approach, and prohibited approach boundaries are all defined based upon the system voltage. No calculations are required for establishing these boundaries. Table 1.9 illustrates the limited approach and restricted approach boundaries based upon NFPA 70E.
1.10.1 Working Distance
Working distance is defined as the closest distance to a worker’s body excluding hands and arms. IEEE 1584 Guide [9] specifies required working distances (Table 1.6). For the 15-kV switchgear, it is 36 in, while for a 480-V MCC, it is 18 in. A larger working distance reduces the incident energy and therefore the HRC. Recommendations of IEEE for the working distances are followed in the calculations in this book. The working distance does exclude hands and arms, which will be much closer to the seat of arc. It is the vital organs like eyes, chest, and heart that are at the working distance from the seat of the arc. This assumes that a worker does not stick his head inside the switchgear door!
1.10.2 Arc Flash Labels
The labeling on the equipment contains the following data:
system voltage
arc flash boundary
PPE category
incident energy release in cal/cm2
working distance
restricted approach boundary
prohibited approach boundary
TABLE 1.10. Arc Flash Boundary and Incident Energy Release for 30 kA of Bolted Fault Current (Arc Flash Current = 28.58 kA rms) in 13.8-kV Switchgear, 13.8-kV System Resistance Grounded, Working Distance = 36″, Gap = 153 mm
Source: Reference [17]. © 2012 National Fire Protection Association.
| Arc Duration in Seconds | Arc Flash Boundary in Inches | Energy, cal/cm2 |
| 0.058 | 74 | |
| 0.5 | 851 | 26 |
| 1.0 | 1736 | 52 |
| 1.5 | 2633 | 78 |
| 2 | 3539 | 104 |
equipment identification
the protective device identification that clears the fault.
The labels can be generated on a variety of media, including plastic weatherproof laminates, and most commercial arc flash analysis program will allow custom designing the labels. A user can choose what goes on the label, including the description of PPE. Even the type fonts can be user selectable.
In NFPA 70E 2012, equipment labeling has been reworded to reflect Article 110.16 of the NEC. Each label must contain the
