8-cal/cm2 arc-rated pants and shirts
4-cal/cm2 treated pants and shirts and 4 cal/cm2 arc-rated overall
8-cal/cm2 arc-rated overall cotton shirts and pants.
It is the total level of arc-rated protection that matters. To achieve 8 cal/cm2 of the example being discussed, any 8 cal/cm2 system of one layer or more is acceptable. Natural fiber clothing like cotton, wool, and silk are still allowed but cannot add to the protection level because of risk of ignition. A note of caution is required in interpretation: a 4 cal/cm2 arc-rated shirt and another 4 cal/cm2 arc-rated shirt put together do not add to 8 cal/cm2. When layers are used, the total system must be tested to get the total arc rating, though most often the testing may prove that even greater protection may be provided than the arithmetic sum of the two layers. Also, layering of clothing that is not arc-rated cannot be used to increase arc rating of the clothing. See appendix H of NFPA 70E.
In NFPA 70E 2012, the hazard risk category (HRC) 2* is eliminated. All HRC 2 tasks will require the use of a balaclava under an arc-rated face shield or the use of an arc-rated hood. Commercially, arc-rated outfits even up to 100 cal/cm2 are available, still for category 4 applications.
1.10 HAZARD BOUNDARIES
The boundaries are defined in NFPA, and the following synopsis is relevant here (Figure 1.9).
The flash protection boundary is the distance at which the threshold of second-degree burns can occur and the incident energy release is 1.2 cal/cm2 (5.0 J/cm2). This is the boundary that is calculated by computer-based programs and IEEE Guide equations. Inside the boundary, the energy level will be higher. This boundary 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