J. C. Das - Arc Flash Hazard Analysis and Mitigation

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/cm 2are available, still for category 4 applications.

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/cm 2(5.0 J/cm 2). 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 cm 2. 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.)

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 1.9illustrates the limited approach and restricted approach boundaries based upon NFPA 70E.

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!

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.

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 incident energy at working distance or the arc rating of the required PPE, or the maximum HRC for that type of equipment. Also, arc flash boundary needs to be specified.

A maximum duration of 2 seconds for the total fault clearance time of an arc flash event is considered, though, in some cases, the fault clearance time can be higher. In IEEE 1584 Guide Annexure B, it is stated that: “if the time is longer than 2 seconds, consider how long a person is likely to remain in the location of the arc-flash. It is likely that a person exposed to arc flash will move away quickly, if it is physically possible and 2 seconds is a reasonable maximum time for calculations. A person in a bucket truck or a person who has crawled into equipment may need more time to move away.”

Tables 1.10and 1.11show the arc flash boundary calculations according to IEEE 1584 Guide equations for a bolted three-phase fault current of 30 kA, in 13.8-kV switchgear, 13.8-kV system resistance grounded, and also in a 480-V MCC, and 480-V system high resistance grounded.

For a 30-kA, 2-second fault in 13.8-kV switchgear, the incident energy boundary is 3539 in equal to 295 ft. For 1-second fault duration, it is 144.6 ft. The arc flash boundary at which a worker can be exposed to 1.2 cal/cm 2of incident energy and sustain threshold of second degree burns seems to be very large.

The working space, in an electrical room switchgear installation in front of electrical equipment, is limited and may be as low as 5–6 ft, and must meet NEC requirements. Many companies, as a policy, keep the electrical rooms locked, and a worker must have the required PPE outfit before entering the electrical rooms.

TABLE 1.11.Arc Flash Boundary and Incident Energy Release for 30 kA of Bolted Fault Current (Arc Flash Current = 16.76 kA rms) in 480-V MCC, 480-V System High Resistance Grounded, Working Distance = 18″, Gap = 25 mm