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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 1.10 and 1.11 show 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/cm² of 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

Arc Duration in Seconds	Arc Flash Boundary in Inches	Incident Energy, cal/cm
0.050	36	3.8
0.5	147	38
1.0	225	75
1.5	288	113
2	343	151

TABLE 1.12. Statistics of Arc Flash Incidents

Accident Occurrence	Percentage
When the operator or worker is working with equipment doors open	65
When the operator happens to be in front of a closed door and the equipment is not arc resistant	10
When the operator is not present at all, and the equipment is not arc resistant	25