Arc Flash – Incident Energy at Working Distance

Table 1—Power circuit breaker operating timesa
Circuit breaker rating and type Opening time at 60 Hz (cycles) Opening time (seconds)
Low voltage (molded case) (< 1000 V) (integral trip) 1.5 0.025
Low voltage (insulated case) (< 1000 V) power circuit breaker (integral trip or relay operated) 3.0 0.050
Medium voltage (1–35 kV) 5.0 0.080
Some high voltage (> 35 kV) 8.0 0.130

aThis table does not include the external relay trip times.

Source: IEEE 1584-2002 IEEE Guide for Performing Arc-Flash Hazard Calculations

Table 3 - Classes of equipment and typical working distances
Classes of equipment Typical working distance (mm)a
15 kV switchgear 910
5 kV switchgear 910
Low-voltage switchgear 610
Low-voltage MCCs and panelboards 455
Cable 455
Other To be determined in field
a Typical working distance is the sum of the distance between the worker standing in front of the equipment, and from the front of the equipment to the potential arc source inside the equipment.

Source: IEEE 1584-2002 IEEE Guide for Performing Arc-Flash Hazard Calculations

Table D.4.2 Factors for Equipment and Voltage Classes
System Voltage (kV) Type of Equipment Typical Conductor Gap (mm) Distance Exponent Factor x
0.208–1 Open air 10–40 2.000
Switchgear 32 1.473
MCCs and panels 25 1.641
Cables 13 2.000
>1–5 Open air 102 2.000
Switchgear 13–102 0.973
Cables 13 2.000
>5–15 Open air 13–153 2.000
Switchgear 153 0.973
Cables 13 2.000

Source:NFPA 70E® - 2015 Standard for Electrical Safety in the Workplace

Note: Same table is also provided in IEEE 1584-2002, Table 6 - Factors for equipment and voltage classes

Table 130.7(C)(15)(A)(b) Arc-Flash Hazard PPE Categories for Alternating Current (ac) Systems [Part only]
Equipment Arc Flash PPE Category Arc-Flash Boundary
Panelboards or other equipment rated 240 V and below
Parameters: Maximum of 25 kA short-circuit current available; maximum of 0.03 sec (2 cycles) fault clearing time; working distance 455 mm (18 in.)
1 485 mm
(19 in.)
Panelboards or other equipment rated >240 V and up to 600 V

Parameters: Maximum of 25 kA short-circuit current available; maximum of 0.03 sec (2 cycles) fault clearing time; working distance 455 mm (18 in.)

2 900 mm
(3 ft)

Source: NFPA 70E® - 2015 Standard for Electrical Safety in the Workplace