Arc Flash – Incident Energy at Working Distance

Posted on April 29, 2017 by Ver Pangonilo

Table 1—Power circuit breaker operating times^a
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

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