COMPARISON OF ARC FLASH CALCULATION METHODS
By Neil Van Geem, PE
General
During recent years the electrical industry has focused attention on the electrical hazard of
arc-flash and the danger of its causing severe burns to electrical workers who are i
n the
vicinity of energized electrical equipment during an arc. Over this time various methods have
been developed to estimate the effects of an electrical arc’s incident energy causing
temperature rise on the human body as a result of the arc and on mitig
ating techniques such
as levels of fire resistant clothing and work location relative to the arc. This paper reports on
the results of a very limited comparison of four methods available for incident energy
calculation. The methods include those developed
and reported with the IEEE Standard
1584, “IEEE Guide for Performing Arc-
Flash Hazard Calculations,” with NFPA 70E,
“Standard for Electrical Safety Requirements for Employee Workplaces –
2000 Edition,” with
a 1981 IEEE paper, “The Other Electrical Hazard:
Electrical Arc Blast Burns,” by Ralph Lee,
a program of heat flux calculation from Duke Power, and with the ARCPRO program by
Kinetrics, Inc. Toronto, Ontario. All calculation methods caution that the testing and
calculations are based on selected conditio
ns. These are methods for predicting or
estimating arc flash hazards and thus actual cases experienced in the field can be expected
to vary from these values.
Comparison of Equation Design
The IEEE Standard, Duke Heat Flux, and NFPA 70E use equations dev
eloped empirically
from tests performed with arcs, while the Lee paper and ARCPRO use equations based on
theoretical analysis and verified by comparison with some measured results.
IEEE 1584 – Calculations consider three-phase arcs in enclosures and in ai
r. Published
input ranges are:
•
Voltage of 208 to 15,000 V
•
Bolted fault current of 0.700 to 106 kA
•
Grounding variations
•
Equipment enclosures of commonly available sizes
•
Gaps between conductors of 13mm to 152 mm (0.5 to 6 inches)
The equations were dev
eloped from curve fitting of results of values measured from
extensive testing performed by the standard’s working group.
Some general conclusions
resulting from their testing are:
•
System X/R ratio, system frequency, and electrode material had little or no effect
•
Incident energy depends primarily on arc current. Bus gap (arc length) is a small
factor
•
Calculations use Lee’s equation for voltages above 15 KV
•
Calculations use bolted fault current input
NFPA 70E – Calculations consider three-phase arcs in
enclosures or in air. Input ranges are
similar to IEEE 1584 including use of bolted fault current input. Tested values were limited to
a distance from the arc of greater than 18” only and for bolted fault currents for the range
from 16 KA to 50 KA and for
system voltages rated 600 V and below. The equations were
developed from curve fitting results of values measured on limited testing (compared to IEEE
1584). Some general conclusions resulting from the testing which developed the equations
and as reported
in an IEEE paper, “Predicting Incident Energy to Better Manage the Electric
Arc Hazard on 600-
V Power Distribution Systems” of the IEEE Transactions on Industry
Applications Vol. 36, No. 1, January/February 2000 were:
