> FOCUS BY JIM PHILLIPS
AC and DC—electrical hazards
The electric shock hazard from both AC and DC power systems has been well documented and understood for decades,
thanks to the research of people such as Charles Dalziel. Even
our understanding of the arc flash hazard has greatly improved,
thanks to years of research by many individuals and the 2002
introduction of IEEE 1584—IEEE Guide for Arc Flash Hazard
Calculations. However, when performing arc flash calculations,
IEEE 1584 only addresses the AC arc flash hazards. Currently,
there are no standards for calculating the arc flash hazard for
DC power systems. DC arc flash is the proverbial elephant in
A work in progress
Two landmark technical papers changed the understanding of
DC arc flash. D.R. Doan’s “Arc Flash Calculations for Exposures
to DC Systems” helped elevate the discussion of DC arc flash
calculations. It was published in IEEE Transactions on Industry
Applications, Vol. 46, No. 6. This paper provides a theoretical
approach to DC incident-energy calculations based on the con-
cept that the maximum possible power in a DC arc flash occurs
when the arcing voltage is 50 percent of the system voltage.
Ultimately, the equations from this paper were included in the
informative annex of the 2012 Edition of NFPA 70E and remain
in Annex D of the 2015 edition.
A subsequent paper, “DC-Arc Models and Incident-Energy
Calculations,” by R.F. Ammerman, T. Gammon, J.P. Nelson and
P.K. Sen, provides a comparison study of the existing body of
research into DC arcs and arc flash modeling that has been conducted over the years. It also provides a series of calculation
methods for determining the incident energy from a DC arc
flash in open air as well as in a box. This second paper is the
basis for DC arc flash calculations that many industry professionals and software packages currently use.
Calculating the incident energy for a DC arc flash begins
with a simple application of Ohm’s Law:
MORE THAN A CENTURY AGO, two giants in the fledgling electrical power industry battled it out for
supremacy. The conflict, sometimes referred to as “The War of the Currents,” would define whether electric
power systems would use alternating current (AC) or direct current (DC). Thomas Edison was well-known
for his Pearl Street generating station in New York City, which was based on DC. George Westinghouse, with
help from Nicola Tesla, was a proponent of AC.
Westinghouse ultimately won the war, and AC is used to power most of the world’s electrical loads. However,
even though they are not as pervasive, DC systems are also common. Examples include rectifiers, traction power
systems, adjustable frequency drives, photovoltaic systems, battery banks and more.
DC arc flash calculations