Ingeniero

Figure 1: Fuse TCC showing long times at steep
portions of the curve.

worst-possible hazard associated with energized work at different locations of the
equipment. In addition, it will cover
methods to reduce the hazard level like
maintenance mode settings and arc flash
sensor relays.

Evaluating the Hazards
of Low-Voltage Arcs
By Albert Marroquin

W

hen it comes to power systemdesign and operation,
there should be no greater
concern than safety. Not
only must electrical system designers implement safeguards to protect equipment
and processes, they must also evaluate the
hazards associated with arc faults.
For example, in many electrical facilities, it’s a common practice to set protective device settings to high-interrupting
fault currents to avoid nuisance trips,which result in undesired interruption and
costly shutdowns and re-starts. However,
protective device settings may perform
poorly when it comes to protecting the
people working on energized equipment
in the event of a low-voltage arc fault.
Protective device trip settings for many
electrical facilities have been set solely
based on bolted three-phase short-circuit
26

criteria.However, low-voltage arc faults
(< 1.0 kV) may produce a current magnitude much smaller than the circuit’s maximum available 3-phase bolted shortcircuit current. Of course, the incident energy expected to be released should be
smaller at lower current magnitudes; however, in some situations it may turn out
that overcurrent devices take much longer
to trip, and thus the release of incident energycould last for seconds or minutes.
Exponentially longer arc fault clearing
times encountered at steep portions of the
time current characteristic curves (TCCs)
translate into much higher amounts of incident energy release (see Figure 1).
This article discusses methods available for calculating the incident energy
released by an arc fault in low-voltage
equipment. It also presentsconsiderations
which should be made to determine the

Electrical Products & Solutions • June 2007

Two Calculation Methods
The majority of the arc flash analyses
are performed using the IEEE 1584 and
NFPA 70E methods. Both methods consider the low-current magnitude phenomenon, but have different ways of
accounting for its effect in the calculation
of the incident energy.
The NFPA 70E 2004 methodrecommends that the incident energy for equipment 600 Volts and below be determined
from the “maximum” and “minimum”
short-circuit currents. In fact, in this model
a 62% reduction of the maximum available short-circuit current is recommended
to determine situations at which the upstream overcurrent device could take seconds or minutes to operate (NFPA 70E
2004 Annex D.6). This reduction percentcorresponds to the industry accepted minimum current level for self sustaining arc
faults. Equation [D.6.2 (a)] can then be
used to calculate the incident energy.
The IEEE 1584TM-2002 and 2004a
“IEEE Guide for Performing Arc-Flash
Hazard Calculations” (sections 5.1 to 5.5)
provides a second method to calculate the
incident energy for low-voltage equipment. The IEEE 1584 empirically-derivedequations can predict very low arc fault
current values. IEEE (Continued on page 28)

Evaluating the Hazards... (Continued from page 26)
1584 2002 equation 1 can be used to determine the magnitude of the actual arc
fault current (instead of the available
short-circuit current as used by the NFPA
70E method).
In fact, for the simple electrical system
described in this article, thecalculated
arcing current magnitude can be as low
as 45% of the maximum available bolted
3-phase short-circuit current. The 45%
value already accounts for the additional
15% reduction recommended by IEEE
1584 for systems with nominal voltages
less than 1000 Volts (section 5.2 of IEEE
1584a 2004).
The lower magnitude of low-voltage
arc faults raises arc flash analysis problems. The results...