Determine the Electrode Configuration and Sensitivity of the Enclosure Dimensions when Performing Arc Flash Analysis

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Transactions on Industry Applications
Determine the Electrode Configuration and Sensitivity of the Enclosure Dimensions
when Performing Arc Flash Analysis 1
Kaynat Zia
Student member, IEEE
University of Texas at Arlington
701 S Nedderman Dr
Arlington, TX 76019, USA
[email protected]
Anusha Papasani
Student member, IEEE
University of Texas at Arlington
701 S Nedderman Dr
Arlington, TX 76019, USA
[email protected]
David Rosewater
Senior member, IEEE
Sandia National Laboratories
1515 Eubank SE
Albuquerque, NM 87123, USA
[email protected]
Wei-Jen Lee
Fellow, IEEE
University of Texas at Arlington
701 S Nedderman Dr
Arlington, TX 76019, USA
[email protected]
Abstract -- Arc flash hazard prediction methods have become
more sophisticated because the knowledge about arc flash
phenomenon has advanced since the publication of IEEE Std.
1584-2002[17]. The IEEE Std. 1584-2018 [1] has added
parameters for more accurate arc flash incident energy, arcing
current and protection boundary estimation. The parameters in
the updated estimation models include electrode configuration,
open circuit voltage, bolted fault current, arc duration, gap width,
working distance, and enclosure dimension. The sensitivity and
effect changes of other parameters have been discussed the
previous literatures [2],[8],[9],[11],[12],[15], this paper explains
the fundamental theory on the selection of electrode
configurations and performs sensitivity analysis of the enclosure
dimension, that have been introduced in the IEEE Std. 1584-2018.
According to the newly published model for incident energy (IE)
estimation, the IE between VCB (Vertical Electrodes inside a
metal Box) and HCB (Horizontal Electrodes inside a metal Box)
can differ by a factor of two with other parameters constant.
Using HCB as the worst-case scenario to determine the personal
protection requirements [7],[10] may not be the best practice in
all circumstances. This paper provides guidance for electrode
configuration selection and a sensitivity analysis for determining
a reasonable engineering margin when actual dimension is not
available.
Index Terms – arc flash, electrode configuration, enclosure
dimensions, incident energy, plasma trajectory.
I.
INTRODUCTION
An electric arc is formed when two physically separated and
energized conducting bodies transfer charge through air [21].
The loss of insulation between conductors, due to ageing;
environmental factors; human errors; and overheating, is one
of the main causes of the electric arc formation [3]. The current
flowing ionizes the air between the conductors, converting it
into plasma and causing a rapid increase in temperature. The
plasma is responsible for giving the arc its characteristic
“flash” and contains the biggest part of the arc energy [20].
The high temperature, often compared to the temperature of
the surface of the sun [16], brings about melting and
evaporation of the conductors and other materials in the
vicinity. This further increase pressure and temperature of the
area near the arc. If the arc is not extinguished, the mounting
pressure and temperature leads to an explosion. Severed
equipment and molten debris move outwards during this
explosion, burning and striking everything around it [14].
An arc starts with a series of transitions signified by their
appearance in high speed film. The glow to arc transition starts
with the “dark discharge” [5] or Townsend discharge [13],
where electric field accelerated free electrons collide with gas
molecules and as a result free more electrons, this causes an
exponential increase in current versus voltage, due to the rapid
ionization of air. The dark discharge stage is followed by
“glow discharge” where voltage drops suddenly as current
increases. The final stage involves the release of large number
of electrons from the cathode [6]. The energy released during
the arc are transferred through radiation, convection, and
conduction. The hazard of an arc flash to humans is
proportional to the temperature rise of skin due to the
absorption of the released energy. If the energy absorbed by
the human skin exceeds 1.2 cal/cm2 [4],[18], it can cause
second degree burns according to the experiments conducted
by Dr. Alice Stoll.
1
This work was partially funded by the U.S. Department of Energy, Office of Electricity, Energy Storage Program. This paper describes objective technical
results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of
Energy or the United States Government. (SAND2020-7245 J)
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Transactions on Industry Applications
Incident energy (IE) is used to quantify the energy incident
on the surfaces, equipment or human, near the arc flash. It is
the area under the curve of the rate of heat transfer to a certain
working distance, over time. By limiting the incident energy,
personnel injury and equipment damage can be prevented.
Electrode configuration can be a compounding factor for IE
because the shape of the plasma explosion is not necessarily
spherical. Right after arc initiation, a rapid increase in
temperature causes the expansion of the hot air/plasma and can
push the trajectory of the plasma to a direction governed by the
orientation of the electrodes. According to IEEE Std. 15842018, an electric arc originating in horizontal conductors in a
metal box (HCB) can have surfaces experiencing twice the IE
of an electric arc originating in vertical conductors in a metal
box (VCB), provided all other parameters remain the same.
As engineers may or may not know the specific dimensions or
electrode configuration of a panel being assessed for arc flash
hazard, they often assume an HCB thinking it will provide a
conservative estimate. However, using HCB as the worst-case
scenario in IE calculations may not be the best practice. The
contribution of this paper is to provide physics-based guidance
to engineers performing arc flash analysis when there is
uncertainty in the actual electrode configuration and detentions
of the enclosure.
II.
Fig. 1 the relationship among current (I), Flux (B), and Force (F)
DETERMINE ELECTRODE CONFIGURATIONS
In order to avoid incorrect risk category estimation due to
the electrode configurations, the right-hand rule can be
employed. Fig. 1 shows the relationship among current (I),
magnetic field (B), and force (F). The right-hand rule state that
if the thumb of the right-hand points in the direction of current,
the index finger points in the direction of the magnetic field
and the middle finger will be the direction of force or the
direction of motion. In this case of an arc flash, the direction
of force will be the direction of the plasma flow or the plasma
trajectory. For AC, the direction of current changes, causing
the magnetic field to change but the direction of force remains
the same. Because of this phenomenon, a worker standing at a
point in the direction of F in Fig. 2, will be exposed to a much
greater arc flash hazard than the one standing at any other point
near the arc flash.
Fig. 3 shows an example of HCB. The trajectory of the
plasma travels outward to the opening of the enclosure. The
red oval circle indicates the arc initiation location. Fig. 4 shows
an example of VCB configuration. The blue oval circle
indicates the arc initiation location. Fig. 5 shows an example
of VCBB (Vertical electrodes terminated in an insulating
barrier inside a metal box) configuration. The arc was initiated
in the lugs of the equipment. These examples of HCB, VCB
and VCBB use 2 pole and single-phase AC for simplicity. The
user can expand this concept to three-phase AC as well.
Fig. 2 Plasma Trajectory. Red Arrow: Current Flow; Green Arrow: Trajectory
of the plasma; Arrowhead : Magnetic field goes out of paper; Arrow tail :
Magnetic field goes into paper
Fig. 3 An example of HCB configuration
Fig. 4 An example of VCB configuration
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Fig. 5 An example of VCBB configuration
III.
IMPACT OF THE ENCLOSURE DIMENSION ON THE IE
ESTIMATION
This section discusses the importance of using the exact
enclosure dimensions and electrode configuration for incident
energy calculation. This is done with the help of equations
provided as the IEEE Std. 1584-2018’s contribution to incident
energy estimation. The incident energy is calculated for the
following range of values (these values are chosen based on
IEEE Std. 1584-2018 model range):
• Gap width: 1 in
• Working distance: 18 in
• Arc duration: 250ms.
• Voltages: 0.48kV, 2.7kV and 14.3kV
• Bolted fault current: 1kA, 50kA and 100kA for
0.48kV; and 1kA, 30kA and 60kA for both 2.7kV and
14.3kV.
• Electrode configurations: VCB, VCBB (Vertical
electrodes terminated in an insulating barrier inside a
metal box) and HCB
• Enclosure Dimensions: 20in x 20in x 20in (smaller
box sizes don’t affect the incident energy values) to
49in x 49in x 49in
The figures shown below indicate the trends of the incident
energy with respect to the enclosure dimensions for different
electrode configurations, bolted fault current, and voltage
values. Some figures also compare the incident energy profiles
for the three electrode configurations for all provided current
categories.
Fig. 6 Enclosure Dimension vs. Incident Energy for 1kA, 50kA and 100kA
VCB (0.48kV)
Fig. 7 Enclosure Dimension vs. Incident Energy for 1kA, 50kA and 100kA
VCBB (0.48kV)
Fig. 8 Enclosure Dimension vs. Incident Energy for 1kA, 50kA and 100kA
HCB (0.48kV)
Fig. 9 Enclosure Dimension vs Incident Energy for 1kA for VCB, VCBB
and HCB (0.48kV)
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Fig. 10 Enclosure Dimension vs Incident Energy for 50kA for VCB, VCBB
and HCB (0.48kV)
Fig. 13 Enclosure Dimension vs Incident Energy for 1kA, 30kA, 60kA
VCBB (2.7kV)
Fig. 11 Enclosure Dimension vs Incident Energy for 100kA for VCB, VCBB
and HCB (0.48kV)
Fig. 14 Enclosure Dimension vs Incident Energy for 1kA, 30kA, 60kA HCB
(2.7kV)
Fig. 12 Enclosure Dimension vs Incident Energy for 1kA, 30kA, 60kA VCB
(2.7kV)
Fig. 15 Enclosure Dimension vs Incident Energy for 1kA for VCB, VCBB
and HCB (2.7kV)
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Fig. 16 Enclosure Dimension vs Incident Energy for 30kA for VCB, VCBB
and HCB (2.7kV)
Fig. 19 Enclosure Dimension vs Incident Energy for 1kA, 30kA, 60kA
VCBB (14.3kV)
Fig. 17 Enclosure Dimension vs Incident Energy for 60kA for VCB, VCBB
and HCB (2.7kV)
Fig. 20 Enclosure Dimension vs Incident Energy for 1kA, 30kA, 60kA HCB
(14.3kV)
Fig. 18 Enclosure Dimension vs Incident Energy for 1kA, 30kA, 60kA VCB
(14.3kV)
Fig. 21 Enclosure Dimension vs Incident Energy for 1kA for VCB, VCBB
and HCB (14.3kV)
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2.7kV
14.3kV
Voltage
0.48kV
2.7kV
14.3kV
Fig. 22 Enclosure Dimension vs Incident Energy for 30kA for VCB, VCBB
and HCB (14.3kV)
Fig. 23 Enclosure Dimension vs Incident Energy for 60kA for VCB, VCBB
and HCB (14.3kV)
TABLE 1 MAGNITUDE CHANGE IN INCIDENT ENERGY WITH CHANGING
ENCLOSURE DIMENSIONS
VCB
Voltage
0.48kV
2.7kV
14.3kV
Voltage
0.48kV
20”→21”
34”→35”
48”→49”
-0.0073
-0.4274
-0.5963
-0.0182
-0.49
-0.9338
-0.0208
-0.54
-1.02
-0.0022
-0.1285
-0.1793
-0.0058
-0.16
-0.2971
-0.0081
-0.21
-0.40
-0.0012
-0.0720
-0.1004
-0.0030
-0.08
-0.1561
-0.0027
-0.07
-0.13
20”→21”
-0.0075
-0.5677
-0.7683
VCBB
34”→35” 48”→49”
-0.0018
-0.1342
-0.1816
-0.0012
-0.0882
-0.1193
Bolted
Fault
Current
1kA
50kA
100kA
1kA
30kA
60kA
1kA
30kA
60kA
Bolted Fault
Current
1kA
50kA
100kA
-0.0167
-0.65
-1.3471
-0.0226
-0.79
-1.5778
20”→21”
-0.0086
-0.4497
-0.5910
-0.0145
-0.56
-1.0885
-0.0179
-0.72
-1.4110
-0.0045
-0.18
-0.3663
-0.0089
-0.31
-0.6202
-0.0027
-0.11
-0.2168
-0.0020
-0.07
-0.1376
HCB
34”→35” 48”→49”
-0.0023
-0.1222
-0.1606
-0.0045
-0.17
-0.3393
-0.0078
-0.31
-0.6138
-0.0015
-0.0759
-0.0997
-0.0024
-0.09
-0.1819
-0.0003
-0.01
-0.0206
1kA
30kA
60kA
1kA
30kA
60kA
Bolted Fault
Current
1kA
50kA
100kA
1kA
30kA
60kA
1kA
30kA
60kA
To examine the sensitivity of incident energy to the
enclosure dimension, the slope or the magnitude change in
incident energy with respect to every 1 inch change in the both
width and height of the enclosure dimensions at 20in, 34in, and
48in, that is, the change in incident energy when the enclosure
dimensions change from 20in x 20in to 21in x 21in, 34in x 34in
to 35in x 35in and 48in x 48in to 49in x 49in, are calculated
for VCB, HCB and VCBB and the results are shown in Table
1.
As one can see from the plotted graphs and the Table 1, the
incident energy measured decreases with the increasing of the
box sizes. This is because plasma expands and the bigger the
box, the more room it has to expand in every direction. Since,
the plasma source is not changing, the plasma intensity
decreases the farther you go away from the source. Thus, the
incident energy recorded at a fixed working distance away
from the arc will show a decrease in magnitude with bigger
box sizes. However, the plasma may not be able to fill the
entire enclosure when the size of the enclosure increases
beyond a certain limit as plasma intensity is a function of
distance from the arc. Hence the sensitivity to the change in
the box size has an inflection point, that is, the incident energy
becomes somewhat independent of the box size. In case of the
enclosure dimension uncertainty a larger engineering margin
should be applied when this sensitivity is higher. For example,
for a VCB configuration at 480V and 100kA (as seen in Fig.
6) bolted fault current, IE is more sensitive to the enclosure
size when it is smaller, at 20in x 20in (magnitude change in
IE is -0.0073), than when it is larger, at 49in x 49in (magnitude
change in IE is -0.0012). This trend has been observed across
all the electrode configurations, voltages, and currents.
Table 2 shows enclosure types for IEEE Std. 1584-2018 arc
flash model. It shows the equipment class and the suitable
enclosure size used. The last three columns of the table show
the magnitude change in incident energy at the given enclosure
size values. The incident energy is calculated for a working
distance of 36in and an arc duration of 1000ms across the three
equipment classes. The incident energy calculated for 15kV
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MCC using the correct enclosure size i.e., 36in x 36in x 36in
and VCB as the electrode configuration is 33.22 cal/cm2,
where the magnitude change is of the order of -0.34. Whereas,
the incident energy estimated at 34in x 34in x 34in is 33.91
cal/cm2 where magnitude change is of the order of -0.38 and
that at 38in x 38in x 38in is 32.61 cal/cm2 with a magnitude
change of -0.29. It can also be seen that mistaking one
electrode configuration for another also has a considerable
impact on the incident energy values calculated. For the 5kV
switchgear, provided the gap length (104mm), enclosure size
(36in x 36in x 36in), bolted fault current (20kA), working
distance (36in) and arc duration (1000ms) is kept constant,
changing the electrode configuration from VCB to HCB
changes the incident energy estimated from 20.21 cal/cm2 to
45.65 cal/cm2 with a magnitude change of -0.17 to -0.19. That
is the change from VCB to HCB changes the incident energy
by a factor of 2 as is evident from Fig. 9-11, Fig. 15-17 and
Fig. 21-23. Incorrect incident energy calculation can have
serious repercussions as it may lead to wrong PPE category
selection.
Human body takes about 0.19s to respond to any visual
stimulus, 0.16s for an audio stimulus and 0.5s to move away
from something that can potentially harm it [19]. The arc
duration used here is representing the worst-case scenario.
judgement and use reasonable estimated parameters when
actual dimension is not readily available.
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TABLE 2. ENCLOSURE SIZES FOR IEEE STD. 1584-2018 ARC-FLASH MODEL
ΔIE per 1in change in size
Equipment
class
15kV
MCC
5kV
Switchgear
5kV
MCC
Gap
(mm)
Enclosure
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(in x in x
in)
IBF
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CONCLUSION
With better understanding on the arc flash phenomena, the
IEEE Std. 1584-2018 [1] has added more parameters to
improve the accuracy of arc flash incident energy. Electrode
configuration and the enclosure dimensions are two
parameters that have drawn substantial discussion among
industry. According to the newly published model for incident
energy (IE) estimation, the difference of the IE between VCB
and HCB can be more than two times with other parameters
remain the same. Considering HCB as the worst-case scenario
and using its results to determine the personal protection
requirements may not be the best practice in every
circumstance. As seen from the results in the last section, the
incident energy values estimated using the IEEE Std. 15842018 have shown a varying degree of sensitivity to the
enclosure dimensions depending on the size and electrode
configuration. The results of the sensitivity analysis from this
paper provide an engineer guidance to make practical
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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIA.2020.3020531, IEEE
Transactions on Industry Applications
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