Incident Energy: A Case Study
Jermichael Beaver
Bruce Luong
David Temple
John Ventura
Date: May 1, 2005
Abstract
Every day, electricians interact with electrical service panels to perform
maintenance and modifications to ensure that an electrical system works properly.
Whenever electricians or other electrical workers interact with live electrical equipment,
they are at risk of being exposed to incident energy or what is more commonly known as
an arc flash. An arc flash results from a short circuit current that flows through the air
instead of conductors, bus bars, and other equipment designed to carry current. The high
amounts of energy present at the location of an arc flash can lead to severe injury or a
slow, painful death. Safety standards and guidelines to protect electricians against arc
flash are referenced to the NFPA 70-E, ASTM F-1506, IEEE 1584, and OSHA
29CFR1910.269.
A three-phase short-circuit current is calculated to determine the worst-case fault
that occurs at various service panels located within a facility. A three-phase short circuit
is the result of all the phases being merged into one point. An example of this would be
if a metal conductor were placed across the horizontal alignment of the three phases in
the main service panel. The only impedances that limit this current are the transformer
and line impedance. The goal of this study is to determine the risk level associated with
electricians performing predictive maintenance on service panels in an industrial
environment and the corresponding set of protective gear necessary to protect employees.
Risk levels will be assessed by the amount of incident energy in J/cm2 calculated at each
panel. The tables in the NFPA 70-E regarding Proper Personal Equipment (PPE) will be
used as a guide in deciding the minimum amount of protection needed at each panel. A
corrective design will be contingent upon the findings of the study. If energy levels are
calculated that fall outside the range covered by acceptable PPE, a design will be
proposed to reduce the incident energy to levels where its effects can be mitigated by
PPE.
1
Introduction
Buckman Laboratories was started in 1945 and has specialized in providing
chemical treatment services for a variety of products and raw materials such as paper,
leather, coatings, plastics, and wood. Buckman Laboratories specializes in controlling
the growth rate of micro-organisms in industrial settings. Today, Buckman Laboratories
is a global company producing over five hundred products in more than seventy
countries.
Similar to other large manufacturing companies, Buckman Laboratories is
committed to achieving a standard of excellence in safety standards. The management at
Buckman Labs is looking for ways to determine the incident energy levels at various
panels in case of a fault at any of the specified locations. By obtaining this information,
Buckman Labs will be able to make informed decisions about the protective wear it
supplies for its workers. In the event of an arc flash, electrical workers can be exposed to
be power levels on the order of several megawatts. Arc temperatures can exceed 5,000
o
F, and can result in electrical workers suffering with first and second-degree burns,
hearing loss, and even brain damage. To better protect its staff, the company needs to
know the energy levels at various locations inside the facility. At the request of Mr.
David Temple, an engineering manager at Buckman, the hazardous energy levels present
at the junction boxes in the facility will be calculated. In the event of a three phase short,
the worst-case scenario in an industrial system, an arc flash may result. The three-phase
short responsible for this is caused by the three phases being merged together.
According to Oberon[4], a leading manufacturer of safety equipment, an electrical
worker dies every day as a result of an arc explosion. References to the guidelines from
the National Fire Protection Organization (NFPA) [1], and Institute of Electrical and
Electronic Engineering (IEEE) [2] & [3] are used to calculate the fault current at each
service panel location.
The management at Buckman Laboratories in particular David Temple has
decided that a short circuit study will be conducted to determine the energy levels at
various service panel locations within the facility. Once the incident energy is calculated
at different junction boxes within the facility, the arc flash suits will be chosen according
to the particular incident energy level present at the service panels. By using the proper
protective apparel, there will be a reduction in the likelihood of death or serious injuries.
The most significant constraints that pertain to the project are social, political, and
health and safety. The first realistic constraint that will be addressed is health and safety.
This will play the most important role in the design, as it is the basis for the project. PPE
will be based on energy levels three times the magnitude of the calculated values. This
means that the protective gear mandated must be rated for energy levels above those
calculated. The energy calculations will then be analyzed to verify that they are
appropriate for the environment at Buckman Labs. For this particular design, the social
aspect may include additional training for electrical workers at Buckman facilities to
show them how to accommodate and to better protect themselves from dangerous fault
currents. As far as implementing the design, social constraints will include developing a
good working relationship with project sponsors and scheduling meetings with our
mentor regularly. Closely related to social constraints are political constraints. Political
constraints will come into play when the project is implemented. Bureaucracies and
members of government agencies will make decisions on which safety regulations should
be used. Ethical constraints that pertain to the project will be taken into account. These
ethical constraints will become a factor when management decides to use the results of
2
the incident energy calculations. They will determine if any new safety precautions will
be taken in order to protect its employees.
Discussion
The magnitude of fault currents is largely dependent on the size of the transformer
that provides power for the load where a short circuit occurs. Power is supplied to the
Buckman Labs plant by a 2000 kVA , 23kV/480V, MLGW, three-phase transformer with
a 5.8 percent impedance. At full load the transformer can supply about 2.4 kA of current
from its secondary. The facility also consists of several other transformers with face-plate
ratings ranging from 20 - 80 kVA. All of the smaller transformers have impedances of 5
percent. The first step in determining incident energy is performing a short circuit study.
The short circuit study will establish the maximum amount of fault current available at
each panel. This study consisted of performing circuit analysis on a three-phase system
and determining the fault currents. Before calculations could be made, detailed
information regarding the plant’s power system had to be collected. Detailed information
regarding wire impedances, transformer faceplate ratings, and rotating machinery was
used to determine the magnitude of short circuit currents at various fault locations. Short
circuit analysis of large dynamic power systems can often be very complicated and
usually require computer assistance. All of the panels and loads were either at 480V or
208V line to line resulting in simplistic calculations that could be performed manually.
The following example illustrates how a fault current would be calculated at some
distance downstream from the main transformer.
A paint wire way is tied directly to a 2 MVA, 23kV/480V three-phase
transformer. The transformer has a percent impedance of 5.8%. The paint wire way is
connected to the transformer by 3 300kcmil conductors per phase. The percent
impedance of the wire per 1000 ft is .0599. The wire way is located at a distance of 500
feet away from the transformer. What will be the value of the maximum amount of fault
current at the wire way. The first step is to determine base values so that the calculations
can be performed in per unit. For this analysis, Vbase = 480 V and S base = 2000 kVA
The secondary base current is calculated below.
S base
2 MVA
I baseSec =
=
= 2.41 kA
Vbase * 3 480 V ⋅ 3
The base impedance of the transformer secondary is found as follows.
480V 2
V b a se2
=
= .1 1 5 Ω
Z b a seS ec =
2 MVA
S base
The next step is determining the equivalent wire impedance and using the transformer
base impedance to find its per unit value. The number of conductors per phase is referred
to as n.
Z wire = Z
Ω
Ω
⋅ l = .0599
⋅ 500 ft = .00998 Ω
1000 ft ⋅ n
1000 ft ⋅ 3
Z wire
. 00998 Ω
+ Z xfmr p .u . =
+ .058 = .145 p.u.
Zbase
.115 Ω
The per unit voltage is easily calculated.
Z p .u total =
V p .u =
Vreal
480 V
=
= 1 p .u .
V base 480 V
3
Using ohm’s law to calculate the short circuit current in per unit,
V p .u .
1
= 6 . 9 0 p .u .
Z p .u
.1 4 5
The per unit current is easily converted to its actual value by just multiplying by the base.
I p .u . =
=
I real = I base ⋅ I p.u. = 6.90 p.u. ⋅ 2.41 kA = 16.63 kA
So at the paint wire way there is a maximum available fault current of 16.63 kA. This
corresponds to the short circuit current available at panel 3-2 on Table 1-2.
This example accentuates the point that fault currents may be many times the normal load
current. This example also helps illustrate how the fault current decreases as the distance
from the main transformer diminishes. This is caused by the increased line impedance
that is a function of distance. While distance away from the transformer can lower fault
currents, rotating machinery can actually increase its value. In the presence of a fault
current, motors will often perform as generators. During this time, the motor may
produce five times its full load current. At the Buckman Labs facility 25 induction
motors are housed within two motor control centers. Each induction motor is rated at
about 5 h.p. There are never more than 5 motors on at any given time. This means that
between the two motor control centers the maximum amount of power consumed by the
motors is 25 h.p or 18.65 kW. The total current that these motors can generate on a 480
V bus is given by the following I =
18.65 kW
480V ⋅ 3
= 22.4 A
Even at 20 times this value, the current contribution would be only 449 A. This is a trivial
amount with respect to fault currents that are typically on the order of 15-35 kA outside
the main box. As a result their contribution in the short circuit calculations is neglected.
This coincides with the methods used in the IEEE red book, as they neglect the
contribution of all induction motors under 50 hp.
After the short circuit study was completed, information regarding the system’s
circuit protection was collected and analyzed. Buckman Labs uses pxf electronic trip
MCC breakers to provide its circuit protection at the main panel. Molded Case Circuit
breakers or MCC’s provide high levels of protection because their trip units are a
combination of thermal and current sensitive devices. MCC’s are also quite popular
because they allow an elevated degree of modification as the thermal magnetic trip units
can be interchanged and replaced. The electronic trip unit used on a MCC breaker
consists mainly of a printed circuit board, a current transformer, and a shunt trip. The
current transformer monitors current levels and steps the current down to an acceptable
level for the circuit board. The circuit board then uses the current ratio input from the
current transformer to decide when the breaker needs to be tripped.
One important characteristic of the MCC’s used at Buckman Labs with respect
incident energy is the clearing time. The clearing time of a breaker is basically the time it
takes for the breaker to open in the presence of a short circuit current. This information
was obtained after reading the trip curve for the pxf 2500 in figure x-x. From the graph
we are able to deduce that at 41 kA, the magnitude of a short inside the main panel, the
breaker clearing time is between .01 and .04 seconds. This is important because the
4
amount of incident energy released is directly proportional to the duration of the arcing
current.
The incident energy at particular location varies due to other parameters such as
the clearing time and available fault current. Worker distance from fault, bus voltage, arc
gap, and the type of enclosure are all contributing factors that determine the how much
incident energy is being released. A working distance of 18 inches was used in
determining incident energy because both the NFPA 70-E and the IEEE 1584 state that
this is the minimum distance that electrical workers should stand away from electrical
equipment when servicing it. The arc gap, or the distance between connectors on the
circuit breaker, is around 25 mm for the px breakers used at Buckman Labs. All breakers
at the plant are on either a 480 V bus or a 208 V bus line. The following example will
help explain how incident energy can be calculated manually at a particular panel inside
the plant.
The available fault current at the secondary of a 2MVA, 23kV/480V three phase
transformer is 41.45 kA. An electrician performs maintenance on a panel at a working
distance of 20 inches. The panel resides on a 480 V bus. Assume the circuit breaker has a
clearing time of .04 seconds and an arc gap of 25 mm. Determine the maximum amount
of incident energy in cal/cm2 that the worker will be exposed to.
The first step is to determine the arcing current at the panel. The following equation is
from the IEEE 1584 pg 10 Eqn(1).
lg( I a ) = K + .662 lg( I bf ) + .0966V + .000526G + .5588V (lg I bf ) − .00304 G (lg I bf )
Where;
lg is the log10
Ia is the arcing current (kA)
K is –0.097 for box configurations
Ibf is the bolted fault current for three-phase faults (kA)
V is the bus voltage (kV)
G is the gap between conductors (mm)
lg Ia = − .097 + .662 lg( 41 .45 ) + (.0966 ⋅ 480 ) + ( .000526 ⋅ 25 ) + (.5588 ⋅ 480 )(lg 41 . 45 ) − . 00304 (lg 41 .45 )
I=
Ia = 22.08 kA
Now that the arcing current is known the Eqn(4) from the IEEE 1584 can be used to
calculate the normalized incident energy.
(4) lg En = K1 + K2 + 1.081 lg Ia + .0011G
where,
En is the incident energy (J/cm2) normalized for time and distance
K1 is –0.555 for box configurations
K2 is –0.113 for grounded systems
G is the gap between conductors (mm)
lg En = -.555 + -.113 + 1.081 * lg(22.08) + (.0011)(25)
En = 6.49 J.
The normalized energy must now be converted to J/cm2.
t 610 x
Eqn(6) E = 4.184C f En ⋅
.2 D x
5
where,
E is the incident energy (J/cm2)
Cf is the calculation factor (1.5 voltages at or below 1kV)
En is the normalized energy
t is the arcing time in seconds
D is the distance from the possible arc point to the person (mm)
x is the distance exponent (1.641 for voltages between .208 and 1 kV)
1.641

 .04  610
 = 12.96 J 2
E = (4.184)(1.5)(6.49)

1.641 
cm
 .2  460

E = 12.96 J/cm2
Converting to cal/cm2 we have
E = 3.095 cal/cm2
A worker at this panel would be exposed to 3.095 cal/cm2 of incident energy.
This is a relatively minute amount of energy exposure with respect to NFPA 70-E
standards. For all energy levels less than 5 cal/cm2, the recommended protection is rated
at level 0. Level 0 protection corresponds to one layer of treated cotton. At level 1 the
required protection consists of a fire retardant shirt and fire retardant pants.
The previous example illustrates the energy level at the main panel. Because
information concerning the model names of the circuit breakers at the facility was
unavailable, data from typical breakers at the rated current protection of the breakers in
the plant were analyzed to determine the clearing times. Breaker characteristics of MCC
breakers were used, as these breakers are the ones that are most likely to appear inside the
panels. The pxf 36 2500-breaker operating at the main panel was also verified to be a
MCC. The smaller rated breakers had clearing times much faster than the breaker at the
main panel. Subsequently a fault current out at one of the loads would be detected first by
the breaker directly upstream and not by the main breaker. This type of coordination
prevents the entire system from being rendered inoperable in the event of a downstream
short circuit.
The trip curve on the following page was used to approximate clearing times of
breakers with ratings on the range of 400-700 amps. The trip curve refers to a ME model
breaker on a 225, 400, or 800 - amp frame. Each frame rating comes with a
corresponding sensor setting. The sensor setting establishes the maximum amount of load
current that the breaker is designed to trip for. In addition to sensors, the breakers also
come equipped with rating plugs, which make it possible to adjust the maximum amount
of load current that will trip the breaker. According to SquareD personnel, rating plugs
generally provide the ability to step down the sensor rating of the breaker to a value of
about 40 percent of its original value. The current levels on the horizontal are in multiples
of the rating plug level. The multiple value is used to find the corresponding clearing
time on the vertical axis. An 18 kA fault current corresponds to 45 multiples of a
breaker with a rating plug set to 400 amperes.
6
7
Incident Energies at Panels
3.5
3
2
1.5
1
0.5
0
M
ai
n
Pa Pan
n e
Pa el 3 l
ne -1
l3
LP - 2
3
LP - 1
Pa 3-2
n
Pa el 4
ne
LP l 5
5
LP - 1
Pa 5-2
n
Pa el 6
n
Pa el 7
n
Pa el 8
Pa nel
ne 9
Pa l 1
ne 0
Pa l 1
n 1
Pa el 1
ne 2
l1
3
cal/cm
2
2.5
Panels
The graph above helps demonstrate the range of energy levels taken from Table
1-3. As seen from the graph, the values of incident energy never exceed level one
protection: 5 cal/cm2. At this level, the protection consists of fire retardant pants and fire
retardant shirt. Table 1.0 [1] provides detailed information regarding the recommended
protection for various incident energies.
Without information regarding the actual breaker model or name, it is impossible
to know how long it will take for a breaker to clear a fault; therefore, there is a great deal
of uncertainty involved in calculating the total amount of incident energy available at the
location of the fault. This information could not be obtained directly because our sponsor
did not think that visits to the plant were necessary. To take into account the large
uncertainty introduced by these factors Table 1-3 provides energy estimates and
protection for incident energies three times the calculated values. When the incident
energy is tripled level 3 protection is needed at the main panel. This protection consists
of cotton underwear along with fire retardant shirts, fire retardant pants, and fire retardant
coveralls. Also, the protection at the other panels will advance to the next level.
8
Recommendations and Conclusions
According to Table 1.0 [1], the maximum protection at the main panel should
consists of fire retardant pants, shirt, and, coverall as well as cotton underwear. Because
of the low incident energies available at the plant, arc flash suits are not necessary. The
bulky nature of the suit itself may also discourage electrical workers from wearing them.
Since the necessary information on the type of circuit breakers could not be obtained
from Buckman Laboratories except that of the main panel, the accurate level of incident
energy at panels downstream from the main panel was an estimate from a “typical” trip
curve. From Square D, trip curves were chosen based on the short circuit current
available at a particular service panels, and the trip time was obtained from those curves.
The adequate protection throughout the facility would consist of level one though level
three PPE. The inability to obtain adequate information from Mr. Temple was a
considerable social constraint. It appears that breaker characteristics are the primary
factor in determining arc flash energy. In closing, Buckman Labs appears to have little to
no hazard level associated with arc flash energy.
References
[1] NFPA 70E Standard for Electrical Safety Requirements for Employee Workplaces.
National Fire Protection Association, Quincy, MA, 2000. Page 61.
[2] IEEE Recommended Practice for Electric Power Distribution for Industrial Plants.
IEEE TK4035.F3 Std 1410. New York: The Institute of Electrical and Electronics
Engineers, Inc., 1994.
[3] IEEE 1584: Guide For Performing Arch-Flash Hazard Calculations. ENGIN
TK277.15 Std 1584. New York: The Institute of Electrical and Electronics
Engineers, Inc., 2002.
[4] Oberon, “Arc Trainer CD.” Computer software. Oberon Company div Paramount
Corporation. PC, 2002.
[5] Mr. David Temple, Engineering Manager, Buckman Laboratories, weekly progress
report on November 18, 2004.
[6] Dr. Fred Terry, Electrical Engineering Professor, Christian Brothers University.
9
Table 1.0 [1]
Hazard Risk
Clothing
Description(number
of clothing layers
given in
parenthesis)
Total Weight
0
untreated cotton (1)
4.5--7
FR shirt and FR
pants (1)
4.5--8
1
9--12
2
Cotton underwear
plus FR shirt and FR
pants(2)
16--20
3
Cotton underwear
plus FR shirt and FR
pants plus FR
coverall(3)
24--30
4
Cotton underwear
plus FR shirt and FR
pants plus double
layer switching coat
and pants
Category
2
oz/yd
Breakopen
Threshhold
Energy(EBT)*
Rating of PPE
cal/cm
N/A
5
10
8
25
40
Table 1.1 Transformer Parameters
Vbase(kV)
Vreal(kV)
Isc(kA)
Zbase Ω
Zreal Ω
Sbase(KVA)
Sreal(kVA)
Spu
Xfmr1
23
23
0.8656
264.5000
15.3410
2000
2000
1
17.24138
1
Main Panel
0.48
0.48
41.4763
0.1152
0.0067
2000
2000
1
17.24138
1
Xfmr2
0.208
0.208
1.6654
1.4421
0.0721
30
30
1
20
1
Xfmr3
0.208
0.208
2.5537
0.9405
0.0470
46
46
1
20
1
Xfmr4
0.208
0.208
1.4434
1.6640
0.0832
26
26
1
20
1
Xfmr5
0.208
0.208
2.5537
0.9405
0.0470
46
46
1
20
1
11
Ipu
Vpu
Table 1.2 System Values
Vbus (kV)
V pu
Ipu
Zwire Ω
ZEQΩ
Zpu
l(ft)
n
Ibase (kA)
Zbase Ω
Z Ω/1000ft
Zxfmr
Isc (kA)
Panel 3-1
0.48
1
7.5952
0.0085
0.0152
0.1317
425
3
2.4056
0.1152
0.0599
0.0067
18.3
Panel 3-2
0.48
1
6.9127
0.0100
0.0167
0.1447
500
3
2.4056
0.1152
0.0599
0.0067
16.6
LP 3-1
0.208
1
19.6351
0.0013
0.0734
0.0509
75
3
0.0833
1.4421
0.0536
0.0721
1.64
LP 3-2
0.208
1
19.4459
0.0013
0.0484
0.0514
75
3
0.1277
0.9405
0.0536
0.0470
2.48
Panel 4
0.48
1
9.6308
0.0053
0.0120
0.1038
360
3
2.4056
0.1152
0.0440
0.0067
23.2
Panel 5
0.48
1
6.3904
0.0113
0.0180
0.1565
635
3
2.4056
0.1152
0.0536
0.0067
15.4
LP 5-1
0.208
1
19.6830
0.0013
0.0845
0.0508
75
3
0.0722
1.6640
0.0536
0.0832
1.42
LP 5-2
0.208
1
19.4459
0.0013
0.0484
0.0514
75
3
0.1277
0.9405
0.0536
0.0470
2.48
Panel 6
0.48
1
6.4854
0.0111
0.0178
0.1542
370
2
2.4056
0.1152
0.0599
0.0067
15.6
Panel 7
0.48
1
5.6471
0.0137
0.0204
0.1771
315
2
2.4056
0.1152
0.0871
0.0067
13.6
Panel 8
0.48
1
12.3056
0.0027
0.0094
0.0813
100
2
2.4056
0.1152
0.0536
0.0067
29.6
Panel 9
0.48
1
11.3323
0.0035
0.0102
0.0882
130
2
2.4056
0.1152
0.0536
0.0067
27.3
Panel 10
0.48
1
10.7647
0.0040
0.0107
0.0929
150
2
2.4056
0.1152
0.0536
0.0067
25.9
Panel 11
0.48
1
14.0873
0.0015
0.0082
0.0710
40
2
2.4056
0.1152
0.0748
0.0067
33.9
Panel 12
0.48
1
10.5447
0.0042
0.0109
0.0948
95
3
2.4056
0.1152
0.1340
0.0067
25.4
Panel 13
0.48
1
8.8695
0.0063
0.0130
0.1127
430
3
2.4056
0.1152
0.0440
0.0067
21.3
12
Table 1.3 Incident Energies at Various Panels
Isc(kA)
Panels
Main
Panel
Panel 3-1
Panel 3-2
LP 3-1
LP 3-2
Panel 4
Panel 5
LP 5-1
LP 5-2
Panel 6
Panel 7
Panel 8
Panel 9
Panel 10
Panel 11
Panel 12
Panel 13
41.5
Vbus LL(kV)
0.48
18.3
16.6
1.64
2.48
23.2
15.4
1.42
2.48
15.6
13.6
29.6
27.3
25.9
33.9
25.4
21.3
0.48
0.48
0.208
0.208
0.48
0.48
0.208
0.208
0.48
0.48
0.48
0.48
0.48
0.48
0.48
0.48
G(mm)
Ia(kA)
En
E(J/cm2)
D(mm)
t(s)
x
Cf
E(cal/cm2)
25
22.10455
6.499571
12.96398
460
0.04
1.641
1.5
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
10.97365
10.12559
1.219524
1.635298
13.44129
9.468383
1.104819
1.635298
9.588386
8.519216
16.57158
15.44533
14.78205
18.60062
14.52353
12.5283
3.048733
2.794857
0.283578
0.389402
3.796161
2.599287
0.254858
0.389402
2.634917
2.318793
4.760278
4.411536
4.207103
5.393358
4.127623
3.518208
6.080971
5.574592
0.565621
0.776697
7.571784
5.18451
0.508337
0.776697
5.255578
4.625041
9.494802
8.799204
8.391444
10.75754
8.232914
7.017382
460
460
460
460
460
460
460
460
460
460
460
460
460
460
460
460
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
1.641
1.641
1.641
1.641
1.641
1.641
1.641
1.641
1.641
1.641
1.641
1.641
1.641
1.641
1.641
1.641
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
13
3.10
3X
E(cal/cm2
9.29
PPE
level
3
1.45
1.33
0.135
0.186
1.81
1.24
0.121
0.186
1.26
1.10
2.27
2.10
2.00
2.57
1.97
1.68
4.36
3.99
0.41
0.56
5.43
3.71
0.36
0.56
3.77
3.31
6.80
6.31
6.01
7.71
5.90
5.03
1
1
1
1
2
1
1
1
1
1
2
2
2
2
2
2
14