Why the focus on Arc
Flash?
In the early 1980's a paper
"The Other Electrical Hazard: Electric Arc Blast Burns" by Ralph
Lee was published in the IEEE Transactions on Industrial Applications. The
effect of this paper was to realize the need to protect people from the hazards
of arc flash. Four separate industry standards concern the prevention of arc
flash incidents:
 OSHA 29 Code of Federal
Regulations (CFR) Part 1910 Subpart S.
 NFPA 702002 National
Electrical Code.
 NFPA 70E2000 Standard
for Electrical Safety Requirements for Employee Workplaces.
 IEEE Standard 15842002
Guide for Performing Arc Flash Hazard Calculations.
Compliance with OSHA involves
adherence to a sixpoint plan:
 A facility must provide,
and be able to demonstrate, a safety program with defined responsibilities.
 Calculations for the
degree of arc flash hazard.
 Correct personal protective
equipment (PPE) for workers.
 Training for workers
on the hazards of arc flash.
 Appropriate tools for
safe working.
 Warning labels on equipment.
Note that the labels are provided by the equipment owners, not the manufacturers.
It is expected that the next revision of the National Electric Code will
require that the labels contain the equipment's flash protection boundary,
its incident energy level, and the required personal protective equipment
(PPE).
Companies will be cited
and fined for not complying with these standards.
Personal Protective Equipment
(PPE)
Categories of PPE as described
in NFPA 70E are:
Category 
Cal/cm^{2} 
Clothing 
0 
1.2 
Untreated Cotton 
1 
5 
Flame retardant
(FR) shirt and FR pants 
2 
8 
Cotton underwear
FR shirt and FR pants 
3 
25 
Cotton underwear
FR shirt, FR pants and FR coveralls 
4 
40 
Cotton underwear
FR shirt, FR pants and double layer switching coat and pants 
Cal/cm2 are the units of
incident energy that the PPE can withstand. Note that a hard hat with fullface
shield and the appropriate gloves are required also.
Steps required for a
flash hazard analysis
To perform an arc flash
hazard analysis, data is collected about the facility's power distribution
system. The data includes the arrangement of components on a oneline drawing
with nameplate specifications of every device. Also required are details of
the lengths and cross section area of all cables. The utility should be contacted
for information including the minimum and maximum fault currents that can
be expected at the entrance to the facility. Once the data has been collected,
a short circuit analysis followed by a coordination study should be performed.
The resultant data can then be fed into the equations described by either
NFPA 70E2000 or IEEE Standard 15842002. These equations will produce the
necessary flash protection boundary distances and incident energy to determine
the minimum PPE requirement.
Flash hazard analysis
 a new approach
Once the data is prepared
and a flash hazard analysis has been performed, most likely it will be discovered
that category 4 PPE will be required in most places. This is most unfortunate
as this type of PPE is very unwieldy and could be costly in terms of time
taken to perform work and the potential for mistakes. Prior to the new arc
flash regulations, coordination studies were targeted at reliability with
all settings adjusted towards the high side. Compliance with the new arc flash
regulations means that not only does the coordination study need to be more
accurate but it also needs to take into account the fact that the arc fault
current is less than the bolted fault current.
The data can be used to
perform a sensitivity study to adjust breaker/fuse characteristics to lower
the PPE requirement. To achieve this goal, the existing breakers may need
to be replaced, generally by more modern counterparts. Old breakers have relatively
slow reaction times and will trip at too high a current. To limit the flash
hazard the breakers are adjusted to trip earlier than before. It is expected
that the outcome of this sensitivity study, when implemented, will result
in most category 4 PPE requirements being decreased to category 1 or 2.
ShortCircuit Study
The shortcircuit study
is based on a review of oneline drawings. The drawings must be created if
they do not exist, and fieldverified if they do. Maximum available fault
current is calculated at each significant point in system. Each interrupting
protective device is then analyzed to determine whether it is appropriately
designed and sized to interrupt the circuit in the event of a bolted type
of short circuit. Next, the associated equipment must be reviewed to insure
that the bus bar is adequately braced to handle the available fault current.
Finally, the bolted fault currents are converted into arc fault currents for
additional analysis.
Coordination Study
A coordination study is
the examination of the electrical system and available documentation with
the goal of ensuring that overcurrent protection devices are properly designed
and coordinated. Overcurrent protective devices are rated, selected and adjusted
so only the fault current carrying device nearest the fault opens to isolate
a faulted circuit from the system. This permits the rest of the system to
remain in operation, providing maximum service continuity. The study consists
of timecurrent coordination curves that illustrate coordination among the
devices shown on the oneline diagram. Note that protective devices are set
or adjusted so that pickup currents and operating times are short but sufficient
to override system transient overloads such as inrush currents experienced
when energizing transformers or starting motors.
The Problems
Now that the hazards associated
with arc flash have been brought to our attention, we face the problem of
trying to eliminate or at least reduce those hazards. The following discusses
some of these problems and the subtleties in implementing corrective actions.
There are several problems
in dealing with Arc Flash Analysis:
 Being overly conservative
in your short circuit analysis may result in the required PPE protection
category being set at a level higher than necessary.


Category
4 PPE 
The above figure is a person
in a full Category 4 suit. This suit will provide the necessary protection,
but it is cumbersome to work in, it is hot, and it provides poor visibility.
The suits will make many
tasks very difficult, if not impossible, to perform. Because of their restrictions
to vision and movement, they may even make some tasks more dangerous. There
are definitely times when this type of protection is both necessary and required,
but being overly conservative will result in excessive stress to workers and
unacceptable time to make repairs or adjustments.
 Relying upon quick
analysis methods can expose you to unexpected liabilities. There are a
number of shortcuts being offered by individuals and companies that can
have disastrous results. Be sure that your methods will stand up to analysis
and peer review.
Cureall solutions are being promoted, such as the installation of currentlimiting
fuses. Pfeiffer Engineering is a firm believer in the use of fuses, particularly
currentlimiting types, but as will be shown below, they are not always
the answer. They are definitely not a quick fix solution.
 Being overly conservative
when performing a short circuit analysis results in the misapplication
of circuit protection equipment, which in turn has the consequence of
calculated Arc Flash levels being higher than they actually are.
 The calculated bolted
fault or short circuit current is a worstcase calculation that assumes
very low short circuit impedance. It is a short circuit connection based
upon two conductors being bolted together to form the short. In reality,
most short circuits are less than ideal resulting in fault currents that
are less than the alculated bolted short circuit condition.
 On the other hand,
the Arc Fault should be a more predictable occurrence. The arc fault calculations
assume that there is a physical gap between conductors that was bridged
by something resulting in the formation of an arc. Once the arc is formed
and plasma is produced, the arc current should closely approximate the
calculated fault levels. The Arc Fault calculations are an approximation
based upon research and testing similar to the short circuit analysis
methods. They are not exact and therefore one needs to be careful when
using the results.
Solution
The solution is to first
perform, as accurately as practical, a short circuit analysis. The goal for
most people performing a short circuit analysis has always been to error toward
the conservative. For example, when a cable length was needed, it is the practice
to always use the shortest practical value, which would result in higher calculated
short circuit current values. When the public utility is contacted, it is
the practice to only ask for the worse case short circuit value.
The overall result is that
the short circuit values are always calculated on the high side. When doing
a short circuit analysis for sizing the interrupting capability of protection
equipment, this is the best practice. But, it is not the best practice when
evaluating equipment for Arc Faults and establishing PPE requirements. This
is extremely significant, and quite nonintuitive, situation.
Arc Fault current (Ifc)
is derived from the available bolted short circuit or fault current (Isc)
and is always substantially less than its corresponding short circuit current.
The IEEE has established formula for calculating (estimating) the Ifc and
they provide a spreadsheet. The following are example results from using their
formula:
Bolted Fault
Current 
Arc Fault Current 
@ 480 V 

10 kA 
= 6.56 kA 
20 kA 
= 11.85 kA 
30 kA 
= 16.76 kA 
40 kA 
= 21.43 kA 
What is now important is
to obtain:
 The expected maximum
(worse case) bolted short circuit current.
 The minimum and maximum
voltage to the facility.
 The minimum expected
short circuit current.
Also needed are definitions
of the operating modes of the facility such as:
 What are the minimum
and maximum motor loads expected during normal operation and offhour
operation.
 Variation in the sources
of supply to the plant, such as alternate feeders or cogeneration.
The data from the public
utility and the determination of the facility's modes of operation should
be converted into the maximum and minimum Arc Fault current at various locations
in the plant. These results are applied to protective device coordination
studies, where the protective devices are evaluated, and adjusted, if necessary,
allowing the proper PPE categories to be determined.
The following coordination
curve illustrates the point:
The figure above shows the
coordination curve for the secondary of a 1000 kVA 480 V transformer. The
curve shows two types of secondary protection, a fuse and a circuit breaker,
each selected based upon the National Electrical Code requirements. The fuse
is a KLPC 1600A and the circuit breaker is a Westinghouse HND breaker with
a Digitrip.
All transformers limit the
amount of fault current that can pass through the transformer. This is a function
of the transformer's impedance. The coordination curve shows a line for the
Isc, the maximum short circuit current that can pass through this transformer
(20,741 amps). The Isc value used assumes that there actually is sufficient
current available at the primary to provide 20,741 amps on the secondary.
Based upon the IEEE formula,
the calculated Arc Fault current Ifc is 12,230 amps. Using these two currents
and the coordination curve one can estimate the time the circuit breaker and
the fuse will take to clear the fault.
Bolted Fault Condition:
 Fuse clears in 0.22
seconds
 Circuit Breaker clears
in 0.02 seconds
Arc Fault Condition
 Fuse clears in 1.80
seconds
 Circuit Breaker clears
in 0.02 seconds
From these current levels
and clearing times the PPE category can be determined.
Emb (Maximum in cubic
box incident energy)
 Fuse 74 cal/cm2 Category
4 PPE
 Circuit Breaker 0.8
cal/cm2 Category 0 PPE
Clearly, in this example
the circuit breaker outperforms the currentlimiting fuse resulting in a minimal
"worker friendly" PPE requirement.
In the above example both
the Arc Fault current and the Bolted Fault current are less than the currentlimiting
point for the fuse, which is approximately 28,000 amps. Thus, there is no
currentlimit effect from using the fuse. Currentlimiting fuses often do
provide additional protection and they are very good devices but they must
be applied properly. In this example, the circuit breaker provides the best
protection.
Studying this example further,
let us assume that the fuse and the circuit breaker are at the main of a facility
and the facility is served by a much larger transformer where the worsecase
bolted short circuit current as reported by the utility is 60,000 amps. Under
this condition the arc fault current would be 30,300 amps. In this case, the
fuse would open in 1/4 cycle and would limit the fault current.
The Emb would equal 1.15
cal/cm2, which falls under a category 0 PPE.
In the next example we have
a fuse and a circuit breaker protecting a 125 Hp motor. The fuse is a LLSRK
200 A and the circuit breaker is a Westinghouse HKD with a Digitrip. There
are three Arc Fault currents analyzed.
Point 1
 Arc Fault Current 3100
Amps
 Bolted Fault Current
4200 Amps
Point 2
 Arc Fault Current 2200
Amps
 Bolted Fault Current
2800 Amps
Point 3
 Arc Fault Current 1800
Amps
 Bolted Fault Current
2200 Amps
 Results:
Point 1
 Circuit Breaker clears
in .02 seconds 1.42 cal/cm2 PPE Cat. 1
 Fuse clears in .02
seconds 1.42 cal/cm2 PPE Cat. 1
Point 2
 Circuit Breaker clears
in .02 seconds 1.42 cal/cm2 PPE Cat. 1
 Fuse clears in .1 seconds
7.7 cal/cm2 PPE Cat. 2
Point 3
 Circuit Breaker clears
in .02 seconds 1.42 cal/cm2 PPE Cat. 1
 Fuse clears in 1.0seconds
78.8 cal/cm2 PPE Cat. 4
At an Arc Fault current
of 4000 amps the fuse will begin to current limit and will open the circuit
in ¼ cycle reducing the PPE category to 0.
The three points analyzed
show that a relatively small change in calculated bolted fault current has
a major effect on the calculated arc fault current. This situation could easily
lead to the misapplication of circuit protection equipment or inappropriate
adjustment of same. It should also be noted that as the calculated arc fault
current is reduced, the clearing time increases, resulting in the incident
energy level increasing and thus, the PPE requirement increases.
In reality, the arc current
is primarily effected by facility operating conditions, i.e. motor contribution
and changes in the fault current coming from the utility. The examples illustrate
that the accuracy required when calculating short currents has to be improved
over traditional methods. Both reliability and arc fault conditions must now
be considered when performing coordination studies.
The Risk
In a study of 33 plants
with 4892 busses or switch points under 600 volts, the median incident energy
was only 2.1 cal/cm2, however many busses had quite high incident energy levels1:
 24% of busses over
8 cal/ cm2 PPE 2
 12% of busses over
40 cal/ cm2 PPE 4
 5% of busses over 85
cal/ cm2 Deadly  no protection
 1% of busses over 205
cal/ cm2 Deadly  no protection
Risks to personnel include2:
 Burns
 Damaging sound levels
 High pressure  720
lbs/ft2 eardrums rupture, 1728 to 2160 lbs/ft2 lung damage
Conclusions
1. Arc Fault Analysis is
in actually Risk Management. There are basically three choices:
 Be very conservative
and require PPE 4 in most cases resulting in higher maintenance cost.
 Do nothing and suffer
the consequences (pay later).· Perform the necessary analysis and
make adjustments to reduce the arc fault conditions resulting in reduced
PPE requirements.
2. A reduction in bolted
fault current and thus a reduction in arc fault current can actually result
in a worse situation. In the motor example above an arc fault current reduction
from 4000 amps to 1800 amps resulted in an increase in arc fault energy from
0.6 cal/cm2 to 78.8 cal/cm2. Exactly the opposite one would expect before
doing the math. In terms of the above example coordination curves, this occurs
because the arc fault current moves from the instantaneous portion at the
bottom of the coordination curve to a point higher up, incurring a the time
delay before the device trips.
3. Overly conservative short
circuit analysis will result in bolted short circuit numbers that may well
result in the misapplication of circuit protection equipment.
4. It is very important
to obtain the minimum available short circuit current as well as the maximum
short circuit current from the electric utility. Voltage fluctuations in the
plant supply should be considered when developing the short circuit calculations.
The arc fault calculations need to be evaluated at more than just the worse
case and the minimum case conditions. In the example above, a reduction in
the arc fault current actually resulted in worse conditions. This represents
a subtle, but extremely significant, change in the methodology of short circuit
analysis.
5. Apart from the fines,
nominal compliance with the regulations will cause workers to have to wear
cumbersome PPE. This will result in little or no high voltage maintenance
being performed, eventually compromising safety, equipment operation, and
ultimately productivity. Arc flash is a risk management issue.
6. Have a registered professional
engineering firm perform the calculations for arc flash hazards for the devices
in your facility and have them recommend any necessary plans that when executed
would result in the lowest category of PPE being required.
Note:
Short circuit analysis is
based upon a number of assumptions; any or all may change over time;
 Available short circuit
current from the utility may vary, particularly in areas where there has
been a significant expansion of, or change to, the electrical systems.
 The number of motors
running at the time of a fault affects the amount of short circuit current
and arc fault current available (motor contribution).
 The facility voltage
often varies as a function of time of day. The utility is often more loaded
during the day.
Similarly, the arc fault
may also be affected by variations in any of the following:
 The available short
circuit current.
 Dirt buildup in the
equipment that may affect the conductive path.
 Moisture (humidity).
 Circuit supply voltage.
 Amount of motor contribution
during a fault
Definitions
Bolted Fault  Short circuit
current resulting from conductors at different potentials becoming solidly
connected together.
Arc Fault  Short circuit
current resulting from conductors at different potentials making a less than
solid contact. This results in a relatively high resistant connection with
respect to a bolted fault.
Author
John C. Pfeiffer, P.E., president, Pfeiffer Engineering Co., Inc., Louisville,
Kentucky
^{1} "A Summary
of Arcflash Hazard Calculations" D.R. Doan & R.A. Sweigart
^{2} "Arcing Flash/Blast Review with Safety Suggestions for Design
and Maintenance" Tim Crnko & Steve Dyrnes 