Why Do You Need An Incident Energy/ARC Flash Analysis?

Why Do You Need An Incident Energy/ARC Flash Analysis?Why Do You Need An Incident Energy/ARC Flash Analysis?



Ryan Downey, PE
AVO Training Institute
In the ever-changing world we live in, a demand for high-efficient electrical equipment is rapidly increasing.  With more electrical equipment to maintain and operate, workers are exposed to numerous hazards each and every day.  One of those hazards is arc flash, or an arc blast, which can have devastating consequences.  This paper will explain:
  • What an arc flash (and arc blast) is
  • How it can affect workers
  • What is involved in an incident energy analysis process
  • What regulations or standards govern the process
  • How to protect personnel
  • How to mitigate high incident energy (or arc flash potential) levels
Protecting workers lives is always the right thing to do.   If there is an incident, the emotional and financial effects can be devastating.  For most places, including the United States (U.S.), electrical safety is also mandated and regulated by the law.  In the U.S., OSHA 1910.132 requires employers to access the workplace to determine if hazards are or are likely to be present.  OSHA references the National Electrical Code, NFPA 70E, and the IEEE standards for compliancy.  Additionally, these standards, as well as the National Electrical Safety Code (which is specifically enforced for utility companies), require an arc flash assessment to be performed.  IEEE 1584 provides a procedure for performing the arc flash hazard/incident energy calculations.

An arc flash is a rapid release of energy due to an electrical arcing fault.  This could be due to a fault that is phase to phase, phase to ground, or phase to neutral.  The arc flash results from an arcing fault, where the air acts as the conductor during a fault.  Also, the temperature from an arc flash can reach 35,000 degrees Fahrenheit, which is approximately four times greater than that of the sun.

Working on energized electrical equipment has become commonplace in many industries so understanding what can happen is very important.  An arc flash typically could be over in less than 6 cycles, which is a tenth of a second, and converts to 100 milliseconds.  Just to put this into perspective, the human eye blinks at 300 to 400 milliseconds, so the blink of an eye is 3 to 4 times slower than an arc flash.

An arc blast is primarily a function of the available short-circuit current.  The incident happens almost instantaneously and the pressure of an arc blast is caused by the expansion of the metal as it vaporizes and the heating of the air by the arc energy.  Copper expands at a factor of 67,000 times when vaporizing, and this accounts for the expulsion of molten metal up to 10 ft. away and faster than 700mph.  Sound waves from the arc blast can reach up to 165dB and may result in hearing damage.

Other types of injuries can include burn injuries, collapsed lungs, eye injuries, and injuries due to shrapnel being ejected from the equipment.  When inhalation injuries due to the toxic fumes are combined with external burns or injuries, the chance of death can increase significantly.

An arc flash hazard is a dangerous condition associated with the possible release of energy caused by an electric arc.  An arc flash hazard may exist when energized electrical conductors or circuit parts are exposed or when they are within equipment in a guarded or enclosed condition, provided a person is interacting in a way that can cause an arc (such as switching on or off a breaker or disconnect switch; racking a breaker…things of this nature).  Also, under normal operating conditions, enclosed energized electrical equipment that has been properly installed and maintained is not likely to pose an arc flash hazard.

Incident energy is the amount of thermal energy impressed on a surface, a certain distance from the source, generated during an electric arc event.  It is based on the available fault current, working distance, and the clearing time of the fault.  The unit of measurement for incident energy is calories per centimeter squared.  To give you an idea of what this means, 1 cal/cm2 is equivalent to the energy produced from a lighter in one second.

Arc Flash Boundary is the distance from the arc at which a person could receive a second degree burn, which begins at 1.2 cal/cm2.   Just as a note, a third degree burn begins at 10.7 cal/cm2.

FIGURE 1: Arc Flash Definitions Diagram
Figure 1 above illustrates the approach boundaries and work spaces as defined in NFPA 70E.  The closer you approach an exposed, energized conductor or circuit part, the greater the chance of an inadvertent contact and the greater the injury that an arc flash event will cause.  When an energized conductor is exposed, you may not approach closer than the arc flash boundary without wearing appropriate PPE.
Some of the things that can cause an arc flash can be the fault of the worker, such as inadequate safe practices, working on energized equipment, or the intentional use of unsafe tools.  For example, while working on energized equipment, a worker drops an uninsulated tool in the equipment that causes a phase-to-phase or phase-to-ground fault that escalates into an arc flash event.

Some of the things that can increase the risk of an arc flash event are also related to the environment, such as moisture building up on the energized equipment, which can increase the conductivity and cause an arc.  Faulty equipment that is not working properly, is defective, or allows exposure to foreign objects can pose an arc flash risk as well.  Looking at the table below, when you start to combine these individual risks with each other, you are really increasing the chances of an arc flash event.

FIGURE 2: Causes of an Arc Flash Incident
Also, proper maintenance of electrical equipment is crucial, as the risk of an arc flash occurring or equipment having exposed energized conductors or circuit parts can be dramatically reduced through the proper maintenance of electrical equipment.

NFPA 70E states that the majority of hospital admissions due to electrical accidents are from arc flash burns and are not from electrical shocks.  In the U.S., each year more than 2,000 people are admitted to burn centers with severe arc flash burns.  As much as 80% of all electrical injuries are burns resulting from the arc flash and ignition of flammable clothing.  If improper clothing burns or melts, clothed areas can be burned more severely than exposed skin as the clothing actually melts onto the skin.  This is why wearing the proper PPE is very important.  Arc flashes can and do kill at distances of 10 feet, or 3 meters.

A common thing that can happen relating to non-compliancy is the misuse of the PPE tables in NFPA 70E.  Something that is often overlooked is that NFPA 70E requires the available fault current and clearing time of the protective devices to be known, which is typically not the case.  Also, NFPA 70E states that an incident energy analysis is to be performed for the following conditions:
  • The worker’s task(s) are not listed in the tables
  • Power systems with greater than the estimated maximum available fault current
  • Power systems with longer than the maximum clearing times
  • Tasks with less than the minimum working distance
When the NFPA 70E tables are used instead of an incident energy analysis, some things to consider are:
  • Notes in the tables that have specific requirements for the PPE are generally ignored
  • The short-circuit current is assumed.
  • The protective device clearing time is assumed.
Also, maintenance of the protective devices is not considered when the tables are used.  This can affect the incident energy in the event a sticky breaker or other protective device is not opening when it should, so the clearing time of the device would be inaccurate.  It is also important to note that the tables and the arc flash calculations are not intended to work together.  This is why NFPA 70E has did away with the PPE values and identifies PPE with actual incident energy values for the analysis.

Insufficient training can also be a problem as workers need to understand the correct use of PPE, they need to be able to recognize electrical hazards, and they also need to understand safe work practices.  This training is required and specified by OSHA, NFPA 70E, and also the National Electrical Code.

For the arc flash hazard analysis to be valid, Section 130.5 in the NFPA 70E requires the consideration of maintenance.  As a case example, consider the following situation:
  • A low-voltage power circuit breaker has not been operated or maintained for several years
  • The lubrication has become sticky or hardened
  • The circuit breaker could take additional time to clear a fault condition
Two flash hazard analyses will be performed using a 20,000-amp short-circuit with the worker 18 inches from the arc:
  •  Based on what the system is supposed to do 0.083 seconds (5 cycles)
  •  Due to a sticky mechanism, the breaker now has an unintentional time delay of 0.5 s (30 cycles)
EMB = maximum 20 in. cubic box incident energy, cal/cm2
DB = distance from arc electrodes, inches (for distances 18 in. and greater)
tA = arc duration (seconds)
F = short circuit current, kA (for the range of 16 kA to 50 kA)
All calculations are formulas from NFPA 70E-2015 Annex D (D.3.3)
(1) DB = 18 in.
(2) tA = 0.083 second (5 cycles)
(3) F = 20kA
EMB = 1038.7DB-1.4738tA[0.0093F2-0.3453F+5.9675]
= 1038x0.0141x0.083[0.0093x400-0.3453x20+5.9675]
= 1.4636x[2.7815]
= 3.5 cal/cm2
NFPA 70E, Section 130.5 requires arc-rated FR clothing and other PPE to be selected based on this incident energy level exposure.Therefore the FR clothing and PPE must have an arc rating of at least 3.5 cal/cm2.
Here is the same calculation considering the sticky breaker:
(1) DB = 18 in.
(2) tA= 0.5 second (30 cycles)
(3) F= 20kA
EMB = 1038.7DB-1.4738tA[0.0093F2-0.3453F+5.9675]
= 1038x0.0141x0.5[0.0093x400-0.3453x20+5.9675]
= 7.3179x[2.7815]
= 20.4 cal/cm2
NFPA 70E, Section 130.5 requires arc-rated FR clothing and other PPE to be selected based on this incident energy level exposure.Therefore the FR clothing and PPE must have an arc rating of at least 20.4 cal/cm2.

It is also very important to use proper signage on the electrical equipment.  It is imperative that workers know the proper PPE to wear before beginning any work on energized electrical equipment.  The label in Figure 3 shown below with an “X” beside it is a generic arc flash label that does not inform the worker of the incident energy present at the equipment, the arc flash boundary, or even what type of PPE is required.  The label with a check mark beside it is a typical Arc Flash label that is based from the requirements in NFPA 70E, section 130.5.  The label shows the voltage, incident energy value, the working distance, and the arc flash boundary. 

FIGURE 3: Arc Flash Warning Labels
You’ll also notice that the Limited Approach and Restricted Approach boundaries are shown.  The limited approach boundary represents that a shock hazard exists within this boundary.  The restricted approach boundary represents an increased shock hazard due to the electric arc over combined with inadvertent movement.

Well, now that we know an arc flash hazard, or incident energy analysis is required, what exactly is it?  In a nutshell, mathematical methods are used to determine and reduce, if possible, the risk of personal injury as a result of exposure to incident energy from an arc flash.  The purpose of the incident energy analysis is to identify the incident energy exposure of the worker, the arc flash boundary, the appropriate working distance, and the required calorie rating of the PPE.

The magnitude of the arc flash hazard is determined by the NFPA 70E equations or the IEEE 1584 standard, which was derived from actual test data that took place.  Arc flash hazard is expressed in incident energy with the units cal/cm2.  Also, arc flash protective clothing is rated in arc thermal performance value (or ATPV) which is also expressed in cal/cm2.  Essentially, you need to be certain that the cal/cm2 rating of the PPE you are wearing is greater than the calculated incident energy (or cal/cm2) of the equipment you are working on.  With a proper arc flash study, this information should be presented on the arc flash label.

How can you be sure you are getting an accurate study?  One of the most frequent questions asked is if an engineer is required to perform an arc flash study.  The correct answer to this should always be “yes”. It is essential to have a qualified and properly trained individual perform the study and that a licensed professional electrical engineer (or PE) should perform the study.  In most cases, the Engineering Board of the state or governing body in which the study was performed requires a PE to certify the work.

It is important to understand people’s lives depend on the information presented in these studies and it is crucial that they are accurate.  If there is an incident, it is a guarantee that OSHA will look into whether the study was accurate and also whether the individual that performed the study was qualified.  It is also recommended to confirm that the study was performed with proven engineering software.

The diagram in Figure 4 below shows the incident energy analysis process.  Each of the tasks listed is a crucial component of a complete analysis and it is very important that each is performed thoroughly and properly in order to create an accurate study that will help implement proper information to protect workers. 

FIGURE 4: Incident Energy Analysis Process
Electrical One-Line Drawings
The process begins with the evaluation of the electrical one-line drawings which should be kept up to date per NFPA 70E.  In order for the study to be accurate, it is very helpful if existing electrical one-line drawings exist that show the full power distribution.  The one-lines should identify the sources of power, voltage levels, and electrical equipment such as transformers, generators, switchgear, motor control centers, panelboards, and the protective devices. 
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FIGURE 5: Typical One-Line Diagram
Data Gathering & System Modeling
Where electrical one-line diagrams are not available, the data gathering process must identify all of this information and essentially a one-line diagram is created with the gathered information.  To properly perform the analysis, the process should be very complete, where all of the information of the equipment is gathered such as: ratings of the equipment, arrangement of components on electrical one line diagram, nameplate of every electrical device, ratings and trip settings of every protective device, as well as sizes and lengths of all conductors.

It is crucial to obtain the information on actual equipment so that the study is accurate.  A lot of times, short-cuts are taken here which can basically cause the study to be inaccurate or invalid. The utility contribution, or available fault current, is also an essential piece of the puzzle for proper analysis.  This can often be a challenging task when trying to get this information from a utility company.  A lot of times, an infinite bus calculation is used when the actual fault current cannot be obtained.  However, when an infinite bus is used, the clearing time would be much faster than it would be with the actual level of fault current, which results in a false calculation.

When an infinite bus calculation is used, the clearing time would be much faster than it would be with the actual level of fault current, as illustrated in the Time-Current Curve in Figure 6 below:

FIGURE 6: Infinite Bus vs. Actual Fault Current TCC
  • Isca = 28kA clears in .1 sec. (Infinite Bus Fault Current)
  • Isca = 9kA clears in 1 sec. (Actual Fault Current)
The gathered information is then typically input into engineering software in the form of a one-line diagram model with the correct information selected for each component.  This provides the basis for comprehensive power system modeling in performing all types of analysis.

Short-Circuit Study
As part of the study, a short-circuit analysis is performed to determine if the protective devices are properly rated to withstand a bolted type short-circuit fault.  To determine this, the maximum available fault current is calculated at each significant point in the system and as an additional analysis, the bolted fault currents are converted into arcing currents.  The results are determined based on the existing rating of the equipment. 

Protective Device Coordination
Another important aspect is the protective device coordination study. This allows the engineer to properly coordinate the protective devices so that you don’t have an upstream breaker tripping before a downstream breaker in the event of a fault.  If this happens it could shut down critical equipment or possibly even an entire facility, depending on the configuration!  In most cases, a protective device coordination study also allows the incident energy levels (or the arc flash hazard) to be reduced at various locations with recommended changes of existing settings on the breakers or relays.

Incident Energy /Arc Flash Hazard Analysis
The arc flash hazard (or incident energy) calculations are also performed as part of the study.  As I mentioned previously, the calculations are typically based on IEEE 1584, however the calculations can be based on the equations depicted in NFPA 70E or NESC, depending on the type of facility and/or electrical equipment involved.

Written Reports & Labels
Of course, as part of the final deliverables, a written report is provided to inform the owner of the results and recommendations.  Labels are also applied onto the electrical equipment, which shows the incident energy, PPE requirement, arc flash boundary and working distance for that piece of equipment.  Electrical one-line drawings are typically provided with the deliverables as well, where the drawings can be customized to show specific incident energy levels, short-circuit current, etc. on the drawings.

Also, as previously mentioned, since this is an engineering report and/or study, the documents are typically required to be certified by a licensed professional engineer.

Also, per NFPA 70E Section 130.5, an incident energy analysis should be updated when major system modifications have taken place.  This accounts for changes in the electrical system that could affect the analysis.  The studies also must be reviewed at a minimum every five years.  Changes to the available fault current or utility equipment could greatly affect the analysis as well.  It is very important to keep the studies up to date.  If the information is not kept current, it is unreliable.

There are good safety practices that can be followed to minimize the risk of being injured during an arc flash event.  One of these practices includes limiting the presence of workers near potential arcing sources, which can be accomplished by increasing the distance between the worker and the source of the arc.  Another good practice includes minimize the arcing energy, which can be accomplished through arc flash mitigation methods and it requires an arc flash study.  Of course the most important method to always prtect yourself is to wear appropriate PPE, which also requires an arc flash study.

Limit the presence of workers near potential arcing sources (by increasing the distance between the worker and arcing source)
Increasing the working distance is very effective as energy levels drop off exponentially as the working distance is increased.  There are several methods that can be used to increase the distance between the worker and the arcing source. 
  • Remote racking - This allows the racking of a circuit breaker outside of the arc flash boundary
  • Chicken switch - This allows the opening or closing of a circuit breaker outside of the arc flash boundary
  • Time Delay Switch - This allows a worker to move outside of the arc flash boundary before the circuit breaker is opened or closed
  • Arc Resistant Switchgear - Any potential arc is shunted away from the worker and extinguished
Arc Flash Mitigation Methods
Mitigating high incident energy levels can often be achieved through several methods, some of which are very cost effective.  Arc flash mitigation will, of course, require an arc flash study and includes engineering methods to determine proper protective device coordination to go along with reducing incident energy levels.  Some of the methods include:
  • Protective Relaying schemes, which are fast-acting control mechanisms.  Common relay schemes include: 
    • Bus or Transformer Differential protection, where a relay compares the primary and secondary currents at a transformer.  Differential protection can be used to protect a specified zone.
    • Arc flash detection relays typically use fiber optics and current detection to sense “true” arcing events.  The use of light and current can reduce or eliminate false tripping.  Some systems also use sound, or ultrasonic devices as a third component.
  • Current limiting fuses are fast-acting fuses in which the clearing time and peak current are greatly reduced.  They may be limited in use, however, since they typically only work for high fault current levels.
  • Adjustable circuit breakers
    • Long-time pickup, short-time pickup, instantaneous (LSI) breakers allow the shaping of tripping curves, and typically offer better performance across wide range of fault current levels.  A protective device coordination study is required to properly coordinate the various upstream and downstream breakers.  Zone-selective interlocking is also an option, which allows trip devices to communicate with each other so a fault is cleared by nearest circuit breaker, with no intentional time delay.
  • Maintenance mode circuit breakers
    • Instantaneous settings are adjusted during routine maintenance to reduce the incident energy levels.
Wear Proper PPE
And of course the thing we all need to make sure we always do:  Wear Proper PPE!  This is the last line of defense!  It is there to protect against life threatening injuries.  Too little protection can result in serious injury or death.  Too much protection can cause limited worker mobility or visibility.  You need to know the exact type of PPE required for the task and this is what an arc flash study will tell you.

So what do you do after the analysis is completed?  Well, you need a plan to keep it maintained!  You need a plan to be certain that the study is reviewed every five years or updated with any system modifications to keep it accurate.  Analyzing the electrical system every five years will identify any changes to the results.  Analyzing the maintenance plan will help integrate any changes to the system.  It is very important to keep the studies up to date.  If the information is not kept current, it is unreliable.


In conclusion, we now understand:
  • What an arc flash is
  • How it can affect workers
  • What is involved in an incident energy analysis process
  • What regulations or standards govern the process
  • How to protect personnel
  • How to mitigate high incident energy levels
Knowing this information, you now have three choices:
You can do nothing and increase the risk of a fatality or injury occurring.  This also increases the risk that fines, litigation, increases in insurance premiums, and that OSHA (or another governing agency) will knock on your door.
You do something, but do not require, or do not know, the correct PPE workers should wear.  Workers could have inadequate or excessive PPE, which could result in an accident, once again increasing the risk of fines, litigation, increases in insurance premiums, and OSHA (or another governing agency) knocking on your door.
Or, you can have an analysis performed by a licensed professional engineer.  You are in compliancy with the codes and standards, and you are helping to protect those that work on electrical equipment.