Friday 22 September 2017
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Arc Flash Study Top 10 FAQs

There are many frequently asked questions about performing an arc flash study (risk assessment) and understanding electrical safety requirements. A careful read of standards such as NPFA 70E or IEEE 1584 can answer some questions. Yet, other questions can be more complex, gray areas can lead to confusion, second-guessing and wondering how everyone else does it.

Sometimes, the responses are predictable and sometimes they are not. Of the more hundreds of questions, I list the top 10 of the most frequently asked queries along with the survey results and a short commentary about the background.


The Top 10 List is broken down into three categories:

  1. Arc flash studies (risk assessments)
  2. Electrical safety practices
  3. Arc flash labels

Although technically not a scientific survey, the results provide an interesting insight into the views about specific topics. It should be noted that the survey results do not mean a particular response is right or wrong; it is simply a reflection of opinion.


  1. What short-circuit data can you obtain from the electric utility company?

The available short-circuit current from the electric utility is a vital part of an arc flash study. Large short-circuit currents can result in an increase in the incident energy. However, lower short circuit currents can lead to protective devices taking longer to operate, which also results in a large incident energy. Quite often, very large incident energy values are due to these long device operating times. The degree to which the utility short-circuit data affects the incident energy is also a function of the equipment’s location. Equipment located farther from the source is generally affected less than equipment closer to the source, because of transformer and conductor impedance.

Question: What data does the utility provide for arc flash studies (assuming data is provided)?

Normal conditions: 18%

Maximum fault current: 28%

Minimum fault current (line out/contingency, etc.): 14%

Infinite bus: 12%

Not sure: 0%

Depends on the utility: 28%

(Respondents were instructed to select all that applied.)


  1. How easy is it to obtain protective-device data from the electric utility company?

The electric utility’s protective device that serves a facility is likely to be the device that defines the arc flash duration at the service equipment. To determine the arc flash duration, data such as fuse size, type of circuit breaker and relay setting information must be known.

Question: Are you able to “easily” obtain protective-device information from the electric utility?

Yes: 15%

No: 27%

Sometimes: 51%

We are the electric utility: 5%

We don’t perform studies: 2%


  1. Do you use the IEEE 1584, 125-kilovolt-ampere (kVA), less-than-240-volt (V) cutoff? What transformer size do you use for the cutoff?

The 2002 edition of IEEE 1584 states, “Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low impedance transformer in its immediate power supply.” This language, sometimes referred to as the “125-kVA exception,” was intended to permit excluding some circuits from the arc flash study. Some use 125 kVA as the upper limit of the cutoff, while others have reduced the limit to even smaller transformers.

Question: What transformer size do you use for the IEEE “125-kVA exception”?

125 kVA (112.5): 67%

75 kVA: 12%

45 kVA: 6%

30 kVA: 0%

We don’t use the exception: 15%


  1. Does your company or client permit energized work where the incident energy is greater than 40 calories per centimeter squared (cal/cm2)?

This question is about the 40 cal/cm2 limit found in Informational Note 3 of NFPA 70E 130.7:“When incident energy exceeds 40 cal/cm2 at the working distance, greater emphasis may be necessary with respect to de-energizing before working within the limited approach boundary of the exposed electrical conductors or circuit parts.” (This language is from the 2012 edition of NFPA 70E, which was in use when this question was asked.)

Although it is only an informational note, many Code users take this value as an absolute go/no-go when it comes to energized work and prohibit work at locations where the incident energy exceeds 40 cal/cm2. The concern is frequently about blast pressure and the legal aspects of a standard containing a reference to 40 cal/cm2 in this context even though it is an informational note. Many times, when there is a large prospective incident energy, it is because of a long arcing time (e.g., a protective device may take a long time to operate).

Question: Does your company or client permit energized work where the incident energy exceeds 40 cal/cm2?

Yes: 9%

No: 59%

It depends: 21%

We don’t perform energized work: 11%

Of the many comments that were also provided in response to this question, it was pointed out that testing for dead means assuming the conductor is live. So, technically, live work is being performed above 40 cal/cm2.


  1. What do you do if an unqualified person enters the limited-approach boundary?

Unqualified people are not permitted within the limited-approach boundary, unless they are advised of the hazards and continuously escorted. Sometimes people, such as managers, owners and others, will show up for various reasons, assuming that only the person performing the work needs to be qualified (and protected). This, of course, is incorrect.

Question: If an unqualified person enters the limited-approach boundary while energized work is being performed, the response should be:

Keep working: 0%

Stop working: 27%

Insist they leave the area: 66%

Report them: 2%

Something else: 5%

(Respondents were instructed to select all that applied.)


  1. How is your company or client’s condition of maintenance of electrical equipment?

This question was asked as a result of the 2015 edition of NFPA 70E placing more emphasis on the condition of the electrical equipment, such as in 110.1(B), which states: “The electrical safety program shall include elements that consider condition of maintenance of electrical equipment and systems.”

Question: What is the condition of maintenance of your/your client’s electrical equipment?

Excellent—Better than most: 2%

Average—Pretty typical: 53%

Poor—What is maintenance?: 30%

We have many clients/locations; we set it all: 15%


  1. What do you list on an arc flash label when 
the prospective incident energy is greater 
than 40 cal/cm2?

This question is a variation of the one about the 40 cal/cm2 limit in Informational Note 3 of NFPA 70E 130.7. However, this question concerns arc flash labels.

Where the calculated incident energy exceeds 40 cal/cm2, many arc flash labels use a red background with white “Danger” as the signal word. However, many of these labels also have language that states, “No Safe PPE Available,” or something similar, even though PPE is available with arc ratings upward of 100 cal/cm2.

 Question: For locations where the incident energy is greater than 40 cal/cm2, which of the following are used for your companies or client’s labels?

Danger: 37%

No safe PPE exists: 29%

Energized work prohibited: 28%

None of these: 1%

Something else: 2%

We don’t have locations > 40 cal/cm2

(Respondents were instructed to select up to three options.)


  1. Do you still have equipment without arc flash labels?

You might think having label requirements defined by NFPA 70E as well as calculation methods from IEEE 1584 for more than a decade that most equipment would have an arc flash warning label by now. That is not the case, according to the answers to this question.

Question: Do you still find electrical equipment without an arc flash label (that should have one)?

Yes: 97%

No: 3%


  1. What do you do with arc flash labels now that the 2015 edition of NFPA 70E has eliminated Category 0?

Many still correlate the calculated incident energy with categories or levels. Often this has meant that, if the prospective incident energy was less than 1.2 cal/cm2 at the working distance, it would be listed as “Level 0.” This question was asked a few months after the 2015 edition of NFPA 70E was published.

Question: How do/will you label equipment with a calculated incident energy less than 1.2 cal/cm2?

We still use “0”: 56%

We will list the incident energy: 25%

We will list a minimum arc rating such as 4 or 8 cal/cm2: 12%

Something else: 7%


  1. Should the date be listed on the arc flash label?

NFPA 70E lists the minimum requirements for information to be included on the arc flash label. The list does not include the date.

Question: Do you feel the date of the study should be included on the arc flash warning labels?

Yes: 93%

No: 7%

So you think you are the only person with a particular question? Highly unlikely. Many questions are quite common and also frequently asked.

Note: The survey responses and accompanying commentary may not necessarily reflect the official policy or position of NFPA, IEEE or other standards and organizations.


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