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Arc Flash Study Top 10 FAQs Part #1

How Does Everyone Else Do This?

By Jim Phillips

3-Part Series

  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.
For the past several years, I have asked a survey question every week at Arc Flash Forum to dig deeper into some of the gray areas. 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. Click here for upcoming scheduled Arc Flash Safety Training and arc flash certification.   ARC FLASH STUDIES
  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%

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