Arc Flash Analysis—Know the Issues: Your questions answered

Webcast presenter Tommy Northcott, PE, CRL, CMRP answered additional questions about topics such as NFPA 70, changes made to the 2018 version, and arc flash best practices.

04/04/2018


Tommy Northcott, PE, CRL, CMRP, JacobsThe "Arc Flash Analysis—Know the Issues?” webcast was presented live on Mar. 27, 2018, by Tommy Northcott, PE, CRL, CMRP. The webcast can be found here.

Northcott has supplied written answers to some of those questions that weren't addressed from the webcast attendees:

Question: How is dc equipment evaluated and what are the arc flash hazards? 

Answer: The concern about arc flash hazards with dc systems comes up regularly and sadly it is not a very easy question for me to answer. First, let me point out that the 70E does include a table for selecting protective personal equipment (PPE) based on dc systems with specific parameters. There are also IEEE technical papers available that discuss methods for calculating arc flash energies on dc systems.

Most notably is D.R. Doan’s technical paper titled “Arc Flash Calculations for Exposures to DC Systems."

The equations from this paper were first included in the 2012 Edition of NFPA 70E and remained in the 2015 and 2018 editions in Annex D. Keep in mind the Annex portion of the 70E is for informational purposes only and not regulatory.

The challenge with any arc flash calculation is the thermal hazard associated with an arcing fault is very complex, with many variables that have an impact on the calculation. There is still much to be learned about arc flash events and how to accurately calculate their potential.

The NFPA and IEEE organizations have developed the “Arc Flash Phenomena Collaborative Research Project." This purpose of this project is to further understand arc flash phenomena, as well as the non-thermal effects of arc blast. It is expected the work they are doing will improve our methods for calculating energy levels for both ac and dc.

Q: What are the best practices when developing arc flash labeling (layout, required information, etc.)

A: The NFPA 70E includes minimum requirements for arc flash labeling.

Article 130.5(H) focuses on:

(1) Nominal system voltage

(2) Arc flash boundary

(3) And, PPE requirement (options – site specific level, minimum arc rating, incident energy level and working distance, or PPE category from the tables).

So you are required to have those elements posted on any electrical equipment that are likely to require examination, adjustment, servicing, or maintenance while energized.

Now, what I would recommend putting on a label is voltage, incident energy, arc flash boundary, required arc flash PPE, shock protections boundaries, VR glove class, and upstream protective device

I prefer to color code the stickers based on site-specific PPE levels and I recommend making the PPE level and AF boundary the most prominent. You can see an example of a sticker format I populated for a client illustrated in the following article: https://www.plantengineering.com/single-article/arc-flash-avoid-the-shock/1fc8e3fd2e96678c1cf173d377558d44.html

Q: How often is it required to review arc flash?

A: According to NFPA 70E Article 130.5(G), the incident energy analysis must be updated when any significant changes are made to the system or at a minimum frequency of 5 years.

Q: Do you recommend a continual training program to insure safety in the workplace?

A: Absolutely. Knowledge is our best weapon against electrical hazards. In fact, the NFPA 70E requires electrical safety training to be repeated at a minimum frequency of 3 years. This lines up with the three year revision cycle, so it is easy to conclude each time the 70E gets revised, we need to provide training to electrical workers with the latest requirements.

Q: Do you need to re-evaluate the entire system for accuracy? How can you assume drawings are update to date?

A: I never assume drawings are accurate when it comes to life altering hazards—not even drawings with my name as the engineer of record. Always field verify as much as possible. Incident energy analysis should not only rely on drawings. Drawings likely will not have enough detail required to effectively complete the analysis. Drawings should only be used as a starting point and then redlines will likely be a secondary benefit of the analysis.

Q: Can we use the original arc flash labels after approved with the 5-year review?

A: The labels are only required to be updated if the information is different. Often the label includes the analysis date to provide a visual reference for when it is time to be updated. However, as long as the employer has documentation that shows the analysis was completed within the last five years and the posted information matches the latest analysis, they are meeting the intent of the requirement.

Q: Any good resource for setting up an effective electrical equipment maintenance program (E3MP)?

A: The E3MP is really a philosophy customized to meet the specific needs of the system. NFPA 70B has some good basic information on reliability centered maintenance principles.

Q: As an owner to understand arc flash hazard, which panels need to be worked on for arc flash?

A: Article 130.5(H) specifies that equipment other than dwelling units likely to require examination, adjustment, servicing, or maintenance while energized are required to have an arc flash warning label on them.

Q: Where can we see examples of actual incident energy calculations?

A: NFPA 70E Annex D has examples of incident energy calculations.

Q: What are overcurrent protection devices according to your presentation? Is it the circuit breaker, the protection relay, or both?

A: An overcurrent protective device is a device(s) that detects an abnormal condition and result in interrupting the circuit.  It could be a breaker or fuse as well as a combination of relay and breaker.

Q: If your equipment and overcurrent devices never change, how do you continue to analyze the system over time?

A: The 5-year analysis would be verifying the system has not changed and all values remain the same. But there must be documentation that shows it was verified in some manner.

Q: You said the energy level must be considered the same regardless of the task. Is walking personnel around a motor starter mounted in the field on a column (for instance) with its door safely closed need to hold the same EPI level? Even if the personal does not need to operate to touch the device (locking for instance).

A: The maximum possible incident energy level that is calculated is the level to protect against regardless of the task you are performing. The context was stated with respect to using the tables. After the analysis is complete, the first question is: "Does this task pose an arc flash hazard?" 

If yes, then you must protect to the maximum level calculated for that device. The two scenarios where the hazard exists are exposed energized conductors and starting/stopping current flow. There are many tasks performed on or around equipment that do not pose an arc flash hazard and therefore do not require arc flash PPE.

Q: As an original equipment manufacturer (OEM), are we required to do the arc flash analysis or is the end user responsible for it?

A: Article 130.5(H) states the owner of the equipment is responsible for the documentation, installation, and maintenance of the arc flash label. They can request it from the manufacturer but the owner will have to provide appropriate details in order for a proper incident energy calculation to be performed. Keep in mind the NEC (NFPA 70) has labeling requirements as well.

Q: Do you see the IE level increase when going through a protective device such as a circuit breaker or fuse?

A: Typically, on a properly coordinated system, you will see the incident energy level decrease on the load side of an OCPD. However, there have been some unique configurations that have resulted in higher than expected values, which adds more emphasis to the importance of performing a detailed incident energy analysis.

Q: What are your thoughts as to specification of an arc flash study as a delegated design requirement for the builder? If completing the arc flash study during design, how do you ensure the installer follows lengths and component specifications are assumed during design?

A: In order to perform an accurate incident energy analysis as part of a design, the owner of the facility must provide the design firm enough information to accurately do the analysis. Also, the analysis must be reviewed after completion to make sure the as-build configuration agrees with the designed configuration and adjust the analysis accordingly. I am a strong advocate of in-house capability of at least being able to review a study for accuracy and thoroughness.

Q: Is an arc rated enclosure different from a "dead front" enclosure?

A: Yes. Dead front only eliminates the exposure to conductor to avoid inadvertent contact. Arc rated is actually rated to contain the maximum rated arc flash energy within the equipment (or plenum) so workers are not exposed to the arc flash energy at any level.

Q: Do you recommend NFPA 70B as a guide for maintenance or use the manufacturer's recommendation?

A: I recommend looking at both as a good starting point and then performing a reliability-centered program such as a E3MP to custom fit the best maintenance plan to the specific asset.

Q: 2015 NFPA70E Table 130.7.C.15.A.a was eliminated in the 2018 version. What was the logic behind the change? What can we do in lieu of the table?

A: Table 130.7 (C)(15)(A)(a) was only slightly modified and renumbered as Table 130.7(C). According to the NFPA 70E notes, this table can be used for either method of arc flash risk assessment, but I recommend reading the * note at the end of the table.

Q: Do transformers require labels?

A: It depends. Article 130.5(H) specifies that equipment other than dwelling units are likely to require examination, adjustment, servicing, or maintenance while energized are required to have an arc flash warning label on them. So, if workers are likely to be inside the arc flash boundary of the transformer for the previously mentioned tasks, then the answer is yes. Otherwise, it is no.

Tommy Northcott, PE, CRL, CMRP, Jacobs

 



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