Arc Flash

Your questions answered: Critical power: Arc flash mitigation

The Sept. 20, 2018, “Critical power: Arc flash mitigation” webcast presenters addressed questions not covered during the live event.
By Brian Martin, PE; and Leslie Fernandez, PE, LEED AP October 1, 2018

Electrical engineers must understand the codes, standards, and design requirements when engineering for arc flash mitigation. Many factors go into mitigating the danger that electrical workers face; engineers must thoughtfully design systems to reduce and mitigate these incidents in the field. Owners, facility managers, engineers, and contractors can all contribute to providing a safer place for personnel to work. The process all starts with safe equipment and engineering design, followed by proper installation of equipment and proper training and maintenance procedures.

Presenters Brian Martin, PE, Jacobs, Portland, Ore.; and Leslie Fernandez, PE, LEED AP, NV5, Las Vegas respond to questions not answered during the live “Critical power: Arc flash mitigation” on Sept. 20, 2018.

Question: Is there a need for labeling equipment if the published company policy is that work shall only be performed on deenergized electrical equipment?

Brian Martin: Yes. Troubleshooting, commissioning, thermography, verification of absence of voltage for lock-out/tag-out are all examples of energized work that an owner may require.

Q: What is the best calculation method for arc flash rating?

Leslie Fernandez: Calculations should be based on IEEE 1584-2002: IEEE Guide for Performing Arc Flash Hazard Calculations. This standard was developed by both NFPA and IEEE. Both organizations worked hard to base the arc flash tables and formulas on actual equipment and test conditions.

Q: Where does NFPA 70E-2018: Standard for Electrical Safety in the Workplace explain how to design a method of preventing arc flash?

Martin: Much like NFPA 70-2017: National Electrical Code (NEC), NFPA 70E is not a design guide. That said, there are many resources available that detail how to prevent arc flash. In addition to the techniques discussed in our webcast, Informational Annex O in NFPA 70E has additional recommendations. In particular, please read Informational Annex O.2.4.

Q: How do you calculate arc flash values? What kind of factors affect arc flash?

Fernandez: You will need to collect data, data, data, and then use the data to input into arc flash software based on IEEE 1584 to determine the arc flash incident energy and create the arc flash labels. Collecting reliable data on the existing system is the most important step. Data needed is from one-line diagrams, utility contributions, generator motor equipment nameplates, voltage, ratings, impedances, cable length and sizes, protective device manufacturer breakers/fuse/relays type, model numbers, settings, amp ratings, sensor amp ratings, current transformer (CT) ratings, clearing times of breakers, etc.

Q: Is high resistance grounding acceptable to utility companies? Utility companies normally solidly-ground their systems.

Martin: The use of high-resistance grounding generally assumes that the owner is taking service at greater than 600 Vac or has his or her own transformer that transforms down to the operational voltage. In those cases, the owner can choose to use a high-resistance ground at the transformers. Note that there are additional code requirements for resistance grounded systems, please refer to NEC Article 250.20(D).

Q: Are current-limiting fuses an acceptable method to reduce fault current by authorities or building officials?

Fernandez: See NEC Article 240.86 A, B, and C. For 240.86(B), there are a lot of breakers that have been tested in series combinations with the generic current-limiting fuses. However, there are not a lot of incidences where the manufacturer has not especially with old breaker types. For these circumstances, NEC Article 240.86(A) allows with very specific provisions that a licensed professional engineer can submit a series calculation application to an authority having jurisdiction (AHJ) using current-limiting fuses. Remember, it is best to ask the AHJ to learn to see if it is agreeable before processing the series calculation application. Of those amiable, be aware of which edition and what, if any, local amendments are in effect for the jurisdiction. Doing so can give you a better understanding of expectations and requirements the AHJ may not accept the series calculation application.

Q: Is zone selective interlocking better than breaker-coordinated protection?

Martin: They are different technologies where one compliments the other. Breaker protection, especially if it is selectively coordinated, may result in very high arc flash incident energy. Zone selective interlocking allows a group of breakers to communicate with each other. If downstream breakers detect a fault, then they operate as normal and breaker coordination is maintained. If the upstream breaker detects a fault and none of the downstream breakers do, then the trip time of the upstream breaker is adjusted to trip without delay.

Q: One more time, could you go over reducing the arc flash by moving the curve closer?

Fernandez: Moving the curves to the left reduces the incident energy time significantly. Typically, a maintenance switch reduces the arcing time in the range of 7 to 15 milliseconds versus 50 to 100 milliseconds. The circuit breaker under maintenance mode breaks the source of power at a rather high speed thereby also quenching arc current.

Q: Are you aware of the industry using room occupancy sensors to automatically enable arc reduction settings? Is this safe?

Martin: I am not aware of this practice and I personally would not be comfortable designing a system that uses a room occupancy sensor. Occupancy in the room does not necessarily indicate that personnel, even unqualified personnel, are in the limited approach boundary or that the equipment is not in a “normal operation condition” (refer to NFPA 70E 130.2(A)(4)). For the market sectors that I work in, compromising selectivity due to the presence of someone in a room would not be acceptable. I also would be concerned about false occupancy indication.

Q: Would a modification to an enclosure with a view window nullify the UL listing of a factory assembly?

Fernandez: If it is done incorrectly, it will nullify the UL listing. Being a specifying engineer, I prefer the equipment manufacture install the viewing windows before installing the equipment in the field. In this manner, the unit will be listed with the viewing windows. If it is done in the field, then coordinate with the equipment manufacturer the placement of the viewing windows and it may be required to also UL field certified the modified equipment.

Q: Can you please explain again on the time-current curve (TCC) curve how changing the instantaneous setting will reduce the incident energy without affecting the selectivity?

Martin: I believe that this question is referring to modification of the breaker trip curve when activating the arc flash reduction mode. Arc flash reduction mode will nearly always compromise selectivity. If the breaker trip curve could be moved so that it was still selective (including motor inrush, transformer inrush, etc.), it would make sense to just have that be the normal setting. The point of arc flash reduction mode is to move the trip curve into a location where arc flash incident energy is reduced and other considerations are secondary.

Q: In that Minimum label, what is the zoning classification?

Fernandez: Good question. The 2015 edition of NFPA 70E eliminated Category 0 and the 2017 edition of NFPA 70E indicates that if you have less than 1.2 cal/cm2, there is no personal protective equipment (PPE) required. However, there still is a shock hazard. It would be prudent that electricians use their arc resistant (AR) PPE even for low-level arc flash equipment.

Q: What is the relationship between available fault current and arc flash incident energy?

Martin: The relationship is roughly the current squared, so the incident energy increases exponentially with an increase in current. That said, arc flash incident energy is an I2t relationship, where t is time, so sometimes an increase in available fault current will reduce the trip/clearing time sufficiently to decrease the arc flash incident energy. Conversely, a decrease in fault current may result in a longer time to trip and therefore a higher arc flash incident energy.

Q: I thought the blast wave over 40 cal/cm2 was deadly. Could you comment?

Fernandez: The 40 cal limit is a cutoff for hazard-risk categories task tables only. Annex H does not appear to have an upper cutoff value. The main arc blast hazard is the pressure wave that occurs-basically it is an explosion. Wearing arc-rated clothing over 40 cal/cm2, although it is available, is not a guarantee of protection against arc blast events. As a consequence, allowing work with incident energy above 40 cal/cm2 also would present a huge liability. One always should try to deenergize the high incident energy levels prior to working on it.

Q: If the equipment has not been properly maintained, how much is the incident energy affected?

Martin: There really is not a method to determine how equipment that has not been maintained will operate in a short circuit. If the protective device completely fails to clear, that is a completely different scenario than a protective device that has a delayed trip or a piece of equipment that is not rated for the available short circuit current. NFPA 70E assumes that equipment has been properly maintained in accordance with manufacturers recommendations. Please refer to NFPA 70E, Table 130.5(C), Informational Note 2. As a result, improperly maintained equipment can’t be relied upon to operate per the manufacturers specifications, which may result in significantly higher arc flash incident energy.


Brian Martin, PE; and Leslie Fernandez, PE, LEED AP