NFPA 70E 130.5(G) Incident Energy Analysis Method requires that “The incident energy analysis shall be updated when changes occur in the electrical distribution system that could affect the results of the analysis. The incident energy analysis shall also be reviewed for accuracy at intervals not to exceed 5 years.” One such change could be the available fault current from the electric utility.
An incident energy analysis, often referred to as an “arc flash study,” involves a series of complex calculations using equations from IEEE 1584: IEEE Guide for Performing Arc-Flash Hazard Calculations. The results include determining the prospective incident energy from an arc flash and the arc flash boundary at each equipment location under study. Input data for the calculations includes the results of a short-circuit study that typically begins with the available fault current from the electric utility service entrance. This sounds simple enough. However, acquiring this data can sometimes feel like trying to hit a moving target.
One of the first steps for the study is to request the fault current data from the electric utility. Just finding the utility contact person can be a challenge. However, once the contact is established, be sure to inform them that the data will be used for an arc flash study. Why? Because sometimes utilities provide short-circuit data based on what is known as an infinite source transformer calculation. This is the worst-case maximum current that could flow through a transformer, assuming the fault current on a transformer primary is infinite. This is a very conservative approach when determining whether overcurrent protective devices have an adequate interrupting rating.
However, for an arc flash study, lower fault current values could be the worst case. A lower current could result in an upstream overcurrent device taking longer to operate, leading to a longer arc flash duration and increased incident energy.
Once you have obtained the utility short-circuit data, you never have to contact them again, right? Not exactly—fault current can change over time for many different reasons. Because of this possibility and how it could affect the incident energy calculations, IEEE 1584 Clause 6.3 states, “It is important to determine the available short circuit current for the mode(s) of operation that provides both the maximum and minimum available short circuit currents.”
The available fault current can increase over time, which may come as a surprise. However, let me explain from personal experience. Earlier in my career, I worked for a large electric utility company, first in the transmission planning department and then heading up the transmission short-circuit studies group.
The planning department would project load growth three and five years into the future. Looking ahead, the loading and voltage profile of the transmission system would be evaluated along with “what if” contingencies. What if this line went out of service? What if that transformer went offline? All of this was to determine where potential weak spots might be in the next year, which would provide time to develop, design and construct a solution.
One specific example was near my house at the time. There had been major residential and commercial development, and with it, an increase in the electrical load in the area. In the planning department, we determined that a new substation was necessary to support future load growth in the area.
I later moved on to head up the transmission short-circuit studies group, where I saw the effect of the new substation. The group would conduct short-circuit studies projected three years in the future and included the new substation. The study indicated that when the substation was brought online, the fault current would increase about 20% on the distribution system nearby.
Any facilities directly connected or “electrically close” to that part of the system would see the current jump. This was a fairly common occurrence when a new substation was added.
The utility fault current can also decrease. How? Utilities may conduct scheduled outages for equipment and circuit maintenance. This isn’t a case of plunging everyone into the dark—that’s not in anyone’s best interest. It is typically relying on the system’s redundancy, which enables temporarily removing a transformer or line from service. As an example, if a substation has two lines serving it, one line may be removed from service for maintenance while the other remains energized to serve the area. The result is with one line out, the fault current in the area decreases.
Because it is possible for the utility short-circuit current to change over time and affect the arc flash study results, it is important to review this data periodically.
Header image: stock.adobe.com / John Takai
About The Author
PHILLIPS, P.E., is founder of brainfiller.com and provides training globally. He is Vice-Chair of IEEE 1584 Arc Flash Working Group, International Chair of IEC TC78 Live Working Standards and Technical Committee Member of NFPA 70E. He can be reached at [email protected].