It goes up. It goes down. Sometimes, it is thought to be infinite (although it isn’t), and other times, it seems impossible to find. The available short-circuit current from the electric utility is one of the more important pieces of information for an arc flash hazard calculation study. Used to help define the severity of an arc flash hazard, it represents the magnitude of current that could flow from the electric utility during a short circuit.
The utility short-circuit current may sometimes increase over time, which can change the results of short-circuit and arc flash studies. How and why does this increase happen? Two words: load growth.
Here is a typical example. A new shopping center is being constructed on the edge of town. At the moment, there is nothing else in the area, just empty fields. To serve the new electrical load, the electric utility usually will construct a feeder circuit from the existing distribution system in the area. Like so many other new developments, the empty fields don’t stay empty. It doesn’t take long before hotels, restaurants and big box stores begin to spring up and further increase the electrical demand for the area.
System upgrades, short-circuit current
To stay ahead of increasing demand, electric utilities develop plans for electrical system upgrades and additions based on load forecasts that project anticipated loads several years into the future. Plans can include anything from constructing a new substation to adding transmission and distribution lines or other creative solutions. Although the primary goal is to serve the increasing electrical demand when upgrades and additions are made to the electrical system, they often cause the available short-circuit current in the area to increase.
Earlier in my career, I worked in the transmission-planning group of a very large electric utility company and eventually headed up the short-circuit studies group. This provided the opportunity to experience the load growth/planning end of the business and witness the effect that new additions and upgrades had on the short-circuit current.
I recall a case where a new distribution substation was being constructed to serve a rapidly developing area with substantial load growth. Before construction was completed, a short-circuit study that included the new substation was conducted. The study indicated that, once the new substation was brought online, short-circuit current in the area would increase by approximately 40 percent, potentially creating a problem for some existing customers.
Is the utility required to automatically inform customers when an increase in short-circuit current occurs? No. The responsibility is with the customer. That is why it is important to review short-circuit and arc flash studies periodically—to verify the data is still valid and no major changes have occurred.
Minimum short-circuit current
If utility short-circuit current generally increases over time, why would it ever decrease (and why would we care)?
Utility systems are made of many substations along with transmission and distribution lines that normally operate in a specific configuration. However, the normal configuration can change. Sometimes, it is planned. Sometimes, it is not.
During storms, unexpected outages may cause a need to reconfigure the system to restore power. Utilities also plan scheduled outages of specific equipment for periodic maintenance, such as taking a transformer or a line out of service. These changes can possibly reduce the available short-circuit current.
Historically, electric utilities often provided short-circuit current data that leaned toward the high side, assuming it was to be used for a short-circuit study where larger values are the worst case. That is not necessarily true for an arc flash study where larger values may not always be worst case. A lower short circuit could lead to protective devices taking more time to operate, resulting in a longer arc flash duration and much more incident energy.
When requesting short-circuit data from the electric utility, make it clear that the data is to be used for an arc flash study and needs to represent present-day normal conditions. Try to avoid obtaining artificially inflated values, such as an infinite bus case. If possible, see if they can also provide minimum values of short-circuit current that represent conditions where equipment may be out of service.
Here’s a clue
If you see a new substation, distribution lines or other electrical infrastructure being constructed in your area, odds are the short-circuit data is about to change. Contact the utility for revised information, so you can review your study and stay ahead of the game.