One of the first steps in performing an arc flash calculation study is to request short-circuit data from the electric utility company. This kind of request is pretty routine, and utilities have been providing this type of data for short-circuit studies for years. The problem is the data used for a short-circuit study may not be suitable for an arc flash study.
Is infinite bus the worst case?
Many utilities provide what is known as an “infinite bus” value of short-circuit current, which assumes there is an infinite amount of short-circuit current available at the primary of the main transformer. Only the transformer impedance limits the short-circuit current available on the customer’s secondary side. This approach usually works well for short-circuit studies since it results in the worst-case maximum current.
Short-circuit studies—infinite bus = good
One of the main reasons utilities use the infinite bus value is that the electrical system is continually changing. New transmission and distribution lines, substations, transformers and similar additions are often necessary to accommodate load growth. Each new addition creates more electrical paths, which can cause the short-circuit current to increase. Basing the short-circuit study on an infinite bus will result in the worst-case maximum short-circuit current, which is a good assumption for short-circuit studies.
Arc flash studies—infinite bus = bad
Infrastructure additions may cause the short-circuit current to increase over time; however, temporary switching changes can cause the current to decrease as well. Equipment can be taken out of service either accidentally or because of maintenance or other switching procedures. Removing equipment from service means there are fewer electrical paths to an area, which can reduce the available short-circuit current. Most of the time, this happens without the customer being notified.
If infinite bus is the worst-case maximum, can’t we just ignore cases that result in lower short-circuit current? If this was a short-circuit study, the answer could be yes—ignore the lower values. However, since this is for an arc flash study, a lower short-circuit current could result in greater incident energy, which could be the worst-case scenario. During an arc flash, a smaller short-circuit current may cause the upstream protective device to take longer to operate. Even though the current is reduced, a longer duration can lead to a greater overall incident energy and a greater hazard.
It’s common that the utility can only provide an infinite bus value, and in such a case, the options become very limited. One method to work around this problem is to begin with the infinite bus data. Yes, I know I just contradicted myself, but the infinite bus is just the beginning and would be used to develop a base case for the study. After the initial infinite bus arc flash calculations have been performed, the calculated incident energy is compared to the protective clothing and personal protective equipment’s arc rating. If the incident energy is less than the arc rating, you are off to a good start.
Next, create several alternate scenarios by slowly reducing the transformer’s primary current from infinite to lower values. This can be done by initially assuming a very large primary source current—large, but less than infinite! With each successive scenario, continue reducing the source value in increments of perhaps 10 or 20 percent. After each reduction, compare the new results to the protective equipment’s arc rating and verify that it is still adequate.
Reaching a lower value of current where the arc rating is no longer sufficient is usually an indication that the short-circuit current fell below the instantaneous trip setting of a protective device. The increase in incident energy can most likely be attributed to a longer duration from the device’s time delay. Continue this process to determine how low the utility short-circuit current can be reduced and still maintain a sufficient arc rating for the protective equipment.
This method may not provide a perfect answer, but in the absence of more specific electric utility data, it will help establish a range of source short-circuit current where the protective equipment is sufficient and creates a confidence factor for the study.