The available short circuit data (fault current) from the electric utility or other source is a very important variable when conducting an arc flash study.  The magnitude of fault current affects the incident energy per unit time and ultimately can affect how an upstream overcurrent protective device responds. In general, the greater the fault current, the faster the device trips.  The lower the fault current, well you get the idea, the slower it may trip. 

A very critical part in all this is what magnitude of current does it take for the overcurrent device to trip instantaneously?  Instantaneous in the electric power world means “no intentional time delay” although there will be some minor delay due to contacts opening and the arc extinguishing.  This “delay” is usually only a few electrical cycles. If a device trips instantaneously, that means the arc duration would be limited to a few cycles.  However, if the magnitude of fault current falls below the device instantaneous function, the device will enter the time delay region – bad for an arc flash because the duration will be much greater, leading to a greater calculated incident energy.

Since the fault current is so important, what if this data is not available, is unreliable or is subject to change?  It is not unusual to contact the electric utility to obtain the available fault current from their system and have an answer such as: 50,000 Amps, 20,000 Amps or some other “round number”, or nothing at all.  If the answer ends with many zeros, time to scratch your head.  Is it really that value?  Probably not.  Also, the fault current can change depending on the configuration of the utility at any given time.  What if a transformer or line is taken out of service?  The fault current will be less.  There are not any perfect answers for these problems but here is a work-around that I have been advocating for years.  It is an iterative approach. 

You begin with the maximum fault current from an infinite bus short circuit calculation. Many will state an infinite source approach is NOT the worst case.  That is INCORRECT.  The correct response is the infinite bus approach MAY NOT be the worst case.  But it could be the worst case if the arc duration does not change.

As an example, let’s say the service is on the 480 Volt secondary of a 1500 kVA, 5.75% transformer.  Using an infinite bus calculation, the calculated fault current on the secondary would be 31,374 Amps.  Begin with that value and run the arc flash study.  Note all the locations where the preferred arc rating is sufficient. i.e. if 8 cal/cm² is to be used, note all of the locations where that arc rating works.

Next, reduce the fault current from 31,374 Amps in increments of 5 or 10%.  Re-run the study.  You will likely see the incident energy decrease at the various locations.  Reduce the fault current again, and again….

At some point after one reduction too many, you will notice the incident energy makes a dramatic increase.  This means the source fault current is now too low causing overcurrent devices to trip in the time delay region.  The greater the arc duration, the greater the incident energy. 

Although not a perfect solution, the iterative approach can be used to determine a range of utility/source fault currents that will result in the arc rating of PPE being sufficient for the calculated incident energy.