The available fault current from an electric utility can change over time creating unique challenges for power system studies such as short circuit and arc flash studies.  Why can this data change? 

The current can increase potentially leading to electrical equipment not having an adequate short circuit interrupting or withstand rating as required by National Electrical Code 110.9 which states: Equipment intended to interrupt current at fault levels shall have an interrupting rating at nominal circuit voltage at least equal to the available fault current at the line terminals of the equipment. 

The fault current also plays an important role with incident energy calculations as part of an arc flash study.

One reason the available fault current can increase is due to the electric utility adding new infrastructure such as new transmission and distribution lines and substations.

Fault current can also be reduced as well.  When a utility removes a line from service whether for maintenance or due to some other cause, the fault current will decrease. This is also true if transformers are taken out of service. 

In this video I discuss just such a situation from my days heading up the transmission short circuit studies group of a very large utility company.