This chapter describes typical functions provided in utility substation automation (SA) systems and some important aspects of the interface between substation equipment and the automation system components. The electric utility SA system uses any number of devices integrated into a functional array by a communication technology for the purpose of monitoring, controlling, and configuring the substation. Choices for locating interfaces also depend on how much of the substation will be modified for automation and the budget allocated for modification. Environmental factors include airborne contamination from dust, dirt, and corrosive atmospheres found at some substation sites. Electric utility SA systems gather power system performance parameters for system generators, transmission lines, transformer banks, station buses, and distribution feeders. Electrical measurements on a high-voltage transmission or distribution network cannot be made practically or safely with direct contact to the power carrying conductors. The most common voltage sources for power system measurements are either wound transformer or capacitive divider devices.