According to the Electrical and Computer Engineering Department of Kansas State University, a single line diagram is a simplified way of diagramming a three-phase power system. It is also called a one-line diagram.
A single line diagram simply shows one circuit of a three-phase power system using standard engineering notation. The diagram represents the entire system because the other loops are easy to find using basic arithmetic after one circuit has been computed. Engineers refer to the diagrammed circuit as the "a-phase." They then compute the b-phase by subtracting 120 degrees from the a-phase. The c-phase is computed by adding 120 degrees to the a-phase. If any of the three phases contains an asymmetric fault, the single line diagram can not be used.