<P> In power engineering, a one - line diagram or single - line diagram (SLD) is a simplified notation for representing a three - phase power system . The one - line diagram has its largest application in power flow studies . Electrical elements such as circuit breakers, transformers, capacitors, bus bars, and conductors are shown by standardized schematic symbols . Instead of representing each of three phases with a separate line or terminal, only one conductor is represented . It is a form of block diagram graphically depicting the paths for power flow between entities of the system . Elements on the diagram do not represent the physical size or location of the electrical equipment, but it is a common convention to organize the diagram with the same left - to - right, top - to - bottom sequence as the switchgear or other apparatus represented . A one - line diagram can also be used to show a high level view of conduit runs for a PLC control system . </P> <P> The theory of three - phase power systems tells us that as long as the loads on each of the three phases are balanced, we can consider each phase separately . In power engineering, this assumption is often useful, and to consider all three phases requires more effort with very little potential advantage . An important and frequent exception is an asymmetric fault on only one or two phases of the system . </P>

Single line diagram of electrical power supply system