All types of electrical systems use one-lines, from electric utility companies to industrial plants, large commercial buildings, oil rigs, and even airplanes.

Understanding one-line diagrams is important for more than electrical engineers, especially in the power grid industry where electrical systems can be quite large and complex and involve cross-discipline teams. Collaboration across disciplines during the design and construction of electrical projects means that other teams like civil engineering, substation engineering, and project management need to have a basic understanding of what a one-line diagram is and how to interpret it.

One-line diagrams, also known as single-line diagrams, show an electrical system’s major components using symbols and single lines. This is the simplest representation of an electrical system, compared to the more detailed three-line diagrams where all connections within the power system are depicted and labeled individually. One-line diagrams are just one type of two-dimensional drawing used to design substations, switchyards, and transmission lines, and they serve as a basic template for load flow and short circuit studies.

One-line diagrams:

1. Provide a simplified overview of an electrical system. The main purpose of a one-line is to provide a high-level view of the equipment and load flow within the electrical system.
2. Are a single-phase picture of a three-phase system. Reducing the three-phase system down to a single-line depiction reduces the confusion of drawing all three phases of the system. This helps focus attention on the most critical pieces of information at this stage in the design process—the major equipment and the flow of power through the system.
3. Show the basic equipment ratings for all the major equipment in an electrical system. Major equipment typically includes generators, step-up transformers, current transformers, voltage transformers, circuit breakers, disconnect switches, and surge arrestors. The lines connecting the major equipment represent the lines through which electricity flows.
4. Serve as a template for relaying. The simplified overview that one-line drawings provide helps engineers identify where switches are needed to protect the electrical system from faults and overloads. After all, the two main components that a one-line includes are what substation relaying and protection focuses on: major equipment that needs to be protected and how load flow will change in the event of faults or other problems within the electrical system.
5. Act as the starting point for the physical layout, also known as the site plan. Before engineers begin to design the physical layout of a site, they first need to understand how and where load flow will occur, what equipment is needed to control the flow, and how the system connects to the overall power grid beyond the immediate substation or switchyard site.
6. Depict equipment arrangement within the electrical system. While one-lines show how major equipment will be laid out and there can be similarities to the physical layout, the electrical one-line is not guaranteed to look identical to the physical layout.

About Beta

Beta Engineering is a substation EPC company with headquarters in Pineville, LA, and an office in San Diego.  Since 1975, Beta has been building high-voltage substation and transmission line projects for power delivery companies across the U.S. Contact our team of experts to leverage our 50 years of problem-solving experience and achieve your long-term goals.