Single-line diagrams are the most essential documents that are developed during the detail design phase of a project. They identify almost all the main items of power equipment and their associated ancillaries. Initially they define the starting point of a project. Finally they are a concise record of the design, from which all the design and purchasing work evolved.
The final single-line diagrams should contain at least the following information. Complicated power systems may require the single-line diagrams to be sub-divided into several companion diagrams, in which aspects such as protection, interlocking and earthing are treated separately. This ensures that the diagrams are not overly congested with information. The end results should be unambiguous and be easily read and understood by the recipient.
The Key Single Line Diagram
Switchboards and motor control centres:
• All switchboards and motor control centre names, bus-section numbers, line voltages, number of phases, number of wires, frequency, busbar continuous current rating.
• Identification of main incoming, bus-section, outgoing and interconnecting circuit breakers, including spare and unequipped cubicles.
• Some diagrams show the cable tag number of the principal cables.
Generators:
• Names and tag numbers.
• Nominal ratings in MVA or kVA and power factor.
• D-axis synchronous reactance in per-unit.
• D-axis transient reactance in per-unit.
• D-axis sub-transient reactance in per-unit.
• Neutral earthing arrangements, e.g. solid, with a neutral earthing resistance (NER), with a common busbar, switches or circuit breakers for isolation.
• Current and time rating of the NER if used, and the voltage ratio of the earthing transformer if used.
Transformer feeders:
• Names and tag numbers.
• Nominal ratings in MVA or kVA.
• Leakage impedance in per-unit.
• Symbolic winding arrangement of the primary and secondary.
• Line voltage ratio.
High voltage and large low voltage motors:
• Names and tag numbers.
• Nominal ratings in kW.
General notes column or box:
Usually several notes are added to the diagram to explain unusual or particular features, such as interlocking, limitations on impedance values for fault currents or voltdrop.