Difference Between Single Line Diagram (SLD ) and Three Line Diagram

Single line diagrams are electrical drawings that provide a basic visual guide to an electrical system detailing various nodes and interconnections which often serve as a base reference for further systems analysis.
A three-line diagram is, however, a more detailed presentation of an electric system with actual cabling information involving positive, negative, ground cables for DC, L1, L2, L3, Neutral and Ground cables on an AC system.

While SLD shows only one line for all electric wires 3LD used 3 or more lines to represent each and exact wires and where they are connected.

Difference in Applications of One Line Diagrams and Three Line Diagrams

SLD is primarily used in panel schedule design, load schedule design, fault analysis design, protection system design whereas 3LD is used in control circuit diagrams, phase sequencing design, relay setting diagrams, meter & transformer connections etc.

The difference in Usage of SLD and 3LD in Project Life Cycle

Single Line Diagram or One Line Diagram is for basic reference of management and engineers for planning and costing phase of any project. A 3 Line Diagram is for actual implementation during the wiring phase of the project and is often provided to electricians who actually do the cabling.