A1: A Single Line Diagram (SLD), also known as a one-line diagram, is a simplified representation of a complex electrical power system. Instead of showing every single wire for all three phases, it uses a single line and standardized symbols to illustrate the main components and their connections, such as transformers, circuit breakers, busbars, and protective devices.

For power line projects, the SLD is a critical "road map" that provides a high-level overview of the entire system. It is essential for safety analysis, helping to identify potential hazards like arc flash and short-circuit risks. It also serves as a fundamental tool for system design, maintenance, and troubleshooting, allowing engineers and technicians to quickly understand the power flow and isolate faults, reducing downtime and ensuring a safe working environment.