Single-line diagram

The single-line (one-line) diagram is the electrical system's P&ID: a deliberately simplified topology in which three-phase circuits collapse to one line each, so that the whole power system — from incoming supply through transformers and switchboards to the loads — can be read as a structure. Electrical engineering designs by it; protection studies, load-flow and fault calculations are organised around it; operations switch and isolate by it; and arc-flash and safety programmes depend on it describing the system that actually exists.

That last dependency is the sharp edge. Power systems are modified continuously through a facility's life, and an SLD that lags reality is not a documentation blemish but a hazard: isolation planned on a diagram that omits a source, protection coordinated against a topology that changed. Keeping the SLD current is a compliance obligation in many regimes and a survival habit in all of them — and on brownfield sites, verifying the SLD against the installed system is a standard, sobering first task.