A basic single-line diagram explains the power network and equipment connections. A protection single-line diagram adds the protection functions and their relationship to instrument transformers and switching equipment. Detailed relay settings, trip logic and wiring remain controlled companion documents.
Two views of the same electrical system
Both drawings simplify a three-phase network so the reader can understand its arrangement. Their level of detail follows their purpose and the project drawing standard. Agree that purpose before preparing the drawing.
| Aspect | Basic SLD | Protection SLD |
|---|---|---|
| Main question | How is power distributed? | How is the network protected? |
| Typical content | Sources, buses, transformers, breakers and feeders | Protection functions, CT/VT inputs and associated breakers |
| Key interfaces | Load list, equipment ratings and network studies | Protection philosophy, studies, relay configuration and trip arrangements |
| Companion documents | Equipment schedules and layout drawings | Settings, logic, wiring and commissioning records |
What belongs in a basic SLD?
Show the agreed network topology and tag equipment consistently with the project schedules. Depending on the drawing purpose, include voltage levels, ratings, normally open points and essential equipment information. Use a clear legend and cross-references for information issued elsewhere.
The drawing should help the reader navigate the system without forcing every cable detail or design calculation into one sheet.
What belongs in a protection SLD?
Show how the selected protection functions relate to the protected equipment and measurement inputs. Identify the relevant relays, CTs, VTs and breakers at the level required by the project standard.
Numerical settings, detailed logic and terminal wiring should have clear references to their controlled documents. A protection SLD alone does not establish that the settings have been calculated, approved or proven by testing.
From design basis to commissioning
- Establish inputs: network arrangement, operating modes, equipment data and utility requirements.
- Develop the philosophy: agree the protection functions and interfaces.
- Coordinate the studies: use the relevant network and protection coordination studies to support decisions.
- Issue a consistent package: drawings, settings, logic and wiring must refer to the same equipment and revision basis.
- Verify implementation: test the issued arrangements and retain the commissioning records.
Check before releasing the drawing
- Do tags and ratings match the latest equipment schedules?
- Are the operating configurations and normally open points clear?
- Are CT/VT references and protection function labels consistent?
- Can a reviewer find the related settings, logic and wiring documents?
- Are open inputs and approval status visible?
Common questions
Does every protection SLD show all settings? No. The drawing convention and purpose determine the detail. Settings should be controlled and traceable, whether shown or referenced.
Can one drawing serve both purposes? Sometimes. Confirm that the combined sheet remains readable and satisfies the project deliverables.
Is the image an approved electrical diagram? No. The cover is an illustrative engineering scene; the graphics on its monitor are conceptual.
Technical references
- SEL — Successfully Designing Integrated Substations, discussion of single-line and supporting communications diagrams.
- SEL-351 documentation, separate manuals and settings sheets illustrating the role of companion records.
Build a consistent drawing and study package
Send the project location, equipment data and available drawings. We’ll help define the engineering scope and required deliverables.
