Quick Facts
What an SLD is
A Single-Line Diagram (SLD) is the schematic electrical drawing that shows the layout of a solar plant’s electrical components in simplified single-line form. The “single line” name comes from the convention of representing each three-phase circuit as a single line on the diagram, rather than showing all three phases separately.
SLDs are the standard documentation format for electrical plants of all sizes. For solar:
Residential 5 kW system: SLD shows panels, inverter, ACDB, building panel, DISCOM meter.
Commercial 100 kW system: SLD shows multiple strings, SCBs, DCDB, inverter, ACDB, meters, and grid connection.
Utility-scale 50 MW plant: SLD shows the entire electrical system from arrays to grid interconnection at the substation.
For all sizes, the SLD is the foundational document that summarises the plant’s electrical architecture. It enables design review, regulatory approval, lender diligence, contractor coordination, and ongoing maintenance.
What an SLD contains
A solar SLD typically includes:
Solar arrays: Strings of panels with count and kWp rating.
DC side components: String Combiner Boxes (SCBs) with fuse ratings, DC Distribution Boxes (DCDBs), DC isolators, surge protection devices.
Inverters: With kW rating, MPPT count, and key specifications.
AC side components: AC Distribution Boxes (ACDBs) with MCB ratings, RCDs, surge protection devices.
Transformers: For HT installations, the step-up transformer rating and configuration.
Switchgear: Circuit breakers, isolators, contactors with their ratings.
Energy meters: Net-metering meter, ABT meter (if applicable), trivector meter.
Grid connection: The interconnection point to the DISCOM grid, with voltage level and capacity.
Earthing system: Earth pits, conductor sizes, equipotential bonding.
Lightning protection: Air terminals, down conductors, surge protection devices.
Cables: Major DC and AC cables with conductor size, length, and material.
Each component is shown with standard symbols (per IEEE Std 315 or IEC 60617) and labelled with its key specifications.
Why SLDs matter for solar
DISCOM net-metering applications: Every solar plant connecting to the grid requires DISCOM approval. The SLD is part of the application documentation, showing the DISCOM how the plant connects to the consumer’s system and the grid.
Plant design review: Designers and reviewers use the SLD to verify the electrical architecture, protection coordination, and code compliance.
Lender’s diligence: Project-financed solar projects undergo technical diligence. The SLD is a key document for the lender’s technical advisor.
Electrical contractor coordination: EPC contractors use the SLD to coordinate field work, ensuring all components are installed and connected correctly.
Field installation guidance: Field electricians refer to the SLD to understand the system’s structure before working on specific connections.
Maintenance and troubleshooting: Years after commissioning, the SLD remains useful for understanding the plant’s architecture and locating components.
Safety: First responders and emergency personnel can quickly understand the plant’s electrical layout from the SLD.
For all these purposes, an accurate, current, and properly drafted SLD is essential.
SLD versus other diagrams
Single-Line Diagram (SLD): Schematic of overall electrical architecture. Simplified single-line representation.
Three-Line Diagram: Detailed three-phase representation, showing all three phases. Used for specific protection or control circuits.
Wiring Diagram: Shows detailed cable connections with terminal numbers. Used for actual installation.
Layout Drawing: Shows physical placement of equipment in space. Different from SLD’s logical arrangement.
Schematic Diagram: General term, sometimes used interchangeably with SLD.
For solar plants, the SLD is typically the primary high-level document. Detailed wiring diagrams complement the SLD for installation work.
SLD creation tools
For SLD creation, designers use various tools:
AutoCAD Electrical: Industry-standard CAD with electrical-specific symbols and intelligence.
EPLAN: Premium European-origin electrical CAD.
PowerCAD: India-developed CAD for power systems.
Lucid Chart: Web-based diagramming tool. Good for simple SLDs.
draw.io: Free web-based diagramming tool. Used for basic SLDs.
Microsoft Visio: Office-suite drawing tool with electrical templates.
Specific software like ETAP or PSS/E for detailed power system analysis.
For most Indian solar EPC contractors, AutoCAD Electrical is the standard. Smaller installations may use simpler tools.
SLD format and content
Standard SLD format conventions:
Title block: Project name, client, date, revision, designer signature.
Symbol legend: Standard symbols used in the diagram.
Components labelled: Each component shows model, rating, and identifier.
Bus arrangement: Clear bus structure with voltage levels indicated.
Phasing: Single-line representation; phase information in notes.
Protection: Circuit breakers, fuses, relays with their ratings.
Grounding: Earthing system shown clearly.
Notes: Important design notes, assumptions, references.
Drawing scale and orientation: Standard sheet sizes (A1, A3) with title block.
For Indian DISCOM applications, specific formats are often required by the DISCOM. EPC contractors typically have templates aligned with major DISCOM requirements.
Common SLD mistakes
Incomplete information: Missing component ratings, missing labels.
Outdated revisions: Not updating SLD after design changes.
Inconsistent symbols: Mixing different symbol conventions.
Missing protection details: Circuit breakers or fuses not labelled with ratings.
Poor legibility: Cramped layouts, small text, illegible to reviewers.
No revision history: Not tracking changes to the SLD over the plant’s life.
Mismatched physical reality: SLD doesn’t match the as-built plant.
For DISCOM acceptance and ongoing usefulness, SLD quality matters.
Best practices
For EPC contractors, develop SLD templates aligned with major DISCOM requirements.
Update SLDs throughout the project lifecycle: design, construction, commissioning, modifications.
Maintain “as-built” SLDs after commissioning that reflect actual installed equipment.
Use standard symbols (IEEE Std 315 or IEC 60617) consistently.
Label all components clearly with their key specifications.
Include adequate protection details and grounding information.
Get SLDs reviewed by experienced electrical engineers before submission.
Archive SLDs throughout the plant’s life as part of the plant’s documentation.
Standards and references
SLD symbols follow IEEE Std 315 (American) or IEC 60617 (international). Indian standards typically reference both. CEA Connectivity Regulations 2019 specify documentation requirements for grid interconnection. State DISCOM net-metering procedures detail SLD-related submissions.
Related glossary terms
Key takeaways
A Single-Line Diagram (SLD) is the schematic electrical drawing that shows a solar plant’s electrical architecture in simplified single-line form. SLDs are essential for DISCOM net-metering applications, design review, lender’s diligence, and field installation. The SLD shows solar arrays, combiner boxes, distribution boxes, inverters, switchgear, meters, and grid connection with standard symbols and component ratings. For Indian solar projects of all sizes (residential to utility-scale), SLDs are required documentation, maintained throughout the plant’s life as “as-built” drawings.