In short. The single-line diagram (SLD) is the master map of the electrical system. Every study, protection setting and operating procedure depends on it, so it must show sources, transformers, switchgear, cables, loads and protection accurately, with ratings, and must match what is installed.
What an SLD must show
| Item | Details to include |
|---|---|
| Sources | Utility supply (voltage, fault level), generators (kVA, kW, voltage, reactances), UPS, BESS |
| Transformers | kVA, voltage ratio, vector group, impedance, tap range, earthing |
| Switchgear | Breaker type and rating, breaking capacity, busbar rating, bus labels |
| Cables and busducts | Size, length, type, number of runs |
| Loads | Motors (kW), static loads, UPS loads |
| Protection | Relay functions, CT and VT ratios, settings references |
| Earthing | Neutral connections, earthing resistors |
| Metering and controls | Key meters, interlocks, transfer devices |
Operating modes
A single drawing can show only one switching state, so list the normal and alternative states: normal, generator supply, maintenance, tie closed, parallel. Studies need to be run for each, and the SLD should make the states clear with breaker status tables or separate versions.
Drawing practice
- Use consistent symbols and a legend.
- Flow power from sources at the top to loads at the bottom.
- Label every bus and device with unique tags.
- Keep one voltage level per band, with transformers marking each step.
- Show ratings in a table or on the drawing, not only in notes.
Making it a valid study base
- Walk the site and compare the drawing to the installation.
- Check nameplates against listed ratings.
- Confirm cable data and lengths.
- Confirm protection devices and settings.
- Mark unverified items and list them as assumptions.
- Issue with revision number, date and author.
Revision control
Update the SLD whenever equipment is added, replaced or reconfigured. Out-of-date SLDs are a common cause of wrong settings, unsafe switching and study errors. Keep a log of changes.
Illustrative example
A site SLD shows a 1,000 kVA generator feeding a main board, but the site survey finds a second generator added years earlier and a tie that is not shown. A short-circuit study based on the old drawing would have underestimated fault level. Updating the SLD changes breaker ratings required at the board.
What a study-grade single-line diagram carries
A drawing that suits an installer is not automatically one that suits a study engineer. For studies the diagram must carry data, not only symbols.
| Element | Data to show |
|---|---|
| Utility incomer | Voltage, fault level (MVA or kA), X/R ratio |
| Transformers | kVA, voltage ratio, impedance percent, vector group, tap range, neutral earthing |
| Generators | kVA, kW, power factor, subtransient and transient reactance, neutral earthing |
| Breakers | Rated current, breaking capacity, normal open or closed state |
| Cables | Size, length, material, number in parallel |
| Loads | kW, kVA, motor starting method, UPS and nonlinear loads |
| CTs, VTs, relays | Ratios and protection functions |
Operating states must be drawn
Show the normal state and every credible alternative: mains supply, generator supply, bus tie closed, one transformer out, parallel operation. Protection and fault levels change with each.
Verification before study
- Walk the site and compare the drawing with what is installed.
- Confirm nameplates against drawing values.
- Mark unknowns clearly and assume conservatively until confirmed.
- Issue the drawing with a revision number and date.
Common defects
- Added loads not on the drawing.
- Tie breaker shown closed when it is operated open.
- Missing neutral earthing information.
See load list and brownfield study.
Common mistakes
- Drawings that show design intent, not as-built state.
- No ratings on the diagram.
- Only the normal state is documented.
What to send us
Send any existing diagrams, photographs and nameplate data. Send your application details. See also brownfield study.
Questions engineers ask
Why does a “single line” show three phases?
One line represents all three phases for clarity. Details appear in notes and schedules.
How accurate must it be?
Accurate enough that studies and switching procedures are safe. Verify against the site.
Who owns the SLD?
Usually the facility owner or operator, with a controlled revision process.
Do I need separate SLDs for each mode?
Not necessarily, but each mode must be clearly defined and studied.
This guide gives general principles. Drawings must reflect the actual installation.