What is a single-line diagram
A Single-Line Diagram (SLD) is a simplified graphical representation of an electrical network where all three phases are shown as a single line. It is the main document of any electrical supply project — from an apartment to a factory.
An SLD shows:
- Power source (transformer, generator, utility feed)
- Transmission lines and cable lines
- Switching devices (breakers, disconnectors, contactors)
- Protective devices (RCDs, fuses, relays)
- Measuring instruments (meters, current transformers)
- Busbars and distribution panels
- Outgoing circuits with consumer labels
Once you learn to read an SLD, you can understand the electrical structure of any facility in 2–3 minutes.
Where single-line diagrams are used
- Electrical design projects for residential, commercial, industrial buildings
- Main switchboard (MSB) passport — on the inside of every apartment building panel door
- Substation auxiliary panels
- Grid connection agreements
- Reconstruction and modernization projects
Symbols: main elements
Per IEC 60617 and IEEE 315 standards:
Switching devices
- Circuit breaker (MCB/MCCB) — code QF. Drawn as a line break with a diagonal stroke (trip unit).
- RCD / RCBO — code QFD. Added differential transformer symbol (oval or I∆).
- Disconnector / Isolator — code QS. Line break without the diagonal stroke (no trip unit).
- Contactor — code KM. Line break with a semicircle (electromagnetic coil).
Measuring devices
- Current Transformer (CT) — code TA. Two intersecting circles on the line. E.g., CT 300/5: 300 A primary = 5 A secondary.
- Voltage Transformer (VT) — code TV.
- Energy meter — code PI. Rectangle with an arrow or Wh (watt-hours).
- Ammeter, Voltmeter — codes PA, PV. Circle with letter A or V.
Other elements
- Busbar — thickened horizontal or vertical line. May be labeled: BB1 (Section I), BB2 (Section II).
- Cable line — thin line with marking: "XLPE 5×10 mm², L=45 m."
- Power transformer — code T. Two intersecting circles on the line.
- Surge arrester (SPD) — code FV. Triangle (varistor) on the line.
Step-by-step: analyzing a residential MSB diagram
Let's analyze a real Main Switchboard (MSB) diagram for a 9-story residential building.
Step 1: From utility feed to main breaker
Look at the top-left corner. Cable from the transformer station: marked "AL armour 4×150 mm², L=120 m" — aluminum armored cable, 4 cores × 150 mm².
Next: disconnector QS1 (400 A) — for visible isolation during maintenance. Then: main circuit breaker QF1 (C400, 400 A) with electronic trip unit.
Step 2: Metering section
After the main breaker: three CTs TA1-TA3 (400/5, class 0.5S). Signal wires go to energy meter PI1 — utility revenue metering.
Step 3: Busbar sectioning
Busbars split into two sections: BB1 (Section I) and BB2 (Section II). Bus-tie breaker QF3 (C250) between them — normally open (N.O.). On loss of one supply, the bus-tie closes.
Step 4: Outgoing circuits
Circuits branching down from the busbars:
- QF4 (C63) → Staircase 1 panel: "FRLS 5×35 mm², L=18 m" (fire-resistant cable to the staircase panel)
- QF5 (C25) → Elevator 1: "5×6 mm², L=55 m" with Type B RCD 30 mA
- QF6 (C40) → Common area lighting panel: "5×10 mm², L=25 m"
- QF7 (C16) → Emergency lighting panel: with ATS (Automatic Transfer Switch)
Step 5: Selectivity check
Chain: QF1 (C400) → QF4 (C63). During a fault in the staircase panel, QF4 (C63) trips first; QF1 (C400) stays on — selectivity achieved. Rating ratio 400/63 ≈ 6.3 — well above the 1.6 minimum.
Common reading mistakes
- Confusing disconnector and breaker: a disconnector (QS) cannot interrupt fault current. If there's no breaker/fuse after a disconnector — that's a design error.
- Ignoring cable type: "XLPE 3×2.5" and "FRLS 3×2.5" are different cables. Fire-resistant (FRLS) is mandatory for escape routes.
- CT/breaker mismatch: a 150/5 CT before a 250 A breaker — the CT will be damaged on overload.
- Missing RCDs: the diagram must clearly show which circuits are RCD-protected. Lighting — typically without RCD; outlets — with 30 mA RCD.
Standards reference
- IEC 60617 — graphical symbols for diagrams
- IEC 61082 — preparation of documents in electrotechnology
- IEEE 315 — graphic symbols for electrical diagrams
- IS 2032 — Indian standard for graphical symbols
How GorkyCAD simplifies reading and creating SLDs
In GorkyCAD, the single-line diagram is generated automatically from the project model. All elements are labeled: codes (QF, QS, TA, PI), ratings, cable types, lengths. Hover over any element for a tooltip with parameters. No guessing what a symbol means — the system explains everything.