How to read single-line diagrams: complete guide

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.