A switchgear single line diagram (SLD) is a simplified drawing that shows how power flows through an electrical system using one line to represent all three phases. To read one, start at the power source at the top, follow the busbar, then read each feeder downwards: every symbol on the way — circuit breakers, disconnectors, earthing switches, current and voltage transformers, protection relays and transformers — tells you how that circuit is switched, measured and protected.
Every electrical project produces a stack of technical drawings. Among them, the single line diagram is the one document that every engineer, technician, project manager, and commissioning team member needs to understand before anything else. It is the map of the electrical system — the document that shows where power comes from, how it flows through the distribution network, what equipment controls and protects each circuit, and where the loads are connected.
And yet, for junior engineers stepping onto their first switchgear project, or maintenance technicians who have worked hands-on with equipment but never spent time interpreting the drawings, a switchgear single line diagram can look intimidating. Circles, squares, diagonal lines, letters, and numbers fill the page with no obvious starting point.
This guide walks through everything you need to read a switchgear single line diagram with confidence — what it shows, how it is structured, what the symbols mean, and how to trace power flow from source to load.
What Is a Single Line Diagram?
Definition and Purpose
A single line diagram — also called a one-line diagram or SLD — is a simplified symbolic representation of a three-phase electrical power system. Rather than drawing all three phase conductors separately, which would make the drawing extremely cluttered and difficult to follow, an SLD uses a single line to represent the three-phase conductors and standardised symbols to represent every component in the system.
The single line diagram is the primary reference document for power system design, protection engineering, load flow studies, fault level calculations, maintenance planning, and operational switching procedures. When an operator needs to perform a switching sequence to isolate a feeder for maintenance, they work from the single line diagram. When a protection engineer is setting relay thresholds, they start from the single line diagram to understand the system topology. When a commissioning team is verifying that the installed equipment matches the design, the single line diagram is the document they check against.
Understanding a single line diagram is therefore not just an academic exercise. It is a practical skill that directly affects how safely and competently you can work with electrical power systems.
Two Types of Single Line Diagrams
There are two types you will encounter on switchgear projects, and they serve different purposes.
The system single line diagram shows the complete power system from the utility supply through all transformation stages, switchgear lineups, distribution boards, and major loads. It gives you the big picture — voltage levels, transformer ratings, bus arrangements, and how the entire facility is connected. This is the document used for power flow studies and fault level analysis.
The equipment single line diagram focuses on a specific piece of equipment — a switchgear lineup, a transformer substation, or a motor control centre. It shows the detailed configuration of that equipment: how each panel is connected, what protection devices are installed, how the busbars are arranged, and what loads are connected to each outgoing circuit. When you are reading a KYN28-12 switchgear lineup drawing, you are typically working with an equipment single line diagram.
How a Single Line Diagram Is Laid Out
Reading Direction
Single line diagrams are read from top to bottom and from left to right, following the direction of power flow. Power enters the diagram at the top — from the utility supply, a generator, or a transformer secondary — and flows downward through the distribution network to the loads at the bottom of the drawing.
This top-to-bottom convention is consistent across IEC and ANSI drawings worldwide, though the specific symbols used differ between the two standard frameworks. Knowing which standard a drawing follows is the first thing to check — look at the drawing title block, which will reference either IEC or ANSI/IEEE standards in the drawing notes or revision history.
Voltage Zones
Transformers divide a single line diagram into voltage zones. When you cross a transformer symbol on the drawing, the voltage changes. Above the transformer symbol you are in one voltage zone — for example 11kV. Below the transformer symbol you are in a lower voltage zone — for example 400V. Each voltage zone is typically drawn as a horizontal band across the page, with all equipment operating at that voltage level shown within that band.
On a switchgear project, the medium voltage zone contains your KYN28-12 incoming feeder panels, bus coupler panels, and outgoing feeder panels. The low voltage zone, shown below the distribution transformers, contains your low voltage distribution boards, motor control centres, and power control centres.
Core Symbols Every Engineer Must Know
The Busbar
The busbar is shown as a heavy horizontal line running across the top of the switchgear lineup representation. It is the common electrical connection point for all panels in the lineup. Every incoming source feeds into this busbar, and every outgoing feeder takes power from it. The current rating and voltage rating of the busbar are typically annotated next to the busbar line.
In systems with two independent busbars — a double busbar arrangement — you will see two parallel horizontal lines with bus section or bus coupler connections between them. This arrangement allows one busbar section to be de-energised for maintenance while the other remains in service, which is critical for facilities that cannot accept a full supply interruption.
Circuit Breakers
A circuit breaker in an SLD is shown as two short diagonal lines crossing a vertical conductor line, forming an X shape. In some drawing conventions, particularly IEC, the circuit breaker is shown as a square or rectangle with a diagonal line through it. The key information annotated next to the circuit breaker symbol includes the rated current, the short-circuit breaking capacity in kiloamperes, and the breaker type designation.
On a medium voltage switchgear drawing, the main breaker in each KYN28-12 panel is shown with this symbol. The draw-out capability of the vacuum circuit breaker may be indicated by a small truck or arrow symbol next to the breaker, distinguishing it from fixed-mounted devices.
Disconnectors and Isolators
Disconnectors — also called isolators or disconnect switches — are shown as a line with a gap and a small angled line indicating the open position of the contact. Disconnectors do not have arc interruption capability and must never be operated under load current. On an SLD, they appear in series with circuit breakers as part of the isolation sequence — the breaker opens to interrupt load current, then the disconnector opens to provide a visible isolation gap for maintenance access.
Earthing Switches
An earthing switch — sometimes called a ground switch — is shown as a disconnector symbol with a connection to the earth symbol, which is a horizontal line with descending lines of decreasing length beneath it. Earthing switches connect a de-energised conductor to earth, providing personal protection for maintenance workers by ensuring that any residual charge is dissipated and that the conductor cannot accidentally become live while work is in progress.
In the five-prevention interlock system of KYN28-12 switchgear, the earthing switch in the cable compartment is mechanically interlocked so it cannot be closed while the cable is live. This interlock relationship is shown on the single line diagram by a dashed mechanical interlock line connecting the earthing switch symbol to the adjacent circuit breaker or isolator symbol.
Fuses
A fuse is shown as a small rectangle inserted in the conductor line, sometimes with the letter F or the fuse rating annotated beside it. In medium voltage systems, current-limiting fuses appear in voltage transformer circuits to protect against fault currents in the instrument transformer secondary. In low voltage distribution, fuses appear as overcurrent protection devices in distribution boards feeding smaller loads.
Transformers
A power transformer is shown as two interlocked circles on the single line diagram, representing the primary and secondary windings. The voltage ratio — for example 11kV/0.4kV — is annotated beside the transformer symbol, along with the rated kVA or MVA, the vector group designation such as Dyn11, and the impedance percentage. The vector group tells the protection engineer how the transformer winding connections affect the behaviour of earth fault currents, which is directly relevant to earth fault protection relay settings.
An instrument transformer — either a current transformer or a voltage transformer — is shown with a smaller version of the same interlocked circle symbol with the appropriate annotation. Current transformers on an SLD are shown in series with the conductor they are measuring, with their ratio annotated. A current transformer marked 200/5A means the primary conductor carries up to 200A and the secondary output to the protection relay is scaled to 5A.
Protection Relays
Protection relays are shown on single line diagrams as rectangles or circles containing the ANSI function number of the protection element they represent. The most common relay function numbers you will encounter on a switchgear SLD are 50 and 51 for instantaneous and time overcurrent protection, 51N or 51G for neutral overcurrent and earth fault protection, 27 for undervoltage, 59 for overvoltage, 87 for differential protection on transformers or busbars, and 79 for auto-reclose on distribution feeders.
Each relay symbol is connected by dashed lines to the current transformer and voltage transformer symbols that feed it measurement signals, and by another dashed line to the circuit breaker it trips. Following these connections on the drawing tells you exactly which measurements each relay uses and which breakers it controls.
Motors and Loads
Major loads on a single line diagram are shown at the bottom of the drawing, connected to outgoing feeder circuits. Motors are represented by a circle with the letter M inside. The motor symbol is typically annotated with the motor rating in kilowatts or horsepower, the supply voltage, and the full load current. In a motor control centre single line diagram, each motor feeder section will show the circuit breaker, contactor, overload relay, and earthing arrangement for that motor as a complete vertical string from the busbar to the motor symbol.
Read More: What Is Metal-Clad Switchgear? Definition, Types & When to Use It
Quick reference for the most common symbols and labels:
| Symbol / label | What it represents | What to look for |
|---|---|---|
| Thick horizontal line | Busbar | Voltage level and bus sections (bus coupler between them) |
| Square box (ANSI) or switch with an ✕ (IEC) | Circuit breaker | Draw-out or fixed, rating, which relay trips it |
| Line with open blade | Disconnector / isolator | Off-load isolation point; interlocked with the breaker |
| Earth symbol with switch | Earthing switch | Which side of the breaker it earths |
| Two overlapping circles | Power transformer | kVA/MVA rating, voltage ratio, vector group |
| Ring or small circle around the conductor | Current transformer (CT) | Ratio (e.g. 400/5 A) and accuracy class |
| Small double-circle symbol connected to the bus or line | Voltage transformer (VT/PT) | Ratio and connection |
| Circle with numbers (50, 51, 87…) | Protection relay function | ANSI/IEEE device numbers (50/51 overcurrent, 87 differential) |
| Circle with M | Motor | kW rating, starter or VFD type |
How to Trace Power Flow Step by Step
Step One: Find the Source
Start at the top of the drawing. Identify the power source — it may be a utility supply shown as a simple annotation with the voltage level, a generator symbol, or a transformer secondary. This is where power enters the system you are reading.
Step Two: Follow the Busbar
From the source, trace the conductor down to the main busbar — the heavy horizontal line. Everything connected to this busbar is at the same voltage. The incoming supply feeds the busbar. All outgoing feeders take their supply from it.
Step Three: Identify Each Feeder
Working across the busbar from left to right, identify each outgoing feeder. Each feeder drops down from the busbar through a circuit breaker, possibly through an isolator and current transformers, to the load or to a downstream distribution panel. The annotation on each feeder tells you where that circuit goes — its load designation, rated current, and cable size may all be shown.
Step Four: Read the Protection
For each feeder, identify the protection relay function numbers shown and understand what they protect against. A feeder with a 51 relay protects against time-delayed overcurrent. A feeder with a 51N relay adds earth fault protection. A transformer differential relay 87T protects the transformer against internal faults. Reading the protection elements on each feeder tells you how the system responds to different fault types.
Step Five: Check the Interlock Relationships
Look for dashed mechanical interlock lines connecting switching devices. These show which devices are mechanically or electrically prevented from operating simultaneously. Understanding the interlock relationships is essential before performing any switching operation, because it tells you what sequence of operations is required to achieve a particular system configuration safely.
Common Mistakes When Reading an SLD
Assuming Symbols Are Universal
They are not. IEC symbols and ANSI symbols for the same component look different. A circuit breaker symbol under IEC 60617 looks different from the same device under ANSI/IEEE standard symbols. Always check the drawing legend before you start reading, and confirm which standard set the drawing follows.
Ignoring the Drawing Revision Number
Power systems change. Equipment gets added, protection settings get revised, and bus arrangements get modified after the original design. A single line diagram with a revision date from five years ago may not reflect the current system configuration. Always verify that you are working from the current issued-for-construction or as-built revision before using an SLD for any operational decision.
Confusing Single Line with Wiring Diagrams
A single line diagram shows system topology and component relationships — it does not show actual wiring connections between terminals. When you need to trace a specific wire between a current transformer and a relay terminal, you need the secondary wiring diagram, not the SLD. The two drawing types work together but serve different purposes.
Using a Switchgear Single Line Diagram on a Project
On a KYN28-12 switchgear project, the equipment single line diagram is one of the core documents supplied by the manufacturer with every panel lineup. At Danli, our documentation package includes a complete equipment single line diagram showing the exact configuration of every panel in the lineup — circuit breaker types and ratings, current transformer ratios, voltage transformer connections, protection relay function numbers, busbar current ratings, and earthing switch positions.
This drawing is the foundation document for the protection engineer setting relay parameters, for the commissioning team verifying that the installed equipment matches the design intent, for the operations team building switching procedures, and for the maintenance team planning future outage work. Getting comfortable reading it is one of the most practical skills anyone working on medium voltage switchgear projects can develop.
Frequently Asked Questions
A single line diagram shows the power system at a high level — sources, buses, breakers and loads — with one line per circuit. A wiring diagram shows every individual wire, terminal and control connection inside the equipment.
Because one line represents all three phases of an AC circuit. This keeps large power systems readable on one sheet.
They are ANSI/IEEE device function numbers for protection relays: 50 is instantaneous overcurrent, 51 is time-delayed overcurrent, and 87 is differential protection.
On the approved single line diagram, plus the panel list, ratings, short-circuit level and protection requirements. A clear SLD is the single most important document for an accurate switchgear quote.
