Single line diagram: how to read an electrical one-line diagram
Deskely Completions team Singapore
A single line diagram (SLD), also called a one-line diagram, shows how electrical power flows through a system, from the source to every load, with one line standing for all three phases of each circuit. It is the first drawing anyone opens to understand a power system, and the drawing the energisation of every switchboard is planned from.
This guide covers what an SLD shows, how to read one from top to bottom, the ratings written beside each symbol, typical substation, solar PV and data centre one-lines, and how an SLD differs from a three-line, schematic or wiring diagram.
- An SLD draws a three-phase system with one line per circuit, so the whole system fits on one sheet.
- Read it from the source down: utility or generator, transformers, busbars, breakers, feeders, loads.
- The ratings beside each symbol (kV, kVA, impedance, vector group, kA, CT ratios) carry as much as the symbols.
- It is the master drawing for protection studies, switching, isolation and the energisation sequence.
What is a single line diagram?
A single line diagram is a simplified drawing of an electrical power system. Each circuit is drawn as one line, even though it carries three phases (and often a neutral and an earth), and each piece of equipment is drawn as a standard symbol: generators, transformers, circuit breakers, disconnectors, fuses, current and voltage transformers, protective relays, busbars and loads.
The point is overview. A three-phase drawing of a whole plant would be unreadable; the one-line shows how power gets from each source to each load, what can be switched where, and how the system is protected, on a handful of sheets.
The names mean the same thing: single line diagram, one-line diagram, SLD, OLD, and in some countries line diagram or key single line. Symbols follow IEEE 315 (ANSI) in North America and IEC 60617 elsewhere. See the single line diagram symbols library for every symbol in both forms.
How to read a single line diagram
- 01Find the sources
Start at the top: the utility incomer, the main generators or the emergency generator. Note each source's voltage, frequency and short-circuit level.
- 02Follow the transformers
Each transformer steps the voltage down (or up). Read its rating: power (kVA or MVA), voltages, vector group and impedance.
- 03Read the busbars
The thick horizontal lines are busbars: each one is a switchboard or switchgear section, labelled with its tag, voltage and rating. Bus couplers or bus ties show how sections can be joined.
- 04Trace the switching devices
On every connection, the circuit breaker, disconnector, contactor or fuse shows where the circuit can be opened and what can interrupt a fault.
- 05Read the protection
Current transformers and voltage transformers feed protective relays, marked with their ANSI device numbers: 50/51 overcurrent, 87 differential, 27 undervoltage and so on.
- 06Follow each feeder to its load
At the bottom are the loads: motors, drives, distribution boards, UPS systems, heaters. Each feeder carries its cable size and the load's rating.
- 07Check the notes
The legend, normal switch positions (normally open or closed), interlocks and references to the schematic and protection drawings are usually in the notes and title block.
Ratings and labels on an SLD
Most of what an SLD tells you is written beside the symbols, not in them:
| Item | What is shown | Example |
|---|---|---|
| Transformer | Rated power, primary and secondary voltage, vector group, percentage impedance, cooling | 2,500 kVA, 11/0.415 kV, Dyn11, 6 %, ONAN |
| Busbar or switchboard | Tag, voltage, rated current, short-time withstand | SWBD-01, 690 V, 4,000 A, 65 kA 1 s |
| Circuit breaker | Rated current, breaking capacity, type, trip unit | 1,250 A, 50 kA, air circuit breaker |
| Current transformer | Ratio, accuracy class and burden | 1,200/1 A, 5P20, 15 VA |
| Protective relay | ANSI device numbers of its functions | 50/51, 50N/51N, 87T |
| Cable | Number of runs, cores and cross section | 3 x (1 x 240 mm²) per phase |
| Motor or load | Power, voltage, starting method | 250 kW, 6.6 kV, DOL |
| Generator | Rated power, voltage, power factor, earthing | 5 MVA, 11 kV, 0.8 pf, NGR earthed |
A vector group such as Dyn11 reads as: delta-connected high-voltage winding (D), star-connected low-voltage winding (y) with the neutral brought out (n), and the low-voltage side displaced by 30 degrees from the high-voltage side (clock position 11). The percentage impedance sets how much fault current the transformer lets through, which is why it matters for the breaker ratings downstream.
Substation single line diagrams
A substation one-line shows the incoming and outgoing lines or cables, the busbar arrangement, the power transformers, and the switching, protection and metering on each bay. The busbar arrangement decides how much of the substation can stay in service when one item is out:
| Arrangement | How it works | Where it is used |
|---|---|---|
| Single busbar | All circuits on one bus, sometimes split by a bus section breaker | Distribution substations, plant switchboards |
| Double busbar | Each circuit can be switched to either of two buses | Larger transmission and industrial substations |
| Ring bus | Breakers connected in a loop, circuits between them | Transmission substations with few circuits |
| Breaker and a half | Three breakers for every two circuits between two buses | High-voltage transmission substations |
Solar PV and data centre single line diagrams
Solar PV. A PV plant one-line runs from the PV strings to combiner boxes, then to the inverters (DC to AC), the step-up transformers, the medium-voltage switchgear and the point of interconnection with its metering and protection. Behind-the-meter systems show the connection to the building's main switchboard instead.
Data centres. A data centre one-line typically shows two utility feeds, medium-voltage switchgear, transformers, low-voltage switchboards, standby generators with automatic transfer switches, UPS systems and the distribution down to the PDUs. The redundancy scheme (N+1, 2N and so on) is visible on the drawing: every critical load traces back to two independent paths.
Offshore and industrial plants. FPSO and process plant one-lines start at the main generators, run through the main switchboard to the medium- and low-voltage distribution, and show the essential and emergency switchboards with the emergency generator and its interlocks.
SLD vs three-line, schematic and wiring diagrams
| Drawing | What it shows | Used for |
|---|---|---|
| Single line diagram | The whole system, one line per circuit, equipment and ratings | Overview, studies, switching and isolation planning, energisation |
| Three-line diagram | All three phases drawn separately, with CT and VT connections | Protection and metering connections, phasing |
| Schematic (elementary) diagram | Control and protection circuits, contacts, coils and relays | How a breaker trips and closes, interlocks, fault finding |
| Wiring or connection diagram | Every wire and terminal, as it is built | Construction, termination checks, loop and circuit checks |
| Cable schedule | Every cable with its from, to, type and size | Cable pulling, termination and test records |
What the SLD is used for
- Studies: load flow, short circuit, protection coordination and arc flash studies are all built on the one-line.
- Equipment specification: switchgear, transformer and cable ratings come from it.
- Isolation and switching: lockout and switching programmes are planned on it, and it is posted in substations and switchrooms.
- Energisation: the order in which switchboards are energised during commissioning is planned and tracked on it.
- Testing: every breaker, relay, CT and transformer on the one-line is a tag with its own test records before it is energised.
Single line diagrams in commissioning
Before a switchboard is energised, every item upstream and on it needs its construction checks and tests done: insulation resistance tests on cables, busbars and transformers, CT ratio and polarity tests, relay testing to the protection settings, breaker timing and function tests, and the earthing checks. Each is an ITR against the tag, and open A punch items on any of them hold the energisation back.
On a large project the one-line is where the electrical commissioning team marks progress: which boards are energised, which feeders are live, and which are still waiting on tests. The same picture comes out of the completions system when every record is held against the tag on the drawing.
In Deskely, every electrical tag on the one-line carries its test records, punch items and certificates, so the team can see what still stands between each board and energisation.
The Deskely products for this
Frequently asked questions
What is a single line diagram?
A simplified drawing of an electrical power system in which each three-phase circuit is drawn as one line and equipment is shown with standard symbols and ratings. It shows how power flows from each source to each load.
Is a one-line diagram the same as a single line diagram?
Yes. One-line diagram, single line diagram and SLD are the same drawing. One-line is more common in North America.
How do you read a single line diagram?
Start at the source at the top, follow the power through transformers and busbars, note the breakers and protection on each connection, and follow each feeder to its load. Read the ratings beside each symbol as you go.
What is the difference between a single line and a three-line diagram?
A single line diagram draws each three-phase circuit as one line for an overview. A three-line diagram draws every phase, with the CT and VT connections, for protection and metering detail.
What does Dyn11 mean on a single line diagram?
A transformer with a delta high-voltage winding, a star low-voltage winding with the neutral brought out, and a 30 degree phase displacement between them (clock position 11).
What symbols are used on a single line diagram?
IEEE 315 (ANSI) symbols in North America and IEC 60617 symbols elsewhere, for breakers, disconnectors, fuses, transformers, CTs, VTs, relays, busbars, generators, motors and loads.
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