Single line diagrams explained: how to read one, what it has to carry, and the work that is done from it
What a single line diagram shows and leaves out, how to read one from the supply down, the elements on it and what each tells you, the levels of single line a site holds, the isolation planning, protection coordination, arc flash studies, maintenance and modifications done from it, and what keeps it true. With an example drawing.
Key takeaways
- A single line diagram shows an electrical system as one line per circuit, from the supply through the switchgear, transformers and buses to the loads, with every device identified and rated. It is the map of how power gets to the equipment and what can interrupt it.
- It is read from the supply downwards. Each bus is a voltage level, each breaker is a point where protection can act and isolation can be made, and each feeder is a drawing reference to follow to the next level of detail.
- A site holds single lines at several levels, from the site key diagram down to each switchboard and MCC, and a schematic is not one of them: a schematic shows every wire for one circuit, the single line shows every circuit with one line.
- Isolation planning, protection coordination, arc flash studies, electrical preventive maintenance, modifications and incident response are all done from it, and each needs the drawing to be current.
- A single line that stops at the project's issued-for-construction revision describes a switchboard that no longer exists, and the first person to find out is the one isolating from it.
An electrician is asked to isolate a conveyor drive. The single line on the switchroom wall shows the feeder breaker, the isolator and the MCC it hangs off, and the isolation is written up from it in a few minutes. Or the single line on the wall is the one issued for construction, the drive was moved to a different cubicle when the VSD was fitted, and the electrician isolates the wrong circuit and finds out when the test-before-touch shows live. The drawing is the same kind of drawing in both cases. The difference is whether anyone kept it true.
The single line diagram is the map of a site's electrical system: one line per circuit, from the supply down through the switchgear, transformers and buses to the loads, with every device identified and rated. This guide sets out what the drawing shows and what it leaves out, how to read one, the elements on it and what each means, the levels of single line a site holds, the work that is done from the drawing and what the drawing has to be for each, and where single lines go wrong. The example drawing below is the crusher branch used across this site. It sits beside the electrical preventive maintenance guide, which covers the equipment the drawing shows.
What the drawing shows, and leaves out
A three-phase system has three phase conductors and often a neutral and an earth, and a drawing that showed all of them for a whole site would be unreadable. The single line draws one line per circuit and trusts the reader to know that the line stands for the set. What it shows is the structure: where the power comes from, what voltage it is at, which devices can interrupt it, how it changes voltage, and where it goes. What it leaves out is everything about any one circuit's wiring, which belongs on that circuit's schematic.
The example above carries what a key single line has to carry. The supply and its fault level, because every breaker's breaking capacity is checked against it. The buses with their voltage and rating. Every breaker identified to match the label on the board, with the protection functions beside it as ANSI device numbers. The transformer with its ratio, rating, vector group and impedance, because the fault studies need them. The earthing. The loads with their ratings. And feeders to other boards with the drawing reference, so the reader knows where to go next. The title block says what the drawing is and that it is as-built.
Reading it
A single line is read from the supply downwards, which on most drawings is top to bottom. Find the incoming supply and its voltage. The first horizontal heavy line is the bus at that voltage, which is a switchboard. Each line leaving the bus is a feeder, and the device on it, a breaker or a fuse, is where that feeder can be interrupted and where protection acts. A feeder to a transformer takes you to the next bus at the next voltage. A feeder to a motor ends at the load. A feeder to another board gives a drawing number and you continue on that drawing.
At each device read two things: its identification, which matches the label on the front of the board, and its rating. A breaker with no identification on the drawing cannot be found on the board, and a breaker with no rating cannot be checked against the fault level, the cable or the load. Protection relays are read by their device numbers: 50 and 51 are overcurrent, 27 is undervoltage, 49 is thermal overload, 87 is differential. The table lists the elements in the order power meets them and what each tells you.
| Element | How it is drawn | What it tells you |
|---|---|---|
| Incoming supply | A circle, an arrow or a labelled line from the utility or the site's generation | Where the power comes from, at what voltage, and the fault level the rest is designed for |
| Circuit breaker | A square box, or an open switch with a cross | A device that can interrupt fault current, controlled by protection. Rated by voltage, continuous current and breaking capacity |
| Isolator or disconnector | An open switch, no cross | A device for isolating, not for breaking load or fault current. Opened after the breaker, locked for work |
| Fuse | A rectangle on the line | Overcurrent protection with no relay; the rating is on the drawing |
| Transformer | Two overlapping circles, or two coils | Changes the voltage; labelled with ratio, rating in kVA or MVA, impedance and vector group |
| Bus | A heavy horizontal line | A common connection point at one voltage: the main switchboard, an MCC, a distribution board. Everything hanging off it is fed from it |
| Bus tie | A breaker between two buses | Lets one incomer feed both sides when the other is out |
| Current and voltage transformers | Small circles on the line, labelled CT and VT | Scale the current and voltage down for the protection relays and the metering |
| Protection relay | A box with the ANSI device numbers | The logic that trips the breaker: 50 and 51 for overcurrent, 87 for differential, 27 for undervoltage and so on |
| Contactor or starter | A box labelled with the starter type | Switches a motor on and off under control; direct on line, star delta, soft starter |
| Variable speed drive | A box labelled VSD, with the motor below it | Controls motor speed; its own protection and its own harmonics |
| Motor | A circle with an M | The load, with its rating in kW and its tag |
| Capacitor bank | Capacitor plates below a breaker | Power factor correction |
| Earthing | The earth symbol, at the transformer neutral and the switchboard | How the system is earthed, which decides fault currents and touch voltages |
| Feeder | A line leaving the bus, labelled with where it goes | A circuit to another board, a drawing reference to follow |
Every device on the drawing has an identification that matches the board and a rating that can be checked. A device with neither is a line, not information.
The levels a site holds
No single drawing shows a site at every level of detail, so a site holds a set: the key single line for the whole site, one per substation or switchboard, one per MCC, schedules for the distribution boards, and the schematics below all of them. Each level references the next, and the references are the only way to navigate. The table sets out the levels and what each is for. The important line is the last one: a schematic is not a single line. Sites that hold schematics and no single lines have the wiring of each circuit and no map of how the circuits relate, which is the map isolation planning needs.
| Level | What it shows | What it is used for |
|---|---|---|
| Site key single line | The whole site on one sheet: the incoming supply, the main substations, the HV distribution and the generation | Fault level and protection studies, the first page in any electrical incident |
| Substation or switchboard single line | One switchboard and everything fed from it, with every breaker, its rating and its protection | Isolation planning, protection settings, arc flash labels, the switching schedule |
| MCC single line | One motor control centre with every starter, its size and its motor | Starter type and rating, motor protection, adding a drive |
| Distribution board schedule | The small power and lighting boards, often as a schedule rather than a drawing | Circuit identification for isolation and testing |
| Schematic | Not a single line: every wire, for one circuit or one starter | Fault finding, modifications, the as-built after a change |
What is done from it
The single line is a working drawing, not a record. The table lists what is done from it and what the drawing has to be for each. Isolation planning needs it current and the identifications matching the board, because the isolation is written against the breaker on the drawing and applied to the breaker on the board. Protection coordination needs the ratings, CT ratios and relay settings, so that a fault on a motor trips the motor's breaker and not the incomer. An arc flash study needs the fault levels and protection clearing times, and its labels go on the boards the drawing shows. Electrical preventive maintenance takes its scope from it: the list of breakers to test and the thermography route are the devices on the drawing.
Modifications are where the drawing most often turns out to be wrong, because a new feeder is designed from the single line and the designer finds the spare way it shows was used two years ago. And incident response is where a wrong drawing costs most: after a trip in the night the single line on the switchroom wall is the first thing read, and it has to say what tripped and what it fed.
| Work | What it needs from the drawing | The drawing has to be |
|---|---|---|
| Isolation planning | Which breaker and isolator to open to make a piece of equipment dead, and what else goes dead with it | Current, with every breaker's identification matching the label on the board |
| Protection coordination | That each device trips before the one upstream for a fault below it, so a motor fault does not drop the board | Ratings, CT ratios, relay types and settings all on the drawing |
| Arc flash study | The incident energy at each board, and the label and PPE that follow from it | Fault levels, cable lengths and sizes, protection settings |
| Load and fault studies | Whether the system can carry the load, start the largest motor and clear a fault | Transformer impedances, cable data, the supply fault level |
| Electrical preventive maintenance | The scope of a switchboard inspection, the list of breakers to test, the thermography route | Every device identified, so the task list can name it |
| Modifications | Where a new drive, feeder or board connects, and what has to change upstream | Current. A modification designed from an old revision is designed for a board that does not exist |
| Commissioning and energisation | The switching sequence to bring a new board live, and the checks before each step | Current, with the protection settings as commissioned |
| Incident response | What tripped, what it fed, and what to check before closing it again | On the wall of the switchroom and current |
Keeping it true
Every change to the electrical system changes the single line: a breaker replaced with a different frame, a feeder added, a drive fitted, protection settings changed, a board extended. Each produces a redline and a reissued revision through management of change, and the change is not closed until the drawing on the switchroom wall is the new one and the old one has been taken down. The document register lists the single lines by the switchboards they show, which is how a change to a board finds the drawings it has to reissue.
Protection settings are the part most often left off. A relay's settings are changed at a coordination study or after a nuisance trip, and if the drawing does not carry them or reference the settings sheet that does, the next study starts from guesses. The convention is to carry the settings on the single line or in a settings schedule the single line references, with the same revision control.
Where single lines go wrong
- The drawing on the wall is the issued-for-construction revision, and the board has changed under it.
- Breaker identifications on the drawing that do not match the labels on the board, so isolation is a matter of interpretation.
- No ratings, so nothing on the drawing can be checked against the fault level or the load.
- Protection settings held in a relay and in someone's laptop, not on the drawing or a schedule it references.
- Feeders to other boards with no drawing reference, so the next level cannot be found.
- A site that holds schematics and no single lines, with the wiring of every circuit and no map of how they relate.
- Two symbol conventions on the same drawing, with no legend.
- A single line kept by the electrical contractor and never handed over, so the site's own copy is a photograph of the wall.
Common questions
What is a single line diagram?
A drawing that represents a three-phase electrical system with a single line for each circuit, showing the source of supply, the switchgear, transformers, buses, protection devices and loads in the order power flows through them, with each device identified and rated. It is also called a one-line diagram. It does not show the individual phase conductors, the neutral or the control wiring, which is what makes it readable: the whole distribution of a site can be followed on a few sheets.
What does a single line diagram show?
The supply and its fault level, each voltage level as a bus, the transformers between them with their ratios and ratings, every circuit breaker, isolator, fuse and contactor with its identification and rating, the current and voltage transformers and the protection relays with their functions, the motors and other loads with their ratings, the earthing arrangement, and feeders to other boards with the drawing reference to follow. On a maintained drawing the protection settings are on it or referenced from it.
What is the difference between a single line diagram and a schematic?
Scope and detail. A single line shows every circuit on a board or a site, one line each, with no control wiring, to show how power is distributed. A schematic shows one circuit, such as a motor starter, in full: every conductor, every contact, every terminal, the control circuit and the protection wiring. The single line is for understanding, planning and isolation. The schematic is for wiring, fault finding and modification of that one circuit.
How do you read a single line diagram?
From the supply downwards. Find the incoming supply and its voltage. Follow the line to the first bus, which is the switchboard at that voltage. Each line leaving the bus is a feeder, through a breaker or a fuse, to a transformer, a motor, a capacitor bank or another board. A transformer takes you to the next bus at the next voltage. A feeder to another board gives you a drawing reference, and you continue on that drawing. At each device read its identification, which matches the label on the board, and its rating.
What is a single line diagram used for?
Isolation planning, which is working out which breaker and isolator make a piece of equipment dead and what else goes dead with it. Protection coordination, so that a fault trips the nearest device and not the whole board. Arc flash studies, which need the fault levels and protection settings the drawing carries. Load and fault studies for a modification. The scope of electrical preventive maintenance. The switching sequence to energise a new board. And incident response, where it is the first page opened after a trip.
What symbols are used on single line diagrams?
Two conventions. IEC 60617 is the international symbol library and is used in Australia and most of the world: a transformer is two circles, a breaker is a square or a switch with a cross, a motor is a circle with an M. IEEE 315 and ANSI conventions are used in North America, with different symbols for some devices and the ANSI device numbers for protection functions, which are also widely used on IEC drawings. Which convention a site uses matters less than using one, consistently, with a legend on the drawing.
Key terms
Plain-language definitions from our glossary for the concepts this article leans on.
Standards and further reading
- IEC 60617 Graphical symbols for diagrams (database) (IEC)
- IEC 61082-1:2014 Preparation of documents used in electrotechnology, part 1: rules (IEC)
- IEC 81346-1:2022 Industrial systems, installations and equipment: structuring principles and reference designations (IEC)
- IEEE 1584-2018 Guide for performing arc-flash hazard calculations (IEEE)
- NFPA 70E Standard for electrical safety in the workplace (NFPA)
- NFPA 70B Standard for electrical equipment maintenance (NFPA)
Related case studies and tools
- Operational readiness for a new plant asset (case study)
Related reading
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