Need a site single-line diagram?
Wondering how to draw a circuit diagram?
Then this article is for you!
Inside I will answer these questions about single-line diagrams and more!
- What is a schematic diagram?
- Why do you need it?
- How to draw a circuit diagram?
- Which electrical symbols do you use?
- What info do you need to include?
- Or maybe you’re just interested in how to read electrical blueprints…
What is a single line diagram?
A single-line diagram (SLD) is a high-level schematic diagram showing how incoming power is distributed to equipment.
A4.1.1 Single-Line (One-Line) Diagram: A diagram which shows, by means of single lines and graphic symbols, the course of an electric circuit or system of circuits and the component devices or parts used therein.
Having a ‘single-line’ allows the diagram to stay readable despite communicating a lot of information about an electrical system.
This diagram is the primary reference for maintenance and operations for lockout/tagout procedures, as well as for any engineering power system studies.
In this post, I will show why you need it and how to make it.
Why do I need a single-line diagram?
There are two main reasons you need it:
For everyday operations and maintenance, as well as engineering power system studies.
Both require that the diagram be kept up-to-date and available.
Operations and maintenance
To plan your lockout/tagout procedures, you need up-to-date primary sources of information.
“184.108.40.206 Lockout procedure
A lockout procedure shall be developed on the basis of the existing electrical equipment and system and shall use suitable documentation, including up-to-date drawings and diagrams.” — CSA Z462
SLDs help verify electrical circuit interlocks will not result in the re-energization of the circuit being worked on.
“220.127.116.11 Electrical circuit interlocks
Suitable documentation, including up-to-date drawings and diagrams, shall be consulted to ensure that no electrical circuit interlock operation can result in re-energizing the circuit being worked on.” — CSA Z462
Use up-to-date diagrams in establishing an electrically safe work condition.
“4.2.5 Process for establishing and verifying an electrically safe work condition
a) Determine all possible sources of electrical supply to the specific equipment. Check applicable up-to-date drawings, diagrams, and identification tags.” — CSA Z462
Having an up-to-date SLD can help avoid longer downtimes and keep everyone safe.
Power system studies
Having an up-to-date SLD is required to complete a power system study.
“6.12.3 Power system studies and single line diagram
Power system studies and one-line drawings are critical to the safe and reliable operation of electrical power systems. The studies and drawings shall be readily available and maintained on a consistent basis.
A main program shall include the continual upkeep and review of the following power system studies and drawings:
- One-line diagrams;
- Short-circuit studies;
- Coordination study;
- Arc flash incident energy study; and
- Load flow study.”
The information on the SLD can be used for different types of power system studies for your site.
- Short circuit study to ensure equipment can withstand a fault.
- Coordination study to ensure the right devices are tripped in time.
- Incident energy study to know the arc flash hazard levels on equipment.
- Load flow study to know the continuous current through the system.
CSA Z463 - Maintenance of Electrical Systems, recommends a single-line diagram be reviewed after 5 years, or when there has been a significant change.
A significant change could be:
- A new installation or system modification
- Change in utility or source
- Change in system impedance, configuration, or loading
- Change in protection devices or settings
How to draw electrical single line diagrams
Ideally, you should not have to draw your own single line diagram.
There would have been a drawing made for the design of the site or one made for a new project.
But maybe you cannot find it or there have been so many untracked changes it isn’t any good.
If you are working on a new SLD, the equipment itself is the best source for data.
Getting the connections right between equipment is the most important part of the diagram.
Between looking at your equipment tags and nameplates you should be able to make all the necessary updates.
Wiring diagram symbols
To start you should know what symbols to use to represent your equipment.
The source for standardized electrical diagram symbols comes from the document IEEE Std 315, ANSI Y32.9, CSA Z99.
Here are the most used symbols you will need to start drawing your system.
Next, I will go through each symbol and look at symbol variations and data to include in your electrical schematic.
|The utility or AC
symbol is used to
show where power
is coming from.
1. Incoming voltage
2. Fault level and impedance (optional)
|These symbols could
an AC generator.
4. # of Phases
transformers can be
either of these symbols
1. connection type (Δ, Y)
4. %Z Impedance
|These are the Delta
and Wye connection
can be added in.
symbols can be used
to represent a disconnect
or transfer switch.
|Fuses can be
either graphic symbol.
Low voltage circuit
1. Rated amperage
3. Trip settings (optional)
symbols, with the
a draw-out type.
1. Rated amperage
|This is the motor
symbol, one with a M
and one with a delta
|1. Power (HP)|
|These symbols show
a current transformer (CT)
above and a potential
on the bottom.
|1. Turns ratio|
|This is the relay symbol
attached to a CT
1. Function number
2. Instrumentation connection
Electrical one line diagram design
There are a few things that make a single-line diagram special and help to keep it readable.
- Remember that you are using a single line to represent multiple conductors.
- Diagrams start at the top of the page with the incoming source of a system’s power.
- Electrical symbols are typically fed from the top and feed from the bottom.
- There is no physical location or size represented of the electrical equipment.
Apart from lots of symbols, the standard also makes note of some drafting practices:
|Orientation: Alterations of the orientation
of the symbol do not alter its meaning.
|Line Width: The line width does not change
the meaning but may be used for emphasis.
|Enlargement or Reduction: There is no meaning
associated with different symbol sizes.
Terminal symbol (o): This symbol can be
You might also wonder how much of the site to include on the diagram.
A common level of detail to stop at is once you have included all the distribution equipment.
This means once you have all your panelboards and MCCs you can transition to using equipment schedules in combination with a single line diagram.
It is also possible to build supporting documents to include more detailed information on equipment and keep the electrical diagram itself readable.
Connect the schematic symbols
To start, connect your electrical symbols using one line.
You can use a horizontal line to indicate a piece of distribution equipment such as switchgear, MCC, splitter, or panel.
You will notice that this diagram is missing the cables, these can be added in without a symbol using some arrows and annotation.
Indicate equipment separation
You can also group symbols using a dash-dotted box to indicate they are part of one piece of equipment.
Here notation was added for the cables using a loop with a line to point to cable data, and dash-dotted boxes for equipment enclosed together.
This shows there are three main parts: an incoming fused disconnect, a transformer, and the main switchgear.
This info is easy to indicate and can be very useful in determining how something should be de-energized.
Add equipment data
Once you have all the symbols and connections figured out you can start adding the equipment info.
Here I have added in the equipment data that is typically found on a wiring diagram.
The amount of information added can vary depending on what is being used for.
Electrical equipment information
The single-line drawing provides the blueprint for communicating different types of information about a power system.
The most important information to include is:
- Incoming service voltage
- Equipment rated current
- Identification names of equipment
- Bus voltage, frequency, phases, and short circuit current withstand ratings
- Cable sizes, #of cables, and lengths
- Transformer connection type, kVA, voltages, and impedance
- Generator voltage and kW
- Motor HP
- Current and Voltage ratios of instrument transformers
- Relay device numbers
Looking at our diagram again we can separate the info into voltage, amperage, and impedance to make sense of what is included for each piece of equipment.
Most of this can be found on the equipment nameplates.
The incoming voltage is 12.47 kV and is down to the transformer primary.
The transformer steps the voltage down to 600 V.
From the transformer secondary to the switchboard is 600 V.
The disconnect, fuse, cables, and circuit breaker do not have voltages shown but should be rated for the associated voltages.
The current rating is the maximum amount of continuous current a piece of equipment can pass without deterioration.
First, let us look for equipment current ratings:
- Disconnect, 150A
- Fuse, 140 A
- Circuit breaker, 2000 A
The transformer does not directly list its rated current, but the info is bundled into the 1500kVA rating.
- 1500kVA / 12.47 kV / √ 3 = 69.5 A
- 1500kVA / 0.6 kV / √ 3 = 1443.5 A
The cables do not list their rated currents, but general info can be looked up in cable ampacity tables from a given size.
Short circuit current rating:
The short circuit current rating is the maximum amount of current a piece of equipment can withstand temporarily without sustaining damage.
At the incoming, the available short circuit fault data for a three-phase fault and a line to ground fault can be added if received from the utility.
This current is then used to calculate the maximum short circuit at any location in the system and compared to the equipment withstand ratings.
On this diagram, only the switchboard short circuit current rating is shown at 86 kA, but each piece of equipment has a limit.
The maximum current that a device can interrupt safely is called the interrupt rating.
On this diagram, none of the interrupt ratings are shown, but the fused disconnect and circuit breaker would both have an interrupt rating.
Impedance affects the amount of current dissipated and is used to determine load flows and short circuit fault levels.
The only equipment that lists the impedance here is the transformer at 5.83 %.
The cable size and length can also be used to approximate impedance.
The other pieces of equipment would have negligible impedance.
In larger systems, relays may be used in combination with circuit breakers.
There are many different relay functions and numbers associated with each type.
Below are a few of the commonly used.
- 50 - Instantaneous Overcurrent Relay
- 51 - AC Time Overcurrent Relay
- 86 - Lock-Out Relay, Master Trip Relay
- 87 - Differential Protective Relay
For complete reference see IEEE Std. C37.2 Standard Electrical Power System Device Function Numbers.
Including the relays and current transformers is important to understand what protections are in place.
Some sites may create a separate document to indicate the relay control signals, but it is also possible to put these signals directly on the SLD.
In the diagram, you can see the use of a draw-out high voltage circuit breaker at 13.8kV marked with double arrows.
Coming into the draw-out breaker is a control signal shown as a red dotted line, which comes from the relays.
The relays show the current transformers (CT) they are connected to, and the CTs show their CT ratio.
Device numbers may be combined if the device has multiple functions (50/51).
There are also suffix letters that may be used with the device number to specify Neutral or Ground protection (50N/50G).
You can also see the differential relay 87 is connected to the relays above and below it. This shows that it is using the same CT data.
The title block helps to manage documentation by tracking changes and dates on a drawing.
It is usually in the lower right corner, but also includes the border around the entire diagram.
One of the first things to check before you begin looking at a single-line drawing is the revisions.
This is a list of the changes that have been made to the document with the date.
It is also worth noting that just because the revision date is recent does not always mean the entire drawing is up-to-date.
In this example, you can see the drawing is being used for tender, for construction, as-built, additions, and removals.
If making revisions a method of communicating exactly what has changed on the drawing is to circle the change with a ‘revision cloud’.
Here the main incoming fuse size has changed to 100A from 80A.
This change would be associated with a revision letter, which could also be placed directly next to the cloud on the diagram.
The next section is to list the reference drawings, allowing you to trace the info to other documents.
There should also be a section that lists the people who drew the drawing, managed the project, the dates, and their company.
The client company, name of the drawing, drawing number, and revision version are also listed in the bottom corner.
Now It’s Your Turn
- Was this article useful?
- Did you need more on how to read wiring diagrams?
- Do you feel like you know how to draw a circuit diagram now?
- Have you got your single line started yet?
Let us know in the comments below!
Looking To Learn More About Arc Flash Studies?
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