How to Make a Single Line Diagram

Need a site electrical single-line diagram?

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Wondering how to draw an electrical 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&#;
• Ready?

Let&#;s go!

 

 

 

 

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. Below is the CSA Z462 single line diagram definition:

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 electrical one line 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 an SLD 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 electrical single line 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. 

&#;4.2.2.2 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. 

&#;4.2.2.4 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:

1. One-line diagrams;
2. Short-circuit studies;
3. Coordination study;
4.  Arc flash incident energy study; and
5. Load flow study.&#;

&#;CSA Z463

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.

SLD Updates

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 electrical schematic 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. 

Equipment Symbols Data The utility or AC
current source
symbol is used to
show where power
is coming from.

1. Incoming voltage 

2. Fault level and impedance (optional)

These symbols could
all represent
an AC generator. 

1. Watts

2. KVA

3. Voltage 

4. # of Phases

5. Frequency 

Two winding
transformers can be
represented with
either of these symbols 

1. connection type (Δ, Y)

2. KVA 

3. Voltages 

4. %Z Impedance

These are the Delta
and Wye connection
configurations, which
can be added in. N/A These switches
symbols can be used
to represent a disconnect
or transfer switch.

1. Rated

Amperage

Fuses can be
represented with
either graphic symbol. 

1. Rated

Amperage

2. Model

Low voltage circuit
breaker symbols,
with the second
indicating a draw-out
type.

1. Rated amperage

2. Model

3. Trip settings (optional)

Medium voltage
circuit breaker
symbols, with the
second indicating
a draw-out type.

1. Rated amperage

2. Model 

This is the motor
symbol, one with a M
and one with a delta
connection symbol.  1. Power (HP) These symbols show
a current transformer (CT)
above and a potential
transformer (PT)
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
added to attachment points of connecting lines
to a graphical symbol.

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. 

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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: 

1. Incoming service voltage 
2. Equipment rated current 
3. Identification names of equipment 
4. Bus voltage, frequency, phases, and short circuit current withstand ratings 
5. Cable sizes, #of cables, and lengths
6. Transformer connection type, kVA, voltages, and impedance
7. Generator voltage and kW 
8. Motor HP
9. Current and Voltage ratios of instrument transformers
10. 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. 

Voltage

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.

Amperage

Current rating:

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, A 

The transformer does not directly list its rated current, but the info is bundled into the kVA rating.

• kVA / 12.47 kV / &#; 3 = 69.5 A 
• kVA / 0.6 kV / &#; 3 = .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.

Interrupting rating: 

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 

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. 

Relays

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.

Title block

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. 

Conclusion

I hope this article has helped to better explain how to make a single line diagram.

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If you have any questions, you can always reach out to me at . 

 

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How to read one-line diagrams

Back to F.A.Q.

We usually depict the electrical distribution system by a graphic representation called a single line diagram (SLD). A single line can show all or part of a system. It is very versatile and comprehensive because it can depict a very complicated three-phase system.

We use universally accepted electrical symbols to represent the different electrical components and their relationship within a circuit or system. To interpret one-lines you first need to be familiar with the electrical symbols. This chart shows the most frequently used symbols.

Symbol Identification Explanation Transformer Represents a variety of transformers from liquid filled to dry types. Additional information is normally printed next to symbol indicating winding connections, primary /secondary voltages and KVA or MVA ratings. Non-drawout circuit breaker Represents a fixed mounted low voltage circuit breaker. Disconnect switch Represents a switch in low or medium/high voltage applications (open position shown) Fuse Represents low voltage and power fuses. Current transformer Represents current transformers mounted in assembled equipment. A ratio of A to 5A shown. Potential or voltage transformer Represents potential transformers usuallymounted in assembled equipment. A ratio of 480V to 120V shown. Ground (earth) Represents a grounding (earthing) point Battery Represents a battery in an equipment package Motor Represents a motor and is also shown with an &#;M&#; inside the circle. Additional motor information is commonly printed next to symbol, such as horsepower, RPMand voltage. Normally open (NO) contact Can represent a single contact or single pole switch in the open position for motor control Normally closed (NC) contact Can represent a single contact or single pole switch in the closedposition for motor control Overload relay Protects a motor should an overload condition develop. Capacitor Represents a variety of capacitors. Emergency generator The symbol is frequently shown in conjuction with a transfer switch. Fused disconnect switch The symbol is a combination of a fuse and disconnect switch with the switch in the open position. Low voltage motor control The symbol is a combination of a normally open contact (switch), overload relay, motor and disconnect device. Meter center A series of circle symbols representing meters usually mounted in a common enclosure. Load center or panelboard One circuit breaker representing a main device and other circuit breakers representing feeder circuits usually in a common enclosure. Transfer switch &#; Circuit breaker type transfer switch
&#;Non-circuit breaker type transfer switch

Lets go through a industrial single line diagram. When interpreting a single line diagram, you should always start at the top where the highest voltage is and work your way down to the lowest voltage. This helps to keep the voltages and their paths straight.

To explain this easier, we have divided the single line into three sections.

The diagram below was created using free online diagram maker located at www.draw.io The draw.io online is a free-to-license web application for everyone. It is free for any usage, there is no premium pay-for functionality, watermarking, etc. You own the content you produce and may use it for any purpose. You can store your draw.io projects in .xml format on your desktop, in Dropbox or Google Drive. Whichever storage option you select, when you start draw.io, you will always be presented with a screen asking whether you want to create a new file or open a new one.

Although draw.io provides an extensive set of default libraries, there may be times when you would like to use symbols that are not provided. Provided you are able to locate and use the relevant symbols, you can incorporate them into a custom library, which can then be used in the same way as any of the existing default libraries. Check draw.io user manual and online tutorials for basic and advanced options.

Area A

Starting at the top, you will notice that a transformer is feeding power to the whole system. The transformer steps the voltage down from 35kV to 15kV, as indicated by the numbers next to the transformer symbol. Once the voltage has been stepped down, a removable circuit breaker (a1) is encountered. Do you recognize the removable circuit breaker symbol? You can assume this circuit breaker can handle 15kV, since it is attached to the 15kV side of the transformer, and nothing different is indicated on the one-line.

Following the drawout circuit breaker (a1) from the transformer, it is attached to a heavier, horizontal line. This horizontal line represents an electrical bus, which is a means used to get electricity to other areas or circuits.

Area B

You will notice that two more removable circuit breakers (b1 and b2) are attached to the bus and feed other circuits, which are at 15kV, since there has been no indication of voltage change in the system. Attached to the removable circuit breaker (b1), a step-down transformer is used to take the voltage in that area of the system from 15kV down to 5kV.

On the 5kV side of this transformer, a disconnect switch is shown. The disconnect is used to connect or isolate the equipment below it from the transformer. The equipment below the disconnect is at 5kV, since nothing indicates the contrary. Do you recognize the equipment attached to the lower side of the disconnect switch as being two medium-voltage motor starters? A number of starters could be connected depending upon the particular system requirements.

Now locate the second removable circuit breaker (b2). This circuit breaker is attached to a fused disconnect switch and it is connected to a step-down transformer. Notice that all the equipment below the transformer is now considered low voltage equipment, because the voltage has been stepped down to a level of 600 volts or lower.

The last piece of electrical equipment in the middle portion of the diagram is another circuit breaker (b3). This time, however, the circuit breaker is a fixed low voltage circuit breaker, as indicated by the symbol. Moving to the bottom area of the one-line, notice that the circuit breaker (b3) in the middle is connected to the bus in the bottom portion.

Area C

To the bottom left and connected to the bus is another fixed circuit breaker. Look carefully at the next grouping of symbols. Do you recognize the automatic transfer switch symbol?

Also, notice that a circle symbol which represents an emergency generator is attached to the automatic transfer switch. This area of the one-line tells us that it is important for the equipment connected below the automatic transfer switch to keep running, even if power from the bus is lost. You can tell from the one-line that the automatic transfer switch would connect the emergency generator into the circuit to keep equipment running, if power from the bus were lost.

A low-voltage motor control circuit is attached to the automatic transfer switch through a low-voltage bus. Make sure you recognize these symbols. Although we do not know the exact function of the low voltage motor control in this circuit, it is obvious that it is important to keep the equipment up and running. A written specification would normally provide the details of the application.

On the right side of the third area there is another fixed circuit breaker connected to the bus. It is attached to a meter center, as indicated by the symbol formed by three circles. This indicates that the electric company is using these meters to keep track of power consumed by the equipment below the meter center.

Below the meter center is a load-center or panel-board that is feeding a number of smaller circuits. This could represent a load center in a building that feeds power to the lights, air conditioning, heat and any other electrical equipment connected to the building.

This over-simplified analysis of a one-line diagram gives you an idea of the kind of story such diagrams tell about electrical system connections and equipment. Just keep in mind that although some one-line diagrams may appear overwhelming by virtue of their size and the wide variety of equipment represented, they can all be analyzed using the same step-by-step method.

Reference // Fundamentals of Electrical Distribution by EATON