10 Questions You Should to Know about single line diagram definition

A single-line diagram (also known as an SLD or one-line diagram) is a simplified representation of an electrical system. Symbols and lines are used to represent the nodes and connections in the system, and electrical characteristics may be included as well.

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In a data center, a single-line diagram is used to visualize the power distribution system to improve planning and troubleshooting, ensure redundancy, and reduce potential outages.

What Does a Single-Line Diagram Look Like?

Single-line diagrams use standard symbols for the different nodes of power systems. The power source is displayed at the top of the diagram so that the power path can easily be followed downstream from node to node and redundant power paths can be visualized side-by-side.

An example of a single-line diagram via Sunbird DCIM.

What Equipment is Included in a Data Center&#;s Single-Line Diagram?

A single-line diagram clearly shows the redundant power paths from the data center facility&#;s power source down to the floor power distribution units.

The nodes of a data center&#;s single-line diagram typically include:

• Utility feed. The main power source from the utility.
• Generators and fuel tanks that deliver backup power during an outage.
• Transformers that convert electrical power from one voltage or current to another voltage or current.
• Switchgears that control, protects, and isolates electrical equipment.
• Switchboards that divide and distribute power into branch circuits.
• Automatic transfer switches (ATS) that switch to a backup power source when there is an outage.
• Uninterruptible power supply (UPS) units that provide emergency power during an outage until the power returns or a longer-term backup system turns on.
• Floor power distribution units (PDUs) that transform raw power feeds into lower capacity feeds that feed into remote power panels or busways and rack PDUs.
• DC power plants that provide uninterrupted DC power.
• DC bays that divide and distribute individual loads from the DC power plant to equipment downstream.

What is a Single-Line Diagrams Used For?

A single-line diagram is used to depict the distribution of power through a facility. The diagram should be kept accurate and updated as equipment in the facility is added, removed, or changed.
The benefits of using a single-line diagram in your data center include:

• Becoming familiar with your power distribution system layout and design
• Documenting redundant power paths to ensure system reliability
• Simplifying planning, troubleshooting, and increasing the efficiency of maintenance activities
• Ensuring compliance with codes and regulations such as NFPA-70E
• Maintaining safe operations to protect personnel

Get Automatically Generated, Dynamic, and Interactive Single-Line Diagrams

Creating and maintaining single-line diagrams is commonly a manual and time-consuming process. However, new functionality in second-generation Data Center Infrastructure Management (DCIM) software dramatically simplifies and enhances single-line diagrams.

DCIM software allows you to centrally track all your IT assets and supporting infrastructure equipment in a single pane of glass and map their relationships and dependencies. Based on the existing asset and circuit information that you have already populated in the tool, the software automatically generates an always-accurate single-line diagram. If you make a change to an item or connection in your system, the single-line diagram will update automatically.

The single-line diagram capability in modern DCIM software is far more useful and versatile than traditional static diagrams because you can overlay real-time power and capacity data which makes data center power management easier.

Both AC and DC power chains are supported, allowing you to visualize and see details of your utility feed, generator and fuel tanks, transformers, load devices, UPS units, AC panels, floor PDUs, DC power plants, and DC bays.

You can track the key electrical characteristics and interconnections for all the facility items in your power chain, track all draw-out breaker and disconnect switches, understand the capacity and load of all nodes, track breaker states, and see a details panel for each node that includes budgeted and actual values for voltage, current, power rating, highest/lowest phase, and more.

All you have to do is create the assets and connections, and the software does all the hard work for you.

These modern single-line diagrams are easily navigable, editable via drag-and-drop, and printable.

Want to see how Sunbird&#;s award-winning DCIM software automates single-line diagrams? Get your free test drive now.

When using the method of symmetrical components, separate one-line diagrams are made for each of the positive, negative and zero-sequence systems. This simplifies the analysis of unbalanced conditions of a polyphase system. Items that have different impedances for the different phase sequences are identified on the diagrams. For example, in general a generator will have different positive and negative sequence impedance, and certain transformer winding connections block zero-sequence currents. The unbalanced system can be resolved into three single line diagrams for each sequence, and interconnected to show how the unbalanced components add in each part of the system.

A secondary advantage to using a one-line diagram is that the simpler diagram leaves more space for non-electrical, such as economic, information to be included.

A one-line diagram is usually used along with other notational simplifications, such as the per-unit system.

The theory of three-phase power systems tells us that as long as the loads on each of the three phases are balanced, the system is fully represented by (and thus calculations can be performed for) any single phase (so called per phase analysis ).[7][8] In power engineering, this assumption is often useful, and to consider all three phases requires more effort with very little potential advantage.[9] An important and frequent exception is an asymmetric fault on only one or two phases of the system.

The lines in the single-line diagram connect nodes &#; points in the system that are "electrically distinct" (i.e., there is nonzero electrical impedance between them). For sufficiently large systems, these points represent physical busbars, so the diagram nodes are frequently called buses . A bus corresponds to a location where the power is either injected into the system (e.g., a generator) or consumed (an electrical load). A steady-state of each bus can be characterized by its voltage phasor; the system state is defined by a vector[4] of voltage phasors for all the buses.[5] In a physical system the state is calculated through power system state estimation, since the end of the 20th century this process involves direct simultaneous measurements (synchrophasor) using the phasor measurement units.[6]

It is a form of block diagram graphically depicting the paths for power flow between entities of the system. Elements on the diagram do not represent the physical size or location of the electrical equipment, but it is a common convention to organize the diagram with the same left-to-right, top-to-bottom sequence as the switchgear or other apparatus represented. A one-line diagram can also be used to show a high level view of conduit runs for a PLC control system.

The one-line diagram has its largest application in power flow studies. Electrical elements such as circuit breakers, transformers, capacitors, bus bars, and conductors are shown by standardized schematic symbols.[2] Instead of representing each of three phases with a separate line or terminal, only one conductor is represented.

In power engineering, a single-line diagram ( SLD ), also sometimes called one-line diagram , is a simplest symbolic representation of an electric power system.[2] A single line in the diagram typically corresponds to more than one physical conductor: in a direct current system the line includes the supply and return paths, in a three-phase system the line represents all three phases (the conductors are both supply and return due to the nature of the alternating current circuits).

A typical one-line diagram with annotated power flows. Red boxes represent circuit breakers , grey lines represent three-phase bus and interconnecting conductors, the orange circle represents an electric generator , the green spiral is an inductor , and the three overlapping blue circles represent a double-wound transformer with a tertiary winding.

Need a site electrical single-line diagram?

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.

The utility or ACcurrent sourcesymbol is used toshow where poweris coming from.

1. Incoming voltage 

2. Fault level and impedance (optional)

These symbols could
all represent
an AC generator. 

These symbols couldall representan AC generator.

1. Watts

2. KVA

3. Voltage 

4. # of Phases

5. Frequency 

Two winding
transformers can be
represented with
either of these symbols 

Two windingtransformers can berepresented witheither 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.

These are the Deltaand Wye connectionconfigurations, whichcan be added in. N/A These switchessymbols can be usedto represent a disconnector transfer switch.

1. Rated

Amperage

Fuses can be
represented with
either graphic symbol. 

Fuses can berepresented witheither graphic symbol.

1. Rated

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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.

Medium voltagecircuit breakersymbols, with thesecond indicatinga 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

This is the motorsymbol, one with a Mand one with a deltaconnection symbol. 1. Power (HP) These symbols showa current transformer (CT)above and a potentialtransformer (PT)on the bottom. 1. Turns ratio This is the relay symbolattached 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. 

Orientation: Alterations of the orientationof the symbol do not alter its meaning. Line Width: The line width does not changethe meaning but may be used for emphasis. Enlargement or Reduction: There is no meaningassociated 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. 

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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