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Home / Technical Articles / Learn To Interpret Single Line Diagram (SLD)

Single line diagram (SLD)

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 very simple DC circuits, or a very complicated three-phase system.

Learn To Interpret Single Line Diagram (SLD)
Learn To Interpret Single Line Diagram - SLD (on photo: An example of 66/6.6kV power substation single line diagram)

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

Individual electrical symbols
Symbol Identification Explanation
Transformer symbol 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.
Drawout circuit breaker symbol Removable or drawout circuit breaker Normally represents a MV drawout circuit breaker 5kV and above.
Future drawout circuit breaker symbol Future removable or drawout circuit breaker position Represents a structure equipped to accept circuit breaker in the future, commonly known as provisions.
Non-drawout circuit breaker symbol Non-drawout circuit breaker Represents a fixed mounted low voltage circuit breaker.
Removable or drawout circuit breaker symbol Removable or drawout circuit breaker Represents a drawout low voltage circuit breaker.
Disconnect switch symbol Disconnect switch Represents a switch in low or medium/high voltage applications (open position shown)
Fuse symbol Fuse Represents low voltage and power fuses.
Bus duct symbol Bus duct Represents low and medium/high voltage bus duct.
Current transformer symbol Current transformer Represents current transformers mounted in assembled equipment. A ratio of 4000A to 5A shown.
 Potential or voltage transformer Potential or voltage transformer Represents potential transformers usually mounted in assembled equipment. A ratio of 480V to 120V shown.
 Ground (earth) symbol Ground (earth) Represents a grounding (earthing) point
 Battery symbol Battery Represents a battery in an equipment package
 Motor symbol 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, RPM and voltage.
 Normally open (NO) contact symbol Normally open (NO) contact Can represent a single contact or single pole switch in the open position for motor control
 Normally closed (NC) contact symbol Normally closed (NC) contact Can represent a single contact or single pole switch in the closed position for motor control
 Indicating light symbol Indicating light The letter inside circle indicates the color. The color red is indicated.
 Overload relay symbol Overload relay Protects a motor should an overload condition develop.
Capacitor symbol Capacitor Represents a variety of capacitors.
Ammeter symbol Ammeter A letter is usually shown to designate the meter type (A = ammeter, V = voltmeter, etc.)
Instantanaeous overcurrent protective relay symbol Instantaneous overcurrent protective relay The device number designates the relay type (50 = instantaneous overcurrent, 59 = overvoltage, 86 = lockout, etc.)
Emergency generator symbol Emergency generator The symbol is frequently shown in conjuction with a transfer switch.
Fused disconnect switch symbol 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 symbol Low voltage motor control The symbol is a combination of a normally open contact (switch), overload relay, motor and disconnect device.
Medium voltage motor starter symbol Medium voltage motor starter The symbol is a combination of a drawout fuse, normally open contact (switch) and motor.
 Meter center symbol Meter center A series of circle symbols representing meters usually mounted in a common enclosure.
 Load center panelboard symbol 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 symbol Transfer switch • Circuit breaker type transfer switch
• Non-circuit breaker type transfer switch
Current transformer with ammeter symbol Current transformer with connected ammeter The instrument connected could be a different instrument or several different instruments identified by the letter.
Protective relay connected to current transformer symbol Protective relays connected to current transformer Device numbers indicate types of relays connected, such as:
• 67 – Directional overcurrent
• 51 – Time overcurrent

Simple electrical circuit

Now, that you are familiar with electrical symbol, let’s look at how they are used in interpreting single line diagrams. Below is a simple electrical circuit.

Simple single line diagram
Figure 1 – Simple single line diagram

You can tell by the symbols that this single line diagram has three resistors and a battery. The electricity flows from the negative side of the battery through the resistors to the positive side of the battery.

Industrial single line diagram

Now, 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.

A typical industrial single line diagram
Figure 2 – A typical industrial single line diagram

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 drawout circuit breaker (a1) is encountered.

Do you recognize the drawout 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 single line diagram. 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 drawout 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 drawout circuit breaker (b1), a step-down transformer is used to take the voltage in that area of the system from 15kV down to 5kV.

SLD area B
SLD area B

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 drawout 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 single line diagram, 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 single line diagram 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 single line diagram that the automatic transfer switch would connect the emergency generator into the circuit to keep equipment running, if power from the bus were lost.

SLD area C
SLD area C

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 panelboard 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.

Few more words //

This over-simplified analysis of a single 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 single 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

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. baryalai tareen
    Oct 13, 2019

    Dear Edward .
    thank you from more information about electrical SLD. you can tell whats first work in distribution projects.

  2. Samuel Olusegun
    Oct 01, 2019

    This is really impactful and easy to understand

  3. Shame Nziramasanga
    Sep 26, 2019

    Hi may you please send me a pdf copy of this info please. Its very helpful

    Aug 07, 2019

    excellent & simple informative

  5. Jim Aquino
    Aug 02, 2019

    Can i get a copy please Sir, thank you very much.

  6. Terry Sanders
    Jul 08, 2019

    Very thorough and informative, great job! Can you please send me PDF file to study? Thank you

  7. mariane
    Apr 27, 2019

    Can you please send me pdf copy. TIA :)

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    Mar 15, 2019

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  9. Charles Ngozi Emmanuel
    Mar 02, 2019

    Please can you send me the pdf copy to my email,

  10. mohamed
    Mar 02, 2019

    fantastic article

  11. Moises M. Meca
    Mar 02, 2019

    Thank you so much EEP & to you Sir For sharing this topic God Bless you all

    Mar 02, 2019

    Please send my email

    Mar 01, 2019


    • Freab demissie
      Aug 11, 2019

      Please soft copy about SLD.

  14. Anil Paul Jacob
    Mar 01, 2019

    Number 52 most commonly represents a circuit breaker especially in drawings made by USA, Mexican, S Korean Engineers. Recently the EU drawings are not with number 52. Also a NO contact or NC contact in an ACB control circuit we names as 52 other than power contacts. Nice Explanation. Ed can you give your email since I had few clarifications to be made, may be you might have published in your earlier versions. My mail is if you send a test mail I would start my mail or else comment here
    Thanks for the informative publication

  15. Somtochukwu
    Mar 01, 2019

    This is real good and educating. Please a PDF copy of this lecture would be appreciated. Thanks

  16. Vivek shaikh
    Mar 01, 2019

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    email –

  17. Aikins larbi kwabena
    Mar 01, 2019

    Kindly send me the pdf through my email

  18. Rahis
    Jan 25, 2019

    Possible send me PDF …

  19. Mike Betts
    Jan 24, 2019


    can someone please tell me the meaning of the number 52 in the square of the drawout circuit breaker symbols on these single line diagrams. It is obviously not the CB number as they are all the same!


    • Edvard
      Jan 24, 2019

      Number 52 represents a circuit breaker according to ANSI/IEEE Standard C37.2 (Standard for electrical power system device function numbers, acronyms, and contact designations).

  20. JR
    Dec 23, 2018

    Good information I need to learn more about this.

  21. Abu Kayonde
    Nov 27, 2018

    Thank you for this insightful and educative information, kindly share with me the PDF.

  22. Lynn
    Nov 03, 2018

    Super helpful, can you please share the pdf with me…..thanks

  23. Abdul kashif
    Oct 08, 2018

    Excellent explanation

  24. Joshua Pacana
    Sep 25, 2018

    Hi Sir Edvard can i please ask for a PDF copy of this? thank you very much.

  25. JHON
    Aug 28, 2018

    Please send me a PDF file form as this article is very useful as reference.

  26. Tanveer
    Aug 25, 2018

    Very useful information can you send me pdf on my

  27. Danish khan
    Jul 28, 2018

    Can you please send me PDF soft copy @

    Jul 15, 2018

    Thanks Edvard Csayani Sir for the excellent work of guidance on Electrical Articles.

  29. Godswill Samuel Benjamin
    Jul 03, 2018

    Very useful information.Credits to EEP and the author i really appreciate.please kindly send it in pdf format for me through my email thanks.

  30. Beryl
    Jun 03, 2018

    Very informative,would appreciate it if this was sent to my email

  31. C Boojum
    May 27, 2018

    Excellent info. For a customized excel template that performs fault calculations, please feel free to email me at

  32. Isaac w Toe jr
    Apr 17, 2018

    Thanks clarity..I love the way you simply the single line’s at the simplest form and,every one can understand it well.
    Pls send me file on Siemens substation and transmissions line protection

  33. Anandadeb Chatterji
    Feb 01, 2018

    Very good. Just add few lines on why & where use of particular type of line breaking equipment( draw-out / non draw-out CBs / Disconnecting Sws. etc. in conjunction in the same line) need to be shown.

  34. sonam ongchuk lepcha
    Jan 11, 2018

    Thank you Edvard Csyani sir for your valuable lessons.EEP has been very helpful,especially for freshers like me who lacks practical experience.I look forward to more important articles.

  35. Zeeshan Ali
    Dec 14, 2017

    I was looking for that stuff,
    Well explained.
    This engineering portal is very helpful for beginner like me.

  36. Akporotu Akpomedaye
    Oct 18, 2017

    i really appreciate you sir, i am a beginner, for this reason, i would appreciate if more training on how to interpret SLD is sent to me. thank you sir.

  37. tarek
    Jun 02, 2017

    if you can provide with this explanation or simillar to it in pdf format.

  38. Shaukat Hussain
    Mar 31, 2017

    This is a great explanation of the ANSI symbols used in SLD’s, I’m used to that standard. However, I’m not so clear on the IEC symbols. Do you have anything that can help out to understand them? A comparison table would be great.

    Great blog!! Thanks!

  39. Thomas Dats
    Mar 02, 2017

    Thanks very much for those information, its very helpful and informative.
    However, could you be able to send me a simple systematic drawing from point of generation to transmission and distribution please.

    I will really appreciate your help.


    Dats Pukma
    Papua New Guinea

  40. Muhammad Saad
    Feb 26, 2017

    Sir, there is this component in a SLD i m given to read which contains a circle with MF written in it, what would that be?

  41. Madhavan.D
    Jan 25, 2017

    I want Editable Electrical single diagram

  42. Sara
    Jan 19, 2017

    The best resource on SLD for beginners. Very lucid!!!!

  43. Muhammad Rafiq
    Dec 05, 2016

    there is not get PDF option

  44. Basant Sharma
    Dec 02, 2016

    Nice blog for beginners, keep updating same.

  45. Bethuel Rmogaela
    Nov 30, 2016

    Is it technically recommended/compulsory to connect lighting system to earth leakage???

  46. James
    Nov 29, 2016

    Mate want to earn some holiday money helping with line diagrams?

  47. Nasir Zaidi
    Nov 24, 2016

    Bravo!! The way you simplified the SLDs I don’t think any one can do better then this.
    Please send me this file if possible.

  48. Víctor Bohórquez
    Nov 08, 2016

    Excelente me gustaría soy técnico en electricidad recidencial y estudio técnico en electricidad industrial me gustaría el Ártico completo gracias

  49. Emmanuel
    Sep 30, 2016

    I really love this site, more informative

  50. Azam
    Jun 10, 2016

    Kindly send me this complete article.
    My email address is:

  51. Ken Sonntag
    May 16, 2016

    Great stuff, please add me on your email list.

  52. hossien
    May 09, 2016

    hi .. i want this file ..(( Learn To Interpret Single Line Diagram (SLD) )) … please send me on my yahoo mail .. thanks …

  53. Mwanyara Mangunda
    Apr 02, 2016

    Very useful information.Credits to EEP and the author i really appreciate.

  54. Ramon Burgos
    Dec 19, 2015

    This is a great explanation of the ANSI symbols used in SLD’s, I’m used to that standard. However, I’m not so clear on the IEC symbols. Do you have anything that can help out to understand them? A comparison table would be great.

    Great blog!! Thanks!

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