How to Interpret a Single Line Diagram (SLD)

Single Line Diagrams (SLD)

When designing an electrical distribution board, it is common practice to use a simplified notation in order to represent the electrical system. This graphical representation is called a single line diagram (SLD).

[Single Line Diagram (SLD) for a triple change over distribution board – Drawn by Sheik Essop, Switchboard Manufacturers]

The single line diagram can represent an entire system or a small part of an electrical circuit. Electrical elements such as circuit breakers, transformers, capacitors, busbars, contactors and conductors are shown by standardized schematic symbols. Single phase or three phase systems can be easily represented as a single conductor with horizontal line strokes indicating the number of phases.

[A SG factory staff member assembling a distribution board based off a Single Line Diagram, Switchboard Manufacturers, JHB]

Universally accepted electrical symbols such as in the SLD’s above are used to represent the different electrical components and their relationship within the circuit. To interpret SLD’s one must familiarise oneself with these symbols. The table below shows some of the most common symbols used when designing distribution boards.

Individual electrical symbols
Symbol Identification Explanation
Transformer Represents a variety of transformers from liquid filled to dry types. Additional information is normally printed next to the symbol indicating winding connections, primary / secondary voltages and KVA or MVA ratings.
   Generator Represents a generator often used for backup supply of essential loads.
   Circuit Breaker Represents a circuit breaker. The 3 horizontal lines indicate that the breaker is a 3 phase breaker. 2 lines would indicate double pole and 1 line, single pole.
Isolator Represents a 3 phase isolator.
Power Factor Correction Represents capacitors that make up a PFC panel.
Surge Arresters Represents a 3 pole + Neutral Surge Arrester for lightning protection.
Fuse Represents low voltage and power fuses.
Earth Leakage Represents an RCD/ Earth Leakage unit with no overload protection.
Meter Metering device with Current Transformers (CT’s) .
  Ground (earth) Represents a grounding (earthing) point.
  Contactor Represents a contactor usually triggered by an external occurrence such as a day/night switch.
  Normally open (NO) contact Represents a single contact or single pole switch in the open position for motor control.
  Normally closed (NC) contact Represents a single contact or single pole switch in the closed position for motor control.
   Change Over  Can be automatic or manual. Can be mechanically or electrically interlocked.
A few more Symbols often found as a legend on the side of an SLD:

Simple Electric Circuit

Now that you are familiar with electrical symbols, let’s look at how they are used in interpreting single line diagrams. Below is a simple electric circuit. As you can see, the layout is like a branched structure where we have a main incomer feeding smaller breakers that regulate the current to the load. Even though the main incomer is a double pole, the diagram does not require two lines throughout the circuit to indicate this. Only one line is required, hence the name “Single” Line Diagram.

[Single Line Diagram for an RDP house DB – Drawn by Raj Rangasami, Switchboard Manufacturers]

You can tell by the symbols that this single line diagram has a Double Pole (DP) 63A Isolator as the main incomer feeding a 63A DP Earth Leakage, a Single Pole (SP) 32A MCB (STOVE), a SP 16A MCB (GEYSER) and a SP 10A MCB (LIGHTS). The Earth Leakage in turn is feeding a SP 20A MCB which is the supply to the plugs as labeled.

Ready Board

[Ready Board available for purchase on the Switchman Products online store]

The components from the above single line would most likely fit into a polycarbonate enclosure like in the image above. This could be used for an RDP board for low cost housing. A ready wired board fitting this description can be purchased here at ‘Switchman Products’.

Drawing Single Line Diagrams

Single line diagrams are often drawn using CAD software such as Autocad. However, these programs are costly and for someone looking for a free program with a full list of components, ABB offers a program called “e-Design”, which can be downloaded here.

I have a SLD, what now?

Once you have an SLD, your accredited board manufacturer can begin the design work for the General Arrangement (GA) of the physical components. The designer must have a full understanding of the physical aspects of the components in order to place them correctly. These drawings can get complicated and an experienced designer or engineer can prevent potential hazards and design faults. In addition, an experienced designer can draft a board in the most cost effective manner, saving you money along the way.

[A general arrangement drawn by one of Switchboard Manufacturers Experienced Designers, Sheik Essop]

Need help understanding or drawing singles line?

Send us a mail at and one of our experienced staff will assist you in designing your board.

Article by Josh Berman, BSc. Elec. Msc Wits,

Electrical Engineer at Switchboard Group.

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