A single line diagram is an electrical system blueprint, a simplified drawing way for representing a three-phase power system. The best fundamental drawing that shows Electrical Installation, rating, and capacity of electrical equipment, Circuits, and protection devices in on a one-line diagram represented by universally accepted electrical symbols. And also it describes how the electrical components are related in a system. Mainly SLDs are used for Planning, Servicing, and maintenance activities. It also helps to change the power system configuration and troubleshooting and rectification of systems that can also be done by relating the subsections to the overall section. The importance of updating/creating SLD is the first step in the preparation of a power system plan that helps an electrical professional to understand the field is new or already existing, can also find the route map of the complete electrical distribution system were before starting their work. It shows the path from the incoming power source to the load connected including the specification of each component so that enables ease of access and safety in the field.

An accurate SLD is used for calculating short circuit current and incident energy. And a small change made in the field should also be updated and the document is a very essential part to keep in every field that ensures safe working condition.

Example of Single Line Diagram from Generation to Distribution Stage

Familiarization of electrical symbols helps to interpret the SLDs each and every symbol represents the different electrical components and their connections within the circuit or a power system. Each symbol carries a meaning/identification of a component such as relays, breakers, meters, transformers, etc., with its explanation that simplifies the learning, reading & analyzing of SLD.

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Fig : 1, Simple circuit

For example the Fig: 1 represents the simple circuit that can be easily recognized by the well-familiarized symbols in its one-line diagram. You can pinpoint the symbols used in the diagram are battery, Switch, and light bulb. The negative side of the battery is connected to the light bulb where electricity flows and the positive side is connected to the switch where the action of the switch controls the flow of the electric light bulb gets ON or OFF accordingly. By the way, you’re right, there goes your understanding of the most commonly used symbols. Refer to the chart below.

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The symbols can be used for making industrial SLDs, always starts from the highest voltage generation/source stage to the lowest voltage distribution/load stage. So that the route map can be easily obtained.


The Fig: 2 represents the Industrial Single line diagram from the generating station to the consumer. Starting from the generating station the generated voltage 11/132kV is stepped up to 132kV/220kV/400kV in the primary transmission at the receiving station the 132kV is stepped down to 33/66kV by step down transformer where the transmission losses can also be reduced. After the secondary transmission substation receives 33/11kV in a substation that’s set to the primary distribution for the Industrial customers and after the secondary distribution by the distribution transformer consumers will receive 11kV/415V respectively.

Can you able to recognize the single line diagram (Fig: 2)?

Simpler the SLDs can provide an idea to learn how it works, and wider the SLDs brings out the expert in you. The significant importance of the Single line diagram is updating it as per the mandate of NFPA – 70E Standard.

As industrial technologies continue to upgrade in technical advancement the intelligent one-line diagram provides an intelligent user interface model to manage the electrical power system with multi-level circuiting of different sub-circuits with unique features such as validation, visualization, and monitoring. The resulting in the next step gives the complete auto-ranging and precision in data formatting accuracy.

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Fig: 3 – Intelligent SLD

Single line diagram takes a major role in the design and development phase. Incorporate with various fields like architectural, Mechanical, Electrical, Plumbing, and fire & safety professionals collaborate with the designing process to develop a better power system one-line diagram. During the various stages of the project development, the one-line documentation gets up-to-date information according to the change/correction often, How Electrical and designing engineers rely on the accurate one-line diagrams to ensure safety.

A single line diagram is the most essential tool for the electrical professional.

Author: Prem Kumar A
Department: Power and Electrical
Linkedin: https://www.linkedin.com/in/prem-kumar-a-3ba958132/


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