HOW TO MAKE ACCURATE SINGLE LINE DIAGRAM FOR POWER SYSTEM TUTORIALS

What are the elements of an accurate single line diagram?

A reliable single-line diagram of an industrial or commercial electrical power distribution system is an invaluable tool. It is also called a one-line diagram. The single-line diagram indicates, by single lines and standard symbols, the course and component parts of an electric circuit or system of circuits. The symbols that are commonly used in one-line diagrams are defined in IEEE Std 315-1975.

The single-line diagram is a road map of the distribution system that traces the ßow of power into and through the system. The single-line drawing identifiers the points at which power is, or can be, supplied into the system and at which power should be disconnected in order to clear, or isolate, any portion of the system.

Characteristics of an accurate diagram
The following characteristics should help to ensure accuracy as well as ease of interpretation:

a) Keep it simple
A fundamental single-line diagram should be made up of short, straight lines and components, similar to the manner in which a block diagram is drawn. It should be relatively easy to get the overall picture of the whole electrical system.

All, or as much as possible, of the system should be kept to one sheet. If the system is very large, and more than one sheet is necessary, then the break should be made at voltage levels or at distribution centers.

b) Maintain relative geographic relations
In many cases, it is possible to superimpose a form of the one-line diagram onto the facility plot plan. This is very helpful toward a quick understanding of the location of the system's major components for operating purposes.

It may, however, be more difficult to comprehend the overall system operation from this drawing. Such a drawing could be used for relatively simple systems. For more complex systems, however, it should be used in addition to the fundamental single-line diagram.

c) Maintain the approximate relative positions of components when producing the single-line diagram
The drawing should be as simple as possible and should be laid out in the same relationship as an operator would view the equipment. The diagram does not need to show geographical relationships at the expense of simplicity.

NOTE: A site plan with equipment locations may be required to accompany the single-line diagram.

d) Avoid duplication
Each symbol, figure, and letter has a definite meaning. The reader should be able to interpret each without any confusion. In this regard, equipment names should be selected before publishing the document; then, these names should be used consistently.

e) Show all known factors
All details shown on the diagram are important. Some of those important details are as follows:

Ñ ManufacturersÕ type designations and ratings of apparatus;
Ñ Ratios of current and potential transformers and taps to be used on multi-ratio transformers;
Ñ Connections of power transformer windings;
Ñ Circuit breaker ratings in volts, amperes, and short-circuit interrupting rating;
Ñ Switch and fuse ratings in volts, amperes, and short-circuit interrupting rating;
Ñ Function of relays. Device functions used should be from IEEE Std C37.2-1991;
Ñ Ratings of motors, generators, and power transformers;
Ñ Number, size, and type of conductors;
Ñ Voltage, phases, frequency, and phase rotation of all incoming circuits. The type
of supply system (wye or delta, grounded or ungrounded) and the available
short-circuit currents should be indicated.

f) Future plans
When future plans are known, they should be shown on the diagram or explained by notes.

g) Other considerations
Refer to IEEE Std 141-1993 for further discussion of singleline diagrams.