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Interpreting Electrical Schematics

Objectives

  • Differentiate between the different types of symbols
  • Explain the basic rules of reading schematics
  • Review examples of electrical schematics

Reading Schematics

Electrical and electronic systems are presented in schematic or diagramatic forms.

The major types of visual representation are:

  • Pictorial
  • Components
  • Block Diagrams
  • Schematic Diagrams
  • Wiring Diagrams

It is the job of a draftsperson to create the schematic so that it is "electrically" correct. It is of equal importance that the schematic is pleasing to the eye and easy to interpret.

A schematic or wiring diagram that illustrates how components are connected is essential to good troubleshooting.

Symbols

Without knowing what component the symbol represents, or how the component operates, it is difficult to understand the operation of the device and therefore repair it.

There are several national and international symbols standards:

  • ANSI Standard Y32 (also known as IEEE Standard 315)
  • IEEE Standard 91/91a
  • IEC Std 60617 (also known as British Standard BS 3939)
  • Russian Standard GOST 2.755-87
  • Australian Standard AS 1102

Some designers use schematic drawing software with libraries that don't conform to any of these standards.

All component and device symbols are usually accompanied by a component index and value or part number.

Labels are located next to their component.

Some components may require extra information such as application, location or pinout indexing.

On integral assemblies or modules, every terminal will have a specific label to describe its function.

Here are some wire and harness symbols commonly used on Caterpillar schematics:

Layout

Components in a schematic are arranged to make the function of the circuit clear, rather than to represent the actual physical location of components relative to each other.

The schematic is laid out to show the function of the circuit, usually with input to output progressing from left to right.

Simple IES schematics will have the positive power supply line at the top and negative (or ground) line at the bottom of the circuit.

For NEMA schematics, the positive line will be on the left and negative will be on the right.

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