Departments› Biomedical Engineering› Module IV› Technical Drawings II› Ch 3: Produce electrical drawings
Biomedical Engineering  ·  Level 5
Technical Drawings II

Chapter 3: Produce electrical drawings

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⚑ What you will be able to do

By the end of this chapter, you will be able to:

  • Identify the key features of AutoCAD software by using the software manual.
  • Operate AutoCAD visual reference commands correctly to view your drawings clearly.
  • Use AutoCAD navigation command options accurately to move around your workspace.
  • Apply AutoCAD ribbon tools correctly to create and edit your drawings.
  • Utilize AutoCAD status bar tools accurately to monitor and adjust drawing settings.
  • Save your AutoCAD drawing files in the proper format following organisational procedures.
  • Print your AutoCAD drawings correctly by following the software manual.

Mastering these skills will help you produce precise and professional electrical drawings, an essential part of success in the electrical trade.

3.1 Electrical symbols and abbreviations

An electrical drawing does not show components as they look. It shows each component by a standard symbol, and it names quantities and parts by short abbreviations, so that a technician in any workshop reads the drawing in the same way. A technician in a biomedical workshop meets these symbols in service manuals, in installation drawings for a ward or imaging room, and on the circuit diagrams of the equipment being repaired. Misreading a symbol, for example confusing a fuse with a resistor or an earth with a neutral, leads to wrong connections and to danger.

Why standard symbols are used

  • A symbol is quick to draw and takes little space on the sheet.
  • The same symbol means the same component to every reader, whatever the language of the notes.
  • Drawings made by different people can be combined and compared.
  • Drawings can be produced in CAD from symbol libraries.
  • The international standard IEC 60617 defines graphical symbols for diagrams, and many manuals and CAD libraries follow it. Some manufacturers use older or national forms, so a drawing carries a key or legend that explains the symbols it uses.

Common electrical and electronic symbols

The table describes the symbols in words, because the shape matters more than the style of drawing.

Component Symbol (described) Usual letter
Resistor A small rectangle with a line at each end (older drawings use a zigzag) R
Capacitor Two short parallel lines with a gap; a polarised capacitor is marked with a plus sign or has one curved plate C
Inductor or coil A row of loops L
Diode A triangle pointing at a bar D
Light-emitting diode A diode symbol with two small arrows pointing away D or LED
Transistor A base line with two leads at an angle to it, the emitter lead carrying an arrow, the collector lead without one; a circle around it is optional Q
Operational amplifier A triangle pointing in the direction of the signal, with two inputs and one output U
Transformer Two coils side by side separated by parallel lines T
Fuse A small rectangle with a line passing through it F
Switch A line that makes or breaks contact between two terminals S
Cell or battery Long and short parallel lines, the long line being positive B or BT
Motor A circle with the letter M M
Earth (ground) A vertical line ending in a set of shortening horizontal lines GND or E

Abbreviations in drawings

Abbreviations are used on drawings for units, terminals and functions. Common ones are:

  • Quantities and units: V for volt, A for ampere, W for watt, Hz for hertz, Ω for ohm, AC for alternating current and DC for direct current.
  • Terminals and supplies: L for line or live, N for neutral, E for earth, GND for ground, Vcc and Vdd for the positive supply of a circuit.
  • Switches and relays: NO for normally open and NC for normally closed.
  • Parts and boards: IC for integrated circuit, PCB for printed circuit board, LED for light-emitting diode.

Reference designators

Each part on a drawing has a label made of its letter and a number, such as R1, R2, C1 and Q1. The label ties the symbol on the diagram to the item on the parts list and to its place on the board, so every part is identified without confusion.

Symbols found on medical electrical equipment

Medical electrical equipment carries safety symbols as well as circuit symbols. They include the protective earth symbol and the symbols for types of applied part, which are the parts that touch the patient and are classified as type B, type BF or type CF. The IEC 60601 series sets out the classification of applied parts and the safety symbols, and a technician reads them on the equipment label and the drawing before servicing it.

Drawing and reading symbols correctly

  1. Draw symbols to a consistent size and with clean lines.
  2. Draw the connection lines horizontal and vertical, and join them to the symbol at its terminals.
  3. Put the reference label next to the symbol and the value beside the label.
  4. Add a legend on the sheet for any symbol that is not in a standard library.
  5. Check each symbol against the legend or the library before the drawing is issued.
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🔒3.2 Meaning of electrical drawings

An electrical drawing is a document that shows how electrical equipment or an installation is arranged, connected and powered. It carries the information that a technician needs to install, test, repair or modify the equipment without having to trace every wir…

🔒3.3 Drawing of electrical diagrams

An electrical diagram represents an electrical system by symbols and lines instead of pictures. Different diagrams are drawn for different purposes: one to explain what a system does, another to show how its parts are connected, another to show how it is wired…

🔒3.4 Drawing of Electrical and Electronic drawings using an appropriate CAD software

Computer-aided design software draws electrical and electronic diagrams from libraries of symbols, keeps every connection as data and checks the drawing for errors, which is faster and more accurate than drawing by hand. Some packages work on circuits and prin…

🔒3.5 Simulation of Electrical and electronic drawings

Simulation uses software to calculate how a drawn circuit would behave, before any part is bought or soldered. The program reads the schematic, replaces each symbol by a mathematical model of the part, solves the equations of the circuit and displays voltages,…

🔒3.6 Installation of Electrical components in a building plan using AutoCAD

An electrical installation drawing shows where each socket, light, switch and distribution board is placed in a building, and how the circuits are routed between them. It is drawn on the architect's floor plan so that the position of every component is related…

Am I competent?

At the start of this chapter we promised you would be able to:

  • Identify the key features of AutoCAD software by using the software manual.
  • Operate AutoCAD visual reference commands correctly to view your drawings clearly.
  • Use AutoCAD navigation command options accurately to move around your workspace.
  • Apply AutoCAD ribbon tools correctly to create and edit your drawings.
  • Utilize AutoCAD status bar tools accurately to monitor and adjust drawing settings.
  • Save your AutoCAD drawing files in the proper format following organisational procedures.
  • Print your AutoCAD drawings correctly by following the software manual.

Tick each one you can genuinely do.

So, are you there yet?

You're competent when you can confidently do 50% or more of what this chapter promised.

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