hampson, j 2013, electrotechnology practice: section 4

PowerPoint

to accompany

Section 4

Drawings, diagrams and schedules

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Mechanical drawing

Mechanical drawing is the main method of communication between all people concerned with the design and manufacture of components, building and constructions and engineering projects.


Drawing standards

Engineering drawings and other technical drawings have to be done in a way that workers in the specific industry can recognise.

These ways are called standards or conventions.


Drawing standards

The most common system of paper sizes in Australia is the ISO standard.

Drawings are done on standard-size sheets, ranging from A0 to A4.

This not only aids storage but is also important in the photocopying of drawings.


Drawing standards


Drawing standards

Drawings may be divided into zones by a grid reference system based on horizontal numbered (L to R) and vertical lettered divisions (A at the top) to assist in readily locating a particular dimension or feature, especially in large drawings.

Note that the letter I is not used, and letters are read from the bottom of the drawing.


Drawing standards


Drawing standards

The intention of an engineering drawing is to convey all the necessary information of how to make the part.

For many engineering parts, the necessary information cannot be communicated in a single view of one side of the part.

Rather than using a number of drawing sheets with different side views of the part, several views can be combined as a single drawing using either first angle or third angle projection.


Drawing standards

Lines in drawings differ in thickness and in the way they are drawn depending on the size of the paper and the job to be done.

However, each kind of line and each thickness must conform to national and international standards.


Drawing standards

Written information on a drawing is always in standard lettering.

The standard regulates the shape and size of letters and numerals.

Symbols are used for items such as dimensions, radius, diameter, tolerance, surface textures, weld details and methods of projection.


Drawing standards


Title block information

The basic requirements for a title block located at the bottom right-hand corner of a drawing are:

the name of the company

the name of what is drawn

the drawing number for storage and reference purposes

the sheet number in a set of drawings

the name of the person who drew the drawing

the name of the person who checked the drawing


Title block information

Basic requirements for a title block (cont.)

the issued date

the size of the original sheet

the scale

any changes

projection symbols

material that the object is made from

grade of finish

tolerance


Drawing standards


Mechanical drawing

Line types

If all lines on a drawing were equally thick, the drawing would be confusing and difficult to interpret, as the outlines would not stand out from the dimension lines.

By varying the thickness and construction lines on a drawing you can express meaning that is otherwise difficult to express.


Drawing standards


Mechanical drawing

Angular lines

Most engineering lines can be drawn by using 30o–60o and 45o set squares in various combinations.

The combinations enable the drawing of lines at angles of 15o, 30o, 45o, 60o and 75o.


Mechanical drawing

Sections and sectional views

Sections and sectional views are used to show hidden detail more clearly.

They are created by using a cutting plane to cut the object.

A section shows the outline of the object at the cutting plane.

Hatching is used in order to aid readability and on sections and sectional views solid areas should be hatched.


Mechanical drawing

Dimensions

The outline of the object indicates the shape of the object; the dimensions indicate the size of the object.

Only those dimensions necessary for the manufacture of the object should be shown on the drawing.


Dimensions

Points to consider with dimensions are:

Place dimensions well clear of drawings and one another.

Place dimensions where they will be best under stood (in relation to a datum).

Use a 0.50 pen for the numbers and arrowheads.

Generally show dimensions once only.


Dimensions

Keep dimensions off the actual view.

Dimension all circles and arcs radially.

Avoid crossing dimension lines.

Avoid dimensioning hidden details.

Dimension along the dimension lines placing sizes so that they will read from the bottom and right-hand side of the sheet.


Dimensions

Dimension lines and projection lines

Projection lines are used to indicate the extremities of a dimension.

They are generally drawn up to 1 mm from the outline of the object.

Dimension lines are used to label a particular dimension.


Drawing standards


Drawing standards

Linear dimensions

Generally linear dimensions are in millimetres. To avoid having to specify ‘mm’ after every dimension, a label such as ‘all dimensions are in mm’ or ‘unless otherwise stated all dimensions are in mm’ is usually contained in the title block.


Drawing standards

Angular dimensions

Angular dimensions are usually specified in decimal degrees, degrees and minutes or degrees, minutes and seconds.


Drawing standards


Drawing standards

Scales

The true size of an object may vary from 1 mm to thousands of mm.

Small objects should be drawn at their natural size.

Larger ones should be drawn to a recognised scale so that they will fit on the drawing sheet.


Drawing standards


Scales

Indication of scale

The scale should normally be noted in the title block of a drawing.

When more than one scale is used they should be shown close to the views to which they refer, and the title block should read ‘scales as shown’.


Drawing standards


Scales

In an engineering drawing to a scale of 1:20:

1 mm will represent 20 mm (one unit of measurement on the drawing equals 20 units of measurement on the actual object)

2 mm will represent 40 mm

10 mm will represent 200 mm

100 mm will represent 2000 mm, or 2 m


Scales

For a scale of 1:10: 100 mm will represent 1000 mm, or 1 m.

For a scale of 1:100: 100 mm will represent 10 000 mm, or 10 m.

For a scale of 1:50: 100 mm will represent 5000 mm, or 5 m.

For a scale of 1:200: 150 mm will represent 30 000 mm, or 30 m.


Scales

It is simply a matter of multiplying the scale measurement by the scale ratio.

1:(10) x 100 mm = 1000 mm or 1 m

1:(50) x 100 mm = 5000 mm or 5 m

1:(200) x 150 mm = 30 000 mm or 30 m


Scales

Tolerance

This is taken to be 0.5 of the unit being measured.

For example, the tolerance of 1000 mm is ± 0.5 mm or the upper tolerance is 1000.5 mm and the lower tolerance is 999.5 mm.


Machining symbols

Machining symbols are used to communicate meaning in a simple form.

Once learned, symbols help in the understanding of exactly what needs to be done.

Engineering drawing symbols are drawn to Australian Standard AS 1101 Part 3.


Machining symbols


Types of drawing representations

Drawings of an object can be drawn as a sequence of side views called orthographic projection or as a figure that looks like the actual appearance and is called a pictorial projection.



Orthogonal projection is a way of drawing an object that shows all the side views of an object drawn as separate distinct drawings but in relationship to one another.

The names given to each side view are the plan view, front elevation, side elevations and bottom view.



There are two different ways of presenting orthogonal projection, based on two different principles called first-angle projection and third-angle projection.

Third angle is used in Australia, Canada and the United States.

First angle is used in Europe.



The principal views are:

Front view—shows the most features or characteristics.

Left-side view—shows what becomes of the left side of the mechanical component after establishing the front-view position.

Right-side view—shows what becomes of the right side of the mechanical component after establishing the front-view position.



Principal views (cont.)

Plan view—shows what becomes of the top of the mechanical component once the position of the front view is established.

Bottom view—shows what becomes of the bottom of the mechanical component once the position of the front view is established.

Rear view—shows what becomes of the rear of the mechanical component once the position of the front view is established.



Detail drawings

The person who makes the mechanical component must clearly understand the shape, size, material and surface finish of a part, what machinery operations are necessary and what limits of accuracy must be observed from the detail drawing.



Assembly drawings

An assembly drawing is a presentation of the individual component put together, showing all parts in their operational positions.



Pictorial views

This is a method of drawing objects in a three-dimensional form. There are three different pictorial projection drawing methods for illustrating objects or buildings:

isometric projection

axonometric projection

oblique projection



Oblique projections can be either:

cavalier

cabinet.


Sketching

Freehand sketching, is a fast reliable method used to record data with respect to electrical fitting components and circuit wiring.

Sketching is a clear way to record technical data that may be needed later for re-assembly, re-connection or for reference.


Sketching

For making dimensional sketches in the field, you will need a measuring tape or rule, depending on the extent of the measurements taken.

In freehand pencil sketching, draw each line with a series of short strokes instead of with one stroke.

Strive for a free and easy movement of your wrist and fingers.

You don’t need to be a draftsman or an artist to prepare good working sketches.


Sketching

Printed grids or papers with lightly ruled lines are often used for sketching.

The grids are used to keep freehand lines straight.

Grids may also be used to create accurately scaled sketches, keeping the drawing in correct proportion.

A sketch is not an rough drawing.

It should be a clear image sketched in proportion, clearly dimensioned and labelled.


Sketching


Architectural drawings

Building structures

An electrician must be familiar with all the common terms used with the various building construction methods.

This knowledge is necessary for correct interpretation of architectural drawings - building plans and specifications.



Brick veneer

Brick veneer construction is a single brick covering tied across the air space to a timber or steel frame with wall ties embedded in the mortar joints.

The timber or steel frame does all the structural load bearing work by supporting all the wall and ceiling linings and the roof trusses and roofing material.



Cavity brick

Cavity brick is two brick walls separated by a 45 mm cavity and tied to each other with wall ties.

The inner brick wall carries out the same structural load bearing work as the timber or metal frame of the brick veneer type construction.



Roof truss

Many roof trusses today are prefabricated for most types of roofing designs such as hip and valley, gable end, Dutch gable and box gable.


Stages of constructionStages of construction include:

setting out

drainer, foundation trenches, temporary power pole and/or meter box position, underground conduit

foundation poured

slab, thickening beam (heating systems or cables may have to be installed) poured or bearers, joists and flooring erected

wall frames erected

roof framing, fascia and barge boards fitted


Stages of construction roof tiled or sheeted

windows and external doors installed

external lining erected

soffits fitted

plumbing installed

electrical fit-out (rough-in, or first fix) installed

ceilings and walls lined and cornice fitted

bath, shower, WCs and kitchen installed

doors, architraves and skirting fitted

plumbing finished off

painting

electrical finish off, or second fix


Electrical plan

The electrical plan is a floor plan which provides the location and type of switches, socket outlets, clocks, lighting points, switchboard, consumer’s mains, water heater, etc.


Electrical plan


Electrical specification schedule

An electrical specification schedule indicates the type of appliance to go into the installation and where the appliance is located.


Drawing scales

A scale drawing is a reproduction diagram of a building or object, but at a standardised fraction of its original size.

When designing a building, you use a specific scale to draw diagrams.

For example, when you need to draw a plan of a building you reduce the size of the diagrams to 1/50 or 1/100 of its actual size, that is, you use a 1:50 or 1:100 scale.


Electrical drawings

Symbols

Derived electrical symbols and their meanings can be sourced from AS/NZS 1102.102:1997 Graphical symbols for electrotechnical documentation.


Electrical drawings


Electrical drawings

Symbols are communications that have specific meaning.

Usually visual, symbols act as communication shortcuts that convey one or more messages that have been previously learned by both the sender and the recipient.

When symbols are standardised a common language is provided which orders and standardises electrotechnical documentation throughout Australia and New Zealand.


Electrical drawings

Block diagrams

Block diagrams are a method of explaining complex systems in a simple form.

In block diagrams various items are shown by a geometrical figure (square or rectangle) and labelled to indicate its purpose.


Electrical drawings


Electrical drawings

How to construct a block diagram

Identify the process. Define the start and end point for the system process to be inspected.

Identify the key process contributors. Identify all key elements that participate in the process operations.

Outline the diagram. For each step, describe the activity, place it in a small box and locate the box in the order the activities occur.


Electrical drawings

Label the diagram. Above each box diagram write the name of the process being examined.

Indicate input and output. Just outside of each block list the input that activates the process and outside the bottom of the diagram list the output that ends the process.

Connect the process steps. Connect the boxes with arrows to show the sequence of activities.

Verify accuracy. Consult with other persons familiar with the system process.


Electrical drawings

Line diagrams

Line diagrams are a simplified notation for representing a power system.

Instead of representing each conductor with a separate line or terminal, only one conductor is represented.


Electrical drawings


Electrical drawings

Wiring diagrams

A wiring diagram is a detailed diagram of a circuit showing all of the conductors and where they are connected, connectors, links, bars and electrical devices of the circuit.

The wiring diagram also identifies the conductors by numbers, letters or colour coding.

Wiring diagrams are necessary to fault-find and repair electrical installation circuits.


Electrical drawings


Electrical drawings

Cable schedule

Cable schedule means the schedule which contains circuit number, core size, current rating, route length and application.

Without a cable schedule you cannot estimate the cable cost and total length of cable required.


Electrical drawings


Lighting circuits

Circuit wiring—TPS cable

Install TPS wiring for lighting circuits at the first-fix stage of installation work with long tails for ease of switch connection and to provide for an adequate drip loop.

A drip loop prevents moisture from flowing down the cable and onto the connector.


Lighting circuits


Lighting circuits

One-way lighting circuits

A lighting circuit consists of a single-pole, double-throw (SPDT) switch and one or more luminaires connected in parallel.


Lighting circuits


Lighting circuits

One-way lighting circuits

Looping at the switch means that the circuit brings the power to the switch first and then to the load.

The next light circuit will be connected from the common and the loop terminal at the switch and also to the earth terminal.

In addition an earth pig tail from the earth termination at each switch must be connected to the building frame if a metal frame construction is used.


Lighting circuits


Lighting circuits

Two-way lighting circuits

A two-way lighting circuit consists of two single-pole, double-throw (SPDT) switches and one or more luminaires.

This circuit allows the luminaires to be switched on and off independently of the other switch.

An example of such a system is in a stairwell where the luminaires can be switched on and off from both upstairs and downstairs.


Lighting circuits


Lighting circuits

Two-way plus intermediate lighting circuits

Two-way plus intermediate switching (also called three-way) uses two single-pole, double-throw (SPDT) switches and a specially designed double-pole, double-throw (DPDT) switch with four operative terminals instead of the usual three operative terminals and a loop terminal as found in an SPDT switch.


Lighting circuits


Lighting circuits

Master on switching

A master on switch is used with respect to two-way plus intermediate switching when control over the ‘on-time’ for the luminaires is required.

This may be at a motel where to attract clients the exterior luminaire would be mastered on thereby preventing individual room occupiers from turning out the exterior light.

At a set time, control over the exterior luminaire is given back to the room occupier.


Lighting circuits


Lighting circuits

Master off for two-way switching

Master off switching using looping at the light method is simply an SPDT switch connected in the active conductor prior to entering the first two-way switch.


Lighting circuits


Lighting circuits

Converting from a circuit diagram to a wiring diagram

It is important to be able to convert a circuit diagram to a wiring diagram in order to build the circuit correctly.

A wiring diagram shows the physical relationship of all the components, as well as the information needed to install the circuit.


END

hampson, j 2013, electrotechnology practice: section 4

Education