12–1 PowerPoint slides t/a Electrical Principles for the Electrical Trades (Machines) Vol 2 6e by...

12–1 PowerPoint slides t/a Electrical Principles for the Electrical Trades (Machines) Vol 2 6e by Jim Jenneson and Bob HarperCopyright © 2012 McGraw-Hill Australia Pty Ltd

Chapter 12Electrical drawing practices


Purpose

• This chapter describes electrical drawing types and conventions.

• It covers circuit diagrams, block diagrams, wiring diagrams and architectural electrical diagrams.


Introduction

• In circuit diagrams, batteries, switches, lamps, resistors, capacitors and inductors are all represented by symbols called ‘graphical symbols’ as they represent some object in a greatly simplified pictorial form.

• Graphical symbols allow people who speak different languages to draw and read the same electrical circuit diagrams.


Introduction (continued)

• Electrical ‘schematic circuit diagrams’ do not necessarily represent the appearance of the circuit.– They do show, in a formal and easy to

interpret manner, is how a circuit operates.

• The flow of electricity can be better understood than it could be by looking at the actual wiring.


Introduction (continued)

• All electrical drawings should conform to the current AS/NZS 1102—Graphical Symbols for Electrotechnology, which is divided into various sections, each covering a specific group of symbols.

• The symbols used are, in most cases, the same as those used in ISO, DIN and other worldwide standards.

• When all drawings are carried out according to the relevant standard practice they become universally easier to read and understand.


Circuit diagrams

• Regardless of how carefully and accurately a pictorial drawing of a circuit is drawn the various connections are hard to view.

• Simplified and stylised connections and the use of symbolised components provides a clearer presentation of the circuit connections.

• The resultant drawing is known as a circuit diagram.


Circuit diagrams (continued)

• The wires and components are separated and drawn apart even though the drawing does not look like the physical layout.

• Instead of a mechanical representation of the structure of the circuit, the circuit diagram is intended to illustrate the connections between components and the power flow (or signal flow) through the circuit.


• Early drawers of electrical circuits made up their own symbols to represent components.

• Quite often, a symbol key was given with the drawing, listing what the symbols represented.



• Local and national standard styles of drawing, and standard symbols were adopted by larger groups who needed to share the drawings.

• Eventually an international body, the International Electrotechnical Committee (IEC), formulated an international standard of graphical representation.

• AS/NZS 1102 is very close to the IEC standard.



Conventions in line work

• The line used to represent a conductor on a circuit diagram is assumed to have zero resistance and it is therefore nothing more than a statement that the two components are connected together.

• The length of the conductor is not important to the circuit diagram.


Conventions in line work (continued)

• Conductors are always represented by straight lines with right angle turns, regardless of the path the actual conductor takes.

• These lines may join to the circuit components or to other conductors.

• When conductors connect to other conductors, the joint is usually indicated by a distinctive dot at the joint.


• When two conductors join to another conductor at the same point, the circuit drawing lines can be offset so that there can be no confusion.

• The connection is indicated by the dot at the point of connection.

• It should be noted that if no dot is shown there is no connection.



• When lines representing conductors must cross in a schematic circuit diagram, they should do so at right angles.

• There is no need to indicate that crossing conductors are not joined.

• In some diagrams, some conductors cross while others join. To avoid any possible confusion the conductors are often offset, or a current alternative method is used.



• Ordinary conductors are a simple single weight line, which in many circuit diagrams is the only type necessary.

• In some diagrams, however, some conductors carry more or less power than ordinary conductors.

• Power conductors are shown as bold or double width lines, while signal conductors are shown as thin or half-weight lines.

• Temporary connections may be shown as dashed or dotted lines.



Electric symbols

• There is no fixed size for symbols but all symbols in the same drawing should be the same relative size and proportion.

• Common AS/NZS 1102 symbols:


Placement of circuit components

• Connections to circuit symbols in schematic circuit diagrams should be made at some distance from the circuit symbols and not on the symbols themselves. – This is done so that the outlines of the

symbols are not confused with closely drawn conductor lines.


Placement of circuit components (continued)

• Symbols should be:– placed in line (aligned), or in the same

relative position, if they are similar– spaced evenly so that they are easy to

see and interpret.

• In the one drawing, repetitive parts of the circuit should be identical to avoid confusion.


• In electrical circuits, two components are often placed in parallel.

• If one symbol has more importance it is placed in the same line as the conductor and the parallel component symbol is offset.

• If they both have the same importance, they are placed evenly each side of the conductor line.



Drawing schematic circuit diagrams

• To make schematic circuit diagrams easier to read, certain conventions are used in their layout.

• One is that the flow of energy, or flow of ‘signal’ and sequence of operation or events, is from left to right and top to bottom.


• A schematic circuit diagram represents the components and the connections between them.

• A circuit diagram is intended to show how an electrical circuit operates whereas wiring diagrams are used to show how a circuit is actually constructed.

Drawing schematic circuit diagrams (continued)


• In Figure 12.8 the left-hand part of the circuit is drawn in lines twice as thick as those on the right-hand side. – This is because this section represents

the power circuit of the motor starter.

• Power passes from the supply at the top to the motor beneath.



• The control circuit is drawn in thinner lines because it takes no part in transferring energy to the motor and the actual currents are small.

• The thinner lines emphasise this factor.



• In Figure 12.8 the coils K1/5 and K2/5 are part of devices called ‘contactors’. – When energised they operate all their associated switch

contacts.

– K1 with the figure 5 under it signifies that it has 5 associated contacts.

• Contact K1.4 is part of the control circuit and, because it is in parallel with the start push-button switch, it takes its place in the circuit.

• The start switch can be released and K1/5 remains energised.– These contacts are commonly referred to as auxiliary hold

in contacts.



• Note in Figure 12.8 that:– the flow of energy and sequence of events was

either from left to right, or top to bottom. – all the contacts of the switches are drawn in

their normal or de-energised condition.

• When switch contacts are operated, either manually or from their associated coils, the line representing the operating part of the switch rotates clockwise.



Other circuit representations

• Other electrical circuit representations include:– block diagrams– wiring diagrams– architectural diagrams.


Other circuit representations (continued)

• A block diagram shows what a circuit does, not how it works.

• It is an overall picture of why a particular circuit is used and the function of groups of components in a circuit.

• Without knowing exactly how a circuit operates, the function of the circuit can be easily explained from the block diagram.


• The block diagram shown in Figure 12.9 represents this complicated circuit:



• Block diagrams usually do not show any power supply.

• They do not represent actual circuit connections but merely explain what the circuit does, so there is no need to include these details.



• A wiring diagram is much closer to the real thing than a schematic circuit diagram.

• A wiring diagram is a stylised true representation of the components and wiring of an electrical circuit.

• A photograph of the wiring of a circuit would be of little use if it was necessary to know exactly how all the connections are made.



• In a wiring diagram, lines representing the conductors are drawn straight and separate.

• They are usually evenly spaced and are all connected to circles representing the terminals on the circuit components.

• In addition, where more than one conductor line terminates at a terminal, all lines are angled to the terminal circles, giving a clear indication where each line (representing a conductor) starts and finishes.



• Circuit components are drawn in the same relative position they would be physically located.



• A wiring diagram is seldom drawn to scale but in some cases it may be in direct proportion to the shape of the object represented.



• Figure 12.14 is a true representation of the motor starter but not drawn to any particular scale.

• The outlines of all components are drawn as dashed lines.

• The schematic circuit diagram symbols have been inserted to show the actual operation of the circuit components.

• Greatest prominence is given to the lines representing the wiring conductors and the circles representing the terminals.



• In wiring diagrams:– the conductors are all separated and

equally spaced. – connections are only made at terminals.

• Wiring diagrams represent the physical circuit layout.



• A wiring diagram is invaluable for actually wiring up a piece of equipment or checking equipment when looking for a fault.

• In many cases the designer of a piece of equipment may first produce a block diagram, then design a schematic circuit diagram and finally produce a wiring diagram.



• An architectural electrical drawing indicates to the installing electrician where the outlets are to be placed.

• It does not give any detail on how wiring and circuit connections are to be carried out.

• In most cases the architectural electrical drawing is accompanied by a set of specifications and a schedule. These may give much more detail on the work to be done.



• An electrical architectural drawing is a drawing of the ‘floor plan’ of a building to which the electrical symbols have been added. – A floor plan of a building is a view looking

down on the building, imagining that the building had been cut through at a level of 1250 mm from the floor and the upper part removed.



• Electrical symbols are drawn on the floor plan to make the architectural electrical drawing.

• The symbols used are different to those used in either schematic circuit diagrams or wiring diagrams.

• They have been derived so that they are easy to draw, easy to recognise and distinctive.



• Dashed lines simply represent which switches control particular luminaires, not the path of the wiring; this is left to the installing electrician who will follow any instructions given in the specifications and will work within the requirements of AS/NZS 3000.



Contactors and relays

• A relay is an electromagnetically operated switch.

• A contactor is an electromagnetically operated switch that controls power to a load.

• A contactor is effectively a large relay.


Contactors and relays (continued)

• A contactor consists of three basic parts: – operating coil– associated magnetic circuit– contacts that are actuated by the

coil.


• When the coil is energised a magnetic field is produced in the magnetic circuit.

• This attracts the hinged armature, against the tension of the spring, to complete the magnetic circuit.

• The movable contact attached to, but insulated from, the armature closes against the fixed contact.



• When the coil is de-energised the armature springs open and also opens the contact.

• Contactors usually have multiple contacts.

• Power contacts are designed to carry the full rated current of the contactor.



• The other contacts on the contactor are designed to carry smaller currents and they are often termed ‘auxiliary contacts’, or sometimes ‘control contacts’.



• AS/NZS 1102 makes no real distinction between power contacts and control contacts. It merely provides two symbols labelled ‘form 1’ and ‘form 2’.



• Both power and control contacts may be either ‘normally open’ or ‘normally closed’.

• In drawings contacts are always drawn in the de-energised state.

• The operation of contact symbols (whether opening or closing) is always considered to be in a clockwise direction.



• Timed contacts will close, or open, after a designated time.

• The timing device may be mechanical or electrical.

• When electrically timed contacts are required, quite often a timing relay is used.

• Any timed contacts are designated by a special symbol.



• Contactors may be drawn in circuits in a number of ways.



• The most common method for drawing contactors is termed ‘detached representation’:– the contactor coil is not only labelled with the

contactor designation but also with the number of contacts it operates.

• This method makes the diagram easier to read.

• The broken line indicates that all three contacts close simultaneously.



• The circuit layout with the flow of energy and sequence of events moves from left to right and top to bottom.

• All components have been spaced across the drawing and kept in line as much as possible.

• Any drawing should be laid out in this manner, even if it is only a pencil sketch. It makes it much easier to read and interpret at a later date.



Control circuit variations

• To provide extra start positions, all that is needed is to connect extra start push-buttons in parallel with the first.

• To stop or de-energise the circuit extra stop push-button switches are placed in series with the first.


Control circuit variations (continued)


• One start position with multiple stop positions can be necessary when a number of emergency stop positions might be required in a plant.

• Only one push-button switch will start the operation but if there is a malfunction in the plant it can be stopped by operators at any number of positions.



• ‘Local’ or ‘remote operation’ avoids the possibility of someone operating the machine at the incorrect position by the inclusion of a changeover switch, so that only one position at a time can be used.



• Automatic motor start control could be from a thermostat on a refrigerator, a float switch on a water tank or a pressure switch on an air compressor.

• Because there is no other stop/start control and only two wires need to be run to the actuating device, is it usually referred to as ‘two-wire control’.



• It may be necessary to start a motor-controlled device using a push-button switch but allow another control to turn it off.



• When a push-button switch is connected so that a circuit operates only while the switch is held depressed it is called ‘jogging control’.



• To reverse a three-phase motor, all that is necessary is to interchange two supply lines to the motor.

• This can be accomplished by using two contactors.



• Two supply lines would be short-circuited if the forward and reverse contactors closed at the same time.

• This can be prevented in two ways:– mechanical interlocking– electrical interlocking.



• Control circuits in particular can be drawn as ‘ladder’ diagrams.

• Ladder diagrams also follow the requirement that energy flow and sequence of events be from left to right and top to bottom.

• Symbols as recommended in AS/NZS 1102 are used.



• Many programmable logic controllers use the National Electrical Manufacturers Association (NEMA) standard for their circuit diagrams.



Quick quiz

1. List 4 types of electrical drawings.

2. Which Australian Standard provides a list of standard drawing symbols?

3. What is the purpose of a block diagram?

4. What is a contactor?

5. What is jogging in a motor control circuit?


Quick quiz—answers

1. Circuit diagrams, block diagrams, wiring diagrams and architectural electrical diagrams

2. AS/NZS 1102

3. A block diagram provides an overall picture of the function of groups of components in a circuit.

4. A contactor is an electromagnetic switch that controls one or more contacts.

5. Jogging is the momentary application of power to a motor to allow small movements of the motor.


Summary

• Schematic circuit diagrams should:– show exactly how a circuit operates– be laid out in a neat and logical manner– show the sequence of events, energy flow

or signal flow from left to right and/or top to bottom

– be drawn using AS/NZS 1102 symbols.


Summary (continued)

• Block diagrams are useful because they:– show what the circuit does– provide an overview of a circuit– are an aid to understanding a circuit– can provide a design starting point for a

circuit.


• A good clear wiring diagram should have the following features:– Prominence should be given to the

conductors and terminals. – All conductors should be evenly spaced.– Where more than one conductor connects

to a terminal, each conductor should be angled to the terminal.

– All changes of direction of conductors should be at right angles.

Summary (continued)


• The features of an architectural electrical drawing are that they:– use special architectural electrical symbols– use a floor plan as a basis– show the positions of luminaires, outlets

and fixed appliances– show which switches control particular

luminaires– give no indication on how a building is to

be wired.

Summary (continued)


• Relays and contactors are both electromagnetically operated switches. – A contactor is used in a power circuit.– A contactor can have both power and

control contacts.– Both contactors and relays consist of an

operating coil, a magnetic circuit and associated contacts.

Summary (continued)


• There are various types of contacts controlled by contactors and relays.

• Some types of contacts are: – normally open– normally closed– timed on closing– timed in opening– timed on both closing and opening.

Summary (continued)


• In detached representation in schematic circuit diagrams:– The contactor coil is designated with both

the contactor designation and the number of contacts it operates.

– The coil designation may be placed on or beside the coil symbol.

– Each contact has the designation of the contactor and a number usually representing its importance or its order of operation.

Summary (continued)


• In push-button switch control:– Extra start push-button switches are all

placed in parallel.– Extra stop push-button switches are all

placed in series.

• Control by some automatically operated switches is termed ‘two-wire control’.

Summary (continued)


• When reversing contactors are used: – They are mechanically interlocked.– They are also electrically interlocked.

• A jogging control will allow the circuit to be energised only while it is held depressed.

Summary (continued)


• When drawing schematic circuit diagrams, the top to bottom and left-to-right rule should be followed.

• Control diagrams are usually drawn as ladder diagrams.

• In ladder diagrams:– stiles are the supply lines– rungs are the various circuit lines.

Summary (continued)

12–1 PowerPoint slides t/a Electrical Principles for the Electrical Trades (Machines) Vol 2 6e by...

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