2PurposeThis chapter describes electrical drawing types and conventions.It covers circuit diagrams, block diagrams, wiring diagrams and architectural electrical diagrams.
3IntroductionIn 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.
4Introduction (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.
5Introduction (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.
6Circuit diagramsRegardless 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.
7Circuit 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.
8Circuit diagrams (continued) 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.
9Circuit diagrams (continued) 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.
10Conventions 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.
11Conventions 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.
12Conventions in line work (continued) 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.
14Conventions in line work (continued) 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.
15Conventions in line work (continued) 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.
16Electric symbolsThere is no fixed size for symbols but all symbols in the same drawing should be the same relative size and proportion.Common AS/NZS symbols:
17Placement 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.
18Placement of circuit components (continued) Symbols should be:placed in line (aligned), or in the same relative position, if they are similarspaced 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.
20Placement of circuit components (continued) 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.
22Drawing 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.
23Drawing schematic circuit diagrams (continued) 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.
25Drawing 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.
26Drawing schematic circuit diagrams (continued) 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.
27Drawing schematic circuit diagrams (continued) 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.
28Drawing schematic circuit diagrams (continued) 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.
30Other 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.
32Other circuit representations (continued) The block diagram shown in Figure represents this complicated circuit:
33Other circuit representations (continued) 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.
34Other circuit representations (continued) 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.
35Other circuit representations (continued) 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.
36Other circuit representations (continued) Circuit components are drawn in the same relative position they would be physically located.
37Other circuit representations (continued) 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.
38Other circuit representations (continued) Figure 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.
39Other circuit representations (continued) In wiring diagrams:the conductors are all separated and equally spaced.connections are only made at terminals.Wiring diagrams represent the physical circuit layout.
40Other circuit representations (continued) 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.
41Other circuit representations (continued) 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.
42Other circuit representations (continued) 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 mm from the floor and the upper part removed.
46Other circuit representations (continued) 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.
48Other circuit representations (continued) 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.
54Contactors and relays (continued) 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.
55Contactors and relays (continued) 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.
56Contactors and relays (continued) The other contacts on the contactor are designed to carry smaller currents and they are often termed ‘auxiliary contacts’, or sometimes ‘control contacts’.
57Contactors and relays (continued) AS/NZS 1102 makes no real distinction between power contacts and control contacts. It merely provides two symbols labelled ‘form 1’ and ‘form 2’.
58Contactors and relays (continued) 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.
59Contactors and relays (continued) 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.
62Contactors and relays (continued) Contactors may be drawn in circuits in a number of ways.
63Contactors and relays (continued) 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.
65Contactors and relays (continued) 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.
66Control 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.
68Control 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.
70Control circuit variations (continued) ‘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.
71Control circuit variations (continued) 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’.
73Control circuit variations (continued) It may be necessary to start a motor- controlled device using a push-button switch but allow another control to turn it off.
74Control circuit variations (continued) When a push-button switch is connected so that a circuit operates only while the switch is held depressed it is called ‘jogging control’.
75Control circuit variations (continued) 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.
76Control circuit variations (continued) 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 interlockingelectrical interlocking.
77Control circuit variations (continued) 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 are used.
79Control circuit variations (continued) Many programmable logic controllers use the National Electrical Manufacturers Association (NEMA) standard for their circuit diagrams.
80Quick quiz List 4 types of electrical drawings. Which Australian Standard provides a list of standard drawing symbols?What is the purpose of a block diagram?What is a contactor?What is jogging in a motor control circuit?
81Quick quiz—answersCircuit diagrams, block diagrams, wiring diagrams and architectural electrical diagramsAS/NZS 1102A block diagram provides an overall picture of the function of groups of components in a circuit.A contactor is an electromagnetic switch that controls one or more contacts.Jogging is the momentary application of power to a motor to allow small movements of the motor.
82Summary Schematic circuit diagrams should: show exactly how a circuit operatesbe laid out in a neat and logical mannershow the sequence of events, energy flow or signal flow from left to right and/or top to bottombe drawn using AS/NZS 1102 symbols.
83Summary (continued) Block diagrams are useful because they: show what the circuit doesprovide an overview of a circuitare an aid to understanding a circuitcan provide a design starting point for a circuit.
84Summary (continued)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.
85Summary (continued)The features of an architectural electrical drawing are that they:use special architectural electrical symbolsuse a floor plan as a basisshow the positions of luminaires, outlets and fixed appliancesshow which switches control particular luminairesgive no indication on how a building is to be wired.
86Summary (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.
87Summary (continued)There are various types of contacts controlled by contactors and relays.Some types of contacts are:normally opennormally closedtimed on closingtimed in openingtimed on both closing and opening.
88Summary (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.
89Summary (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’.
90Summary (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.
91Summary (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 linesrungs are the various circuit lines.