# Chapter 3 Electrical Circuits and PLCs

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Chapter 3 Electrical Circuits and PLCs
Electrical Symbols and Diagrams • Standard Electrical Symbols • PLC Programming Symbols • Pictorial Drawings • Wiring Diagrams • Line (Ladder) Diagrams • Logic Functions • AND Circuit Logic • OR Circuit Logic • NOT Circuit Logic • NOR Circuit Logic • NAND Circuit Logic • Electrical Wiring Methods • Direct Hardwiring • Hardwiring Using Terminal Strips • PLC Wiring

Switch symbols are drawn to provide a visual indication of how each switch operates in a circuit.
A symbol is a graphic element that represents a quantity, unit, device, or component. Standard electrical diagrams use very specific symbols to represent the electrical equipment used in electrical circuits. Each symbol is drawn to provide a visual indication of what the device or component is and to show how the equipment operates. For example, devices such as pushbuttons, limit switches, temperature switches, pressure switches, flow switches, level switches, and foot switches all have specific symbols that are easily recognized. See Figure 3-1. See Appendix.

Any normally open (NO) switch that is manually operated has the operator of the switch drawn above the terminals, and any normally closed (NC) switch that is manually operated has the operator of the switch drawn below the terminals. Understanding whether a switch is manually, mechanically, or automatically operated is important because each switch has a symbol that is used on electrical prints. Any normally open (NO) switch that is manually operated will have the operator of the switch (part that moves) drawn above the terminals of the switch. See Figure 3-2. The symbol for a normally open switch shows the manual operator moving downward when activated.

Electrical switches are drawn in normal condition or activated condition on electrical prints.
Manually operated switches are drawn on electrical diagrams in the “normal” condition, which is the nonactivated condition (no one touching the switch). Because mechanical and automatic switches can be in an activated or nonactivated condition after the switches are installed in a circuit, the switches can be drawn in either condition on a print. For example, a normally open limit switch can be drawn on a print as either normally open or normally open, held closed. See Figure 3-3.

Switches are drawn on prints in the operating condition the switch is most likely to be in at any point in time. The reason for drawing a switch on an electrical diagram in the activated position is to better understand a circuit’s operation when troubleshooting. For example, a refrigerator has a lamp inside that is turned ON when the refrigerator door is opened and turned OFF when the door is closed. The opening of the door releases a limit switch so the switch can be returned to normal condition (condition of switch before activation). The closing of the door activates the limit switch so that the normally closed contacts open. See Figure 3-4.

Not understanding how manual, mechanical, and automatic switch symbols are drawn makes circuit understanding and troubleshooting difficult. In a refrigeration control circuit, two pressure switches are used to provide safety and protect the refrigeration unit and circuits. See Figure 3-5. A high-pressure switch (HPS or HPC) is used to prevent a higher-than-normal system pressure from damaging the compressor and/or burning out the electric motor by causing an overload condition.

Solid-state switches start and stop the flow of electricity in a circuit without the use of moving parts. Solid-state switch symbols show the actual switch symbol in a diamond shape. Originally all electrical switches used mechanically operated contacts to start and stop the flow of electricity in a circuit. Today, both mechanically operated and solid-state (no moving parts) switches are used to start and stop the flow of electricity in a circuit. The symbol difference between a mechanical switch and a solid-state switch is that a solid-state switch includes a diamond shape around the switch symbol. See Figure 3-6.

Programming symbols for input devices are drawn as generic normally open or normally closed contacts, and all output components are drawn as parentheses. When designing a program for controlling a circuit using a PLC, device and component symbols are selected and displayed on a computer screen as the electrical circuit is developed. The symbols are typically selected from a tool palette that is displayed on the screen. Basic symbols used to program a PLC include normally open inputs, normally closed inputs, and standard outputs. See Figure 3-7.

Special symbols are used in PLC programming when a PLC has advance control functions.
All expanded input devices and output components have a specific symbol that represents the device or component. Like any PLC programming input or output symbol, words are typically added to the symbol to further explain what the symbol represents. Although exact symbols differ slightly from manufacturer to manufacturer, most symbols have typical shapes and designations. See Figure 3-8.

Pictorial drawings are used to provide a visual picture of devices, components, and wiring.
A pictorial drawing is a drawing that resembles a photograph or three-dimensional picture. A pictorial drawing shows what a device looks like, but does not give accurate dimensions or other specific detail information. PLC pictorial drawings show the layout and position of devices and components used for an application. See Figure 3-9.

Wiring diagrams show the connection of all devices and components to a PLC.
A wiring diagram is a drawing that shows the connection of all devices and components to a PLC. Wiring diagrams show the connections as closely as possible. Internal and external connections can be shown in detail to allow tracings. Wiring diagrams are used when installing equipment and when troubleshooting. See Figure 3-10.

Thermocouples are connected to analog PLC input terminals because thermocouples produce a voltage proportional to the measured temperature. Wiring diagrams are also used to show smaller more specific parts of PLC wiring. For example, thermocouples can be connected to the analog input section of a PLC to provide temperature information to the PLC. A thermocouple is a temperature sensor made of two dissimilar metals that are joined at the end where heat is to be measured. The two dissimilar metals produce a voltage output at the nonjoined end that is proportional to the measured temperature. See Figure 3-11.

Line diagrams use standard electrical symbols to indicate what types of input devices and output components are being used in a circuit. Line (ladder) diagrams use standard electrical symbols to indicate what types of input devices and output components are being used. PLC programming diagrams use generic symbols that are identified with words to indicate what types of input devices and output components are being used. See Figure 3-12.

Standard line diagrams and PLC programming diagrams are similar, although different symbols are used to represent the same device or component. Except for a few differences, PLC programming diagrams are similar to ladder diagrams. PLC programming diagrams have two vertical power lines that represent L1 and L2 (X1 and X2 when a control transformer is used); however, no voltage potential exists between the two lines. Horizontal lines (rungs) represent the current flow paths between the vertical power lines. Each rung can represent several input devices and an output component. See Figure 3-13.

All electrical control circuits that use any type of switches to control loads are comprised of switching logic functions. All electrical control circuits (hardwired or PLC) that use switches to control loads are comprised of basic switching logic functions. The switching logic functions are used individually or in combination. The five basic switching logic functions are AND, OR, NOT, NOR, and NAND. Normally open (NO) and normally closed (NC) AND and OR switches or contacts connected in series or parallel are used to create the five logic functions. The five basic logic functions are used in ladder diagrams, PLC programming diagrams, and digital logic diagrams. See Figure 3-14.

All input devices of an AND logic circuit must be activated in order to energize the output component. AND circuit logic is control logic developed when two or more normally open switches or contacts are connected in series to control a load. A logic circuit is AND logic because input 1 AND input 2 must be activated to energize the output component. The input devices that develop AND logic circuits can be any number of normally open switches (contacts) connected in series. See Figure 3-15.

Only one input of an OR logic circuit is activated in order to energize the output component.
OR circuit logic is control logic developed when two or more normally open switches or contacts are connected in parallel to control a load. A logic circuit is OR logic because input 1 OR input 2 is activated to energize the output component. The input devices that develop OR logic can be any number of normally open switches (contacts) connected in parallel. See Figure 3-16.

The input of a NOT logic circuit must be deactivated in order to energize the output component.
NOT circuit logic is control logic developed when a normally closed switch or contact is connected to control a load. A logic circuit is NOT logic because the output component is ON when the input device is not activated. Only one normally closed switch (contact) is used to develop NOT logic. When two or more normally closed contacts are used together, NOR logic or NAND logic is developed. See Figure 3-17.

Only one input of a NOR logic circuit must be deactivated in order to energize the output component.
NOR circuit logic is control logic developed when two or more normally closed switches or contacts are connected in series to control a load. A circuit logic is NOR logic because the output component is ON when input 1 OR input 2 is not activated. The input devices that develop NOR logic can be any number of normally closed switches (contacts) connected in series. See Figure 3-18.

All input devices of a NAND logic circuit must be deactivated in order to energize the output component. NAND circuit logic is control logic developed when two or more normally closed switching contacts are connected in parallel to control a load. A circuit logic is NAND logic because the output component is ON when input 1 and input 2 are not activated. The input devices that develop NAND logic can be any number of normally closed switches (contacts) connected in parallel. See Figure 3-19.

In direct hardwired circuits, the power circuit and control circuit are wired point-to-point.
Direct hardwiring is the oldest and most straightforward wiring method used. Direct hardwiring is a wiring method where the power circuit and the control circuit are wired point-to-point. See Figure Point-to-point wiring is a wiring style where each component in a circuit is connected (wired) directly to the next component as specified by wiring or ladder diagrams.

To add forward rotation indicator lamps and reverse rotation indicator lamps to a direct hardwired circuit is difficult because the exact connection points for the lamps must be found. A direct hardwired circuit may be difficult to modify. See Figure For example, when a forward rotation indicator lamp and a reverse rotation indicator lamp are to be added to a motor control circuit, the exact connection points for the lamps must be located. Once the exact connection points are found, the lamps can be wired into the control enclosure. However, problems can arise when making the actual connection. For example, there may not be enough room under a terminal screw for more wires to be connected.

To make modifications to direct hardwired circuits typically requires the removal and/or addition of circuit wiring. A hardwired circuit modification such as adding forward and reverse indicator lamps may not be a problem if the lamps only require the addition of a couple of new wires. See Figure When performing an indicator lamp modification, old wires are not required to be moved or removed.

Hardwiring using terminal strips dramatically simplifies circuit troubleshooting.
Hardwiring to a terminal strip allows for easier circuit modifications and simplifies circuit troubleshooting. When using terminal strips for wiring, each wire in the control circuit is assigned a reference number (terminal screw wire number) on the ladder diagram to identify the various wires that connect the components in a circuit. Each reference point is assigned a wire reference number. See Figure 3-23.

Modifications are easier with terminal strips because most, if not all, of the wires required to make a change are disconnected and reconnected at the terminal strip. Terminal strips with wire reference numbers help in troubleshooting. Terminal strips also make circuit modifications easier to perform. Modifications are easier because most, if not all, of the wires required to make a change are disconnected and reconnected at the terminal strip. See Figure 3-24.

When using a PLC to control a circuit, all input devices are wired to the input module of the PLC, and all output components are wired to the output module of the PLC. When using a PLC to control a circuit, the PLC is part of the control circuit. The power circuit to a component does not change. What does change is that the control circuit input devices (pushbuttons, limit switches, and overload contacts) are wired to a PLC input module, and the control circuit output components (motor starter coils and indicator lamps) are wired to a PLC output module. See Figure 3-25.