The Basics of Reading PLC Panels and Wiring Diagrams

PLC Panel Overview 

In industrial applications, an electrical panel is basically a service box that connects the main power line to an electrical device and distributes electric currents to various other circuits within the system. In an industrial setting, you don’t simply plug a PLC controller into a wall socket, instead, an electrical panel is used. A PLC panel is simply an electrical control panel consisting of electrical components which use electric power to control a variety of mechanical functions of industrial machinery or equipment.

In order for industrial machinery and equipment to accomplish their various process goals, they require user-defined functions and well-organized control. Thus, electrical control panels like the PLC panel are used to perform these functions within manufacturing machinery. Any electrical control panel you’ll ever come across will always consist of two main categories: panel structure and electrical components. Similarly, a PLC panel consists of a special stainless steel box containing electrical components required to run a machine or process in a factory setting. The electrical components are of two types: power and control components.

Structure of a PLC Panel 

The PLC panel structure is a combination of an enclosure and a back panel. You can relate a PLC control panel to an electrical breaker box in an office or home.  

  • Panel Enclosure: This is typically a stainless steel or aluminum metal box and it comes in varying sizes. In most industrial applications, the size of the enclosures is determined by the numbers on the enclosure doors of the PLC panel. Moreover, all enclosures have a UL safety rating, IP rating, and/ or a NEEMA classification which is printed on a metal plate attached to the enclosure. 
  • Back Panel: This is a metal sheet that is mounted on the inside of the enclosure to provide structural support for wiring ducts and DIN rail mounting. The wiring ducts provide routing options and allow for neat organization of the wires used while assisting in controlling electrical noise between the electrical components inside the PLC panel. DIN rails feature standardized dimensions and provide a mounting structure for the electrical components. 

Components of a PLC Panel 

As previously stated, a PLC panel consists of two types of electrical components: power components and control components. To be able to read an electrical wiring diagram of a PLC panel, you should have an understanding of these components. Let’s take a look at these two types of electrical components: 

A) PLC Panel Power Components  

  • Rotary Disconnect: It is used for connecting incoming power lines/wires. It can include fuses or not. To turn the power ON or OFF, it is usually rotated using the black or yellow PLC panel door handle. 
  • Power Distribution Block: It is mainly made of machined aluminum. The top of this block has a couple of holes for larger connecting wires, while its bottom has multiple holes for smaller connecting wires. The power distribution block is used to split a large connecting wire into smaller wires to be used with many other different electrical components in the PLC panel. 
  • Relays and Contactors: These are ON/OFF switches that control mechanized functions based on control commands from the PLC controller. Smaller relays control simple functions such as fans and lights. The larger relays are known as contactors, and they control more advanced functions like three-phase motors.  
  • Main Circuit Breaker: We are all familiar with the main disconnect of electric circuits in our homes or offices. Well, the main circuit breaker in a PLC panel is similar to such an electrical disconnect. In most industrial control applications, the main circuit breakers in PLC panels handle between 120V to 480V. 
  • Branch Circuit Breakers: These provide short circuit protection and in some cases, they prevent overload conditions for each type of load controlled by the PLC, such as a heater, motor, etc. 
  •  Surge arresters: They are used to prevent utility power surges or lightning strikes from damaging the electrical components inside a PLC panel due to overvoltage. 
  • Transformer: Normally, a step-down transformer is used to reduce the incoming AC voltage to 120V for various components while in other cases it is used to step down the incoming voltage to 24V. This is applicable in instances where the PLC panel is connected to a 120V AC power line. 
  • Power Supply: This is used to convert high power AC voltage, normally 120V or 240V AC to a lower and safe control DC voltage (24V DC) for the various control components in the PLC panel like HMI, or PLC controller.  
  • Power Contacts: These are used to manually disable/enable power flow to a machine using e-stop buttons. 
  • Motor Starter: Also known as a motor contactor, it turns ON motors at full speed and full voltage. 
  • Variable Frequency Drive (VFD): This is a type of motor controller which is used to adjust the motor speed as well as monitor other motor data.  
  • Motor Soft Starter: This is also a type of motor controller which is used to start a motor gradually over time, then ramp up to full motor speed. 
  • Grounding: This connection is necessary as it provides a current flow pathway in the event of an electrical fault.  

B) PLC Panel Control Components 

  • Supplementary Circuit Breaker: It is used to protect high-end and expensive control devices and components of the PLC panel.  
  • Master Control Relay (MCR): It is used to implement a safety circuit that transfers power from all output devices in case of an emergency. In most cases, the MCR will be paired mushroom-head Stop pushbutton. 
  • Network Switches: They make up the communication hub of the PLC panel. They facilitate communication between the PLC system and a variety of other network-compatible devices on the assembly line. An example of a network switch is the Ethernet switch which is used for network communications between the PLC, the HMI, and other smart devices. 
  • Programmable Logic Controller (PLC): This is essentially the CPU of the PLC contained inside the panel. This unit has the Arithmetic and Logic Unit (ALU) which is responsible for data manipulation, arithmetic functions, and logic operations. A control unit is also included to regulate the timing of the PLC control operations. 
  • Human Machine Interface (HMI): HMIs provide a Graphical User Interface (GUI) that enables operators to interact with the PLC control system. So an operator can use an HMI graphical display to monitor and review real-time input and operational data or configure and control certain functions of machinery or process. Examples of HMIs include keypads, text-readouts, joysticks, video monitors, or large touch screen panels like Liquid Crystal Displays (LCDs).  
  • Input/Output Module: I/O modules provide an interface from the PLC to the input field devices and the components or devices being controlled. The output module connects the PLC to the input devices such as sensors, start/stop pushbuttons, and switches. On the other hand, the output module is used by the PLC to control output field devices like relays, solenoid valves, motors, pumps, and electric heaters. You can have either analog or Digital Input/Output modules. 
  • Operator Pushbutton: This is located on the front of the PLC panel and is used by the operator to control the process or machine.  
  • Terminal Blocks: They are used to connect and splice field device wiring and internal PLC panel wiring. They also assist in organizing and distributing the array of wires emanating from various sources to different electrical devices. 

How do you read PLC Panel Wiring Diagrams? 

Also known as an electrical schematic, circuit diagram, elementary diagram, or electrical diagram, a wiring diagram is simply a graphical representation of an electric circuit. A schematic wiring diagram represents the electrical components and interconnections of a circuit using standardized symbols, while a pictorial wiring diagram uses simple images to represent circuit components. In this article, we’ll discuss both schematic and pictorial electrical wiring diagrams of a PLC panel.

All the PLC panel power and control components discussed above, exist within an electrical wiring diagram. They define and organize the various different functions performed by the PLC panel. In an electrical schematic, those components are represented by standard electrical symbols. Hence, a foreknowledge of what symbol represents which component is necessary for you to be able to read a PLC panel wiring diagram.

With an understanding of the various electrical components that are contained in a PLC panel and the symbols that represent them in a wiring diagram, we can now learn how to read PLC panel wiring diagrams using some examples. But before that, here are some rules to be followed whenever you’re reading a PLC panel wiring diagram:
Rule #1: You should read a PLC Panel wiring diagram from left-to-right and top-to-down, just like when you’re reading a book.
Rule #2: To understand the addressing system of a PLC panel wiring diagram use the combination of the provided column numbers and page numbers. For example, if you find a number like 38.7 below or beside an electrical component in a PLC panel wiring diagram, it means that the component has been used on page 38, column 7. This rule applies when you’re reading an actual PLC panel wiring diagram in a booklet with multiple pages.
Okay, now let’s look at some examples of PLC Panel wiring diagrams.

Example 1: A PLC Controlled Three-phase Motor System 

In this example, the motor is required to operate in both directions, which is only possible through a Forward/Reverse Control Logic circuit or Relay circuit. A simple Forward/Reverse PLC control logic would be the most viable solution in this case. Thus, a PLC system is used for Forward and Reverse control of the three-phase asynchronous motor.

Two relays or contactors for motor control are used because two different directions are needed. The first contactor is for Forward Direction control while the Second contactor is for providing Reverse Direction control of the Motor. Also, three pushbuttons are used for Stop, Forward and Reverse functions of the motor. So, the operator will use the Forward Pushbutton (FWD PB) for forward motor operation, and Reverse Pushbutton (REV PB) for reverse motor operation and Stop Pushbutton (STOP PB) for stop function. The resulting wiring diagram is as shown below:

Figure 1: A Pictorial Wiring Diagram of a PLC Controlled Motor 

Note: The dashed lines in the wiring diagram above (Figure 1) indicate a single purchased component, which in this case is the PLC system. 

Example 2: PLC-based Motor Controller System 

In this motor controller system, three phase AC power is connected at the terminal block and then supplied to a power interrupter (the main circuit breaker). After which all the three phases (L1, L2, and L3) are supplied to a motor starter with three contacts, labeled as M. Next, it is connected to three thermal overload relays(branch circuit breakers). Two phases(L2 and L3) of the three phase AC power are then connected to a step-down transformer, which is connected to a PLC system for powering the logic. The resulting electrical wiring diagram is as shown below:

Figure 2: Electrical Schematic of a PLC-based Motor Controller System

Example 3: PLC Panel Ladder Logic Wiring Diagram 

To better understand the logic control provided by the PLC system, a ladder logic circuit diagram is included in the motor electrical schematic in Figure 2 in place of the PLC system. Generally, a Ladder Logic wiring diagram of a PLC panel can be sub-divided into two distinct parts. The first part is the power circuit which shows the flow of power to the system. The power circuit is normally denoted by bold lines. The second portion is usually denoted by thin lines and is the control circuit. In cases where an external power supply is used to power the PLC control panel, this is usually not shown. The figure below shows a good example of a ladder logic wiring diagram with both power circuit and control circuit:  

Figure 3: Electrical Schematic and Ladder Logic Diagram of a PLC Controlled Motor

In the above example (Figure 3), the power circuit shows how a three phase AC (L1, L2, and L3) supply voltage flows to the motor; first, it flows to the terminals, it is then connected to a power interrupter(main circuit breaker), the three phases are then supplied to three contacts, M, and three thermal overload relays(branch circuit breakers). In this case, an external PLC power supply is not required as two phases (L2 and L3) of the three phase AC power are connected to a step-down transformer that powers the PLC system. Figure 3 also includes a control circuit (shown as a Ladder Logic diagram) that focuses on how the motor is being controlled.

In the PLC-based control portion of the electrical schematic in Figure 3, standard Ladder Diagram symbols are used. However, electrical components like fuses and disconnect devices can also be used in control circuits. In Figure 3, the control circuit includes a fuse. Look at some of the standard Ladder Diagram symbols shown below, which will be helpful in reading the PLC-based control circuit in Figure 3.

Figure 4: Most Common Ladder Diagram Symbols 

Reading Ladder Diagrams: Interpretation of Figure

In Figure 3, the power circuit can be read as described above, but to be able to read the PLC-based control circuit we’ll have to look at several conventions of reading ladder diagrams. 

Conventions of Ladder Logic Wiring Diagrams 

As the name suggests, the physical layout of a Ladder Diagram (LD) resembles a ladder; whereby two vertical power rails are built among a series of horizontal rungs.  

A) In a Ladder Diagram, the vertical lines indicate the power rails between which the PLC circuit is connected. On the far left, there is the Positive bar while on the right-end there is the Negative or Neutral power bar. So, the flow of electric current is from the vertical power rail on the left-end across the horizontal rung to the right-end vertical rail.  

B) The horizontal rungs show pushbutton switches, relay coils, relay contacts, switch contacts, and the elements being controlled by the PLC such as lamps, motors, and solenoid coils. All these components are shown in between the vertical power rails.  

C) Inputs are located on the left side of each horizontal rung and they should be true to energize the connected outputs. Hence, each horizontal rung starts with input and ends with an output.  

 D) Another very important convention is that a Ladder Diagram program is read from left-to-right, and top-to-bottom. The PLC processor reads the top rung first from left-to-right, then down to the second rung also read from left-to-right, and so on. This process is referred to as the program scan cycle of a PLC.  

E) Each horizontal rung of the ladder logic circuit defines a single control operation of the PLC. Thus, when reading a ladder logic wiring diagram you should be able to logically visualize the PLC controlled process across the horizontal rungs; as data flows from the inputs on the left end to the control components and unto the output devices being controlled. 

F) Lastly, in Ladder Logic circuits, physical electrical components of an electric circuit are depicted in their normal condition. For example, if a relay contact switch is usually Normally Open (NO) till a certain condition is met to close it, then it will be shown as Normally Open on the ladder wiring diagram. Similarly, a Normally Closed (NC) component will be shown as normally closed.  

Having understood the conventions of reading a ladder diagram, we can now interpret the PLC-based control circuit of Figure 5 (shown below), as follows: 

Figure 5: PLC-based Control Circuit

First, Figure 5 shown above indicates a single-rung ladder diagram with the vertical rails indicating the power supply from the step-down transformer to the PLC system. The vertical power rail on the left-end is protected from over-current conditions with a fuse while the vertical power rail on the right-end is grounded. The horizontal rung is read as follows: 

Rung #1: Reading from left to right, we have the following inputs and outputs: (i) NO (Normally Open) Start Pushbutton labeled as a start; (ii) NC (Normally Open) mushroom-head Stop Pushbutton labeled as stop; (iii) Relay Coil MM representing the motor output. The three components are connected in series. Thus, the motor will run (controlled output) if the Start Pushbutton is pressed, and the mushroom-head Stop Pushbutton is not pressed. 

Note: the condition of the motor input M depends upon motor output M. This means that when the motor output M is HIGH, the motor input M will also be HIGH.

Also, Figure 5 shows the number of wires used in the PLC control panel wiring diagram. This is very essential for industrial PLC controlled systems containing thousands or hundreds of wires. The number schemes are unique to each industrial setup or factory facility.

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