Relay Logic for Industrial Control Panels Part 1

Why study relay-based circuits?

Relays are commonly used in industrial control and automation. This includes the physical DIN mounted relays located in your industrial control panels. It also includes the ladder logic representation of relays programmed into your Programmable Logic Controllers (PLC). This interplay between the physical relay and the software abstraction is important as learning one strengthens your knowledge of the other.

As a technician, you will spend a significant amount of your time troubleshooting defective field devices and wiring. You will occasionally examine the PLC programs. As an engineer, you should study physical relay patterns to better understand how the PLC interfaces with the real world and to build PLC independent failsafe circuitry. It will also help you write better PLC code providing future technicians the ability to quickly troubleshoot and restore the equipment to operational status.

Hands-on experiments are the best way to learn about relays

This is the first installment of a multipart education piece in which we will explore common patterns / combinations of relays and switches. You are encouraged to learn about these circuits using hands-on experiments as shown in Figure 1. This is one of the best ways to learn how the circuits function. It will also improve your troubleshooting skills as you will certainly make mistakes as you wire the circuit. The experiments were completed using this small trainer pictured in Figure 1 with additional information located in this article.

Please bookmark this page and return often as we are adding additional content. This is a placeholder for links to future installments.

Figure 1: Picture of the switch wiring with highlighted wire labels. Close inspection of the switch body shows the central red and green tabs corresponding to N.C. and N.O. respectively.

Tech Tip: Wire diagrams are a shared language between the technician, assembler, engineer, and component manufacturer. This is a complex shorthand using standardized symbols to represent specific devices and the action taken or acted upon those devices. An introduction to those symbols is provided by NEMA as standard ICS19-2002 with the last revision in 2021. Recommend you download and study the free electronics version of the document.

Introduction to relay-based ladder logic

The first step to mastering industrial control is to understand the structure of relay-based ladder logic. Figure 2 presents a small “ladder.” The sides of the ladder provide power to the circuitry. In this example, 24 VDC is on the left side and return is on the right.

Each ladder has one or more rungs that are powered by the sides of the ladder. In Figure 2 there are two rungs:

  • Rung 1 contains a selector switch, a normally closed (N.C.) pushbutton (red), a normally open (N.O.) pushbutton (green) and a relay coil

  • Rung 2 contains a relay contact and a green panel lamp.

Notice that the relay and switch contacts are to the left with outputs such as relay coils and panel lamps to the right. This is the convention followed for most circuits with a few exceptions such as the overload contacts of a motor starter.

Figure 2: Wire diagram showing the AND operation (rung 1) and a relay contact to turn on a green panel lamp (rung 2).

Ladder logic orientation

In the Figure 2 example, the ladder is set upright, as if leaning against a wall. This conventional upright orientation can be challenging as the rungs start at the top and work down. This is reflected in Figure 2 with the rungs #1 (label on the left) at the top and rung #2 at the bottom.

Occasionally, we find ladders that have been laid on their sides. There are benefits for both orientations. However, the beginner may prefer to lay the ladder on its side as the rungs now read left to right which is natural for an English reader. As you learn to read and troubleshoot diagrams, it may be helpful to rotate the paper schematic 90 degrees to better understand the circuit.

Continuity across a rung

The term continuity is very important for understanding ladder logic. We say that the switches and contacts to the left provide continuity – a closed circuit – providing power to the coil or panel lamp on the right.

Troubleshooting continuity across a rung

As we troubleshoot a relay circuit, we typically start with the thing that is broken. For example, let’s suppose the Figure 2 panel lamp is not activated. Our first action is to attempt to establish continuity for the things to the left of the panel lamp. For example, is K1 active?

Assuming K1 is inactive, we jump upstream to rung 1. We then repeat the process attempting to establish continuity across the left-hand components of rung 1. It’s possible that we are experiencing “operator error” as the selector switch may not be turned on or the N.O. pushbutton may not have been pressed. It’s also possible that the circuit is damaged with a broken switch or loose wiring.

After we have completed these initial assessments, we may need to troubleshoot the circuit using a multimeter. Rung #1 wire 3 is a good place to start as it is the first step for a systematic half-step troubleshooting method. We can test for DC voltage between wire #3 and the DC return. If voltage is present, we next test wire is #4. If voltage is absent, our next test step is wire #2. Also, at some point, we should have checked the 24 VDC power supply.

Tech Tip: We use the term continuity to describe the flow of current across a rung. We say that the rung has continuity when the left-hand switches and contacts provide a complete electrical path thereby activating the right-hand devices. This is related to but different than the “continuity test” performed by your multimeter. The questions at the end of this document encourage you to consider if your multimeter’s continuity test function may be used to test for continuity across a rung.

The logic AND pattern for relay logic

The logic AND pattern describes a series-connected circuit with a single path for continuity. Two or more left-hand inputs are series connected. When all the devices are activated the corresponding right-hand side output is activated.

A three-input AND example is shown in Figure 1 and 2 featuring two pushbuttons and a relay. In this example, the control relay (K1) coil will activate when the selector is active, the red normally closed switch is not active, and the green N.O. pushbutton is active. The physical connection to the relay are shown in Figure 3.

Tech Tip: The concept of “active” is a challenging concept especially when we add the term normally closed and N.O.:

  • An activated N.O. switch or relay contact will have continuity.
  • A N.C. contact must be inactive (not pressed) to have continuity.

This discussion is further complicated in a PLC’s ladder logic abstraction. In the PLC the N.C. and N.O. elements are not real. Instead, they are functions that operate on a single bit of PLC memory. The relay-based ladder logic and terminology as introduced in this article will serve you well, but there are times when the analogy breaks down. It’s like using water analogy for Ohm’s law at some point you will need to stop talking about the flow of water and use a more sophisticated model.

Figure 3: Connections to the Weidmüller DPDT relay. Note that the common connections for the relay’s DTDT connection (pins 11 and 21) are located the second deck.

The logic OR pattern for relay logic

The logic OR operation provides two or more paths for continuity. An example is presented in Figures 4 through 6. Here we see two N.O. pushbuttons connected in parallel. This provides two paths for continuity – current can take the path through PB1 OR through PB2.

Figure 4: Picture of the switch wiring with a OR function constructed by the two switches on the left. Close inspection of the switch body shows the central white and black tabs corresponding to N.O. and N.C. respectively.

Figure 5: Wire diagram showing the OR operation performed by pushbuttons 1 and 2.

Figure 6: Connections to the Weidmüller 4PDT relay. Terminal block with 4-position jumpers may be seen in the background. These block act as the “sides” of the ladder providing power and ground to the circuit.

The combinational pattern for relay logic

A close examination of Figure 4 through 6 reveals that this is a combinational circuit as it features both AND plus OR logic. In Figure 5 we see that either pushbutton will activate the circuit. However, the continuity is broken if the normally closed pushbutton is pressed. In Boolean terms the circuit’s operation is described as PB1 OR PB2 with not PB3.

As we will see in future articles, combinational circuit provide a wide range of circuit function. We will also see that these primitive combinational circuits may be combined to make sequential circuits – that is to say, circuits with memory. This mixture of combinational and sequential circuits provides a robust foundation for industrial controls, especially when we consider how these circuits are integrated into the PLC. One must-know example of this sequential circuit is the classic 3-wire start stop control circuit.

Tips for circuit construction

Industrial control panels must be methodically constructed to reliability, safety, and to facilitate maintenance. Before concluding this article, let’s explore wire color codes and numbering.

What color wire should be used in an industrial control panel?

Wire color is an important consideration for industrial control panels. It’s like a language spoken between the technician, assembler, and engineer. Some colors are associated with specific voltages. For example, in this article, blue and white with blue stripe wires are used. Blue is used for any wire that is directly associated with the 24 VDC supply, or any wire that could be at a 24 VDC potential. The contrasting white-with-blue-stripe wires are reserved for connection that 100% associate with the 24 VDC return.

Additional information for color coding and machine safety are detailed in the IEC 60204-1 standard. Unfortunately, the cost of this standard is beyond ready for most students. However, companies such as Siemens provide reference manuals relating back to the IEC standard with additional information here at Siemens Control Panel Tips.

Note that wire color code is not fixed as it may vary by region and by equipment manufacturer. Be sure to identify and observe the colors used in your machine.

Safety Tip: Do not place 100% confidence in wire color. For example, the blue wire may be used for DC control circuitry. Blue or light blue may also be used for the neutral of a three-phase system. Reliance on color alone would lead to confusion and delay while troubleshooting.

How should we identify and label wires in an indistrial control panel?

There are many ways to label wires. You will find that the complexity increases with the complexity of the system to include wire diagram page number, type of signal, and identification of the associated connection point. In this article we are using a simple rung and wire number. For example, in Figure #3 R1 / W4 stands for rung #1 Wire #4.

With this system, the number change when continuity is broken by a contact or coil. Note that two wires may have the same designation as shown in Figure 4 and 5. For example, on the top left we see rung 1 wire 1 (R1 / W1) in two locations. Note that the wires are electrically on the same node as they both connect to the upper screw terminal of the left-most pushbutton.

Parting Thoughts

This article provides a brief introduction to relay-based ladder logic. As you can see, there are many interwoven concepts required to successfully construct control circuits. Attention to detail for seemingly unimportant aspects such as wire color can have significant impacts down the road.

As you continue your journey be sure to consider the question that appear at the end of this note. Also, don’t forget to bookmark this page as we intend to expand the content to include combinational circuits and a variety or relay control and PLC interface subjects in the future.

Your comments and recommendations are welcomed.

Best Wishes,


About the author

Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this. LinkedIn | Aaron Dahlen - Application Engineer - DigiKey


  1. What is ladder logic?

  2. How is relay-based ladder logic different than the IEC 61131-3 PLC-based ladder logic?

  3. Define the term continuity as related to ladder logic.

  4. What is the purpose of the NEMA Standards Publication ICS 19-2002 (R2007, R2011, R2016, R2022)?

  5. According to the NEMA standard, what is the symbol for a normally open limit switch?

  6. Present the ladder logic for a panel lamp that will activate when switches A and B are activated or when switch C is not activated.

  7. Describe the half-step troubleshooting method.

  8. True / False: In conventional ladder logic, a switch may be placed to the right of a coil.

  9. True / False: A normally closed momentary pushbutton will provide continuity when it is activated.

  10. True / False: Given a green detachable switch body or a switch block with a green actuator tab, we can be reasonably confident that the switch contacts are normally open.

Critical thinking questions

  1. Use DigiKey and supporting manufacturer web pages to locate a 22 mm momentary pushbutton featuring dual contacts such that one set is normally open and the other is normally closed. Note that most manufactures offer pushbuttons as part of a family of interrelated components. Your cart for a single pushbutton could contain a single assembly or up to 4 of 5 individual components. To get started, please consider this introduction using AEO switches.

  2. Modern multimeters feature a continuity test that will beep when the probes encounter a low resistance. Locate the technical manual for your meter to determine if this is a suitable way to test rung continuity in a 24 VDC or a 120 VAC industrial system.

  3. Modern multimeters are designed to measure both AC and DC voltage – but not simultaneous. Describe how this may be a problem while troubleshooting a control panel with mixed AC and DC voltages.

  4. True / False: For a rung featuring a panel lamp, the lamp will always be lit if the rung has continuity. Hint: how to you define continuity, specifically what is and what is not included.

  5. What is meant by a control panel sign that says, “DANGER This equipment is powered by multiple sources”? What precautions must you take to protect you and the technician working on the next shift?

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