What is an electrical interlocking circuit?

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The electrical interlocking circuit is used in an electrical system containing two mutually exclusive states. The three-phase reversing motor contactor shown in Figure 1 is a common example featured in industrial control and automation systems. This motor starter features two contactors (one for forward and one for reverse). They are mutually exclusive as they must never activate at the same time. If they were simultaneously activated, it would cause a destructive phase-to-phase short.

This engineering brief presents a progression of electrical interlocks starting with a pair of pushbuttons followed by a relay-based circuit. Note that the components pictured in this document are identified in DigiKey’s Small Relay and PLC Trainer.

Figure 1: The reversing motor with an electrical interlocking circuit formed through the normally closed contacts of the auxiliary contact block.
Figure 1: The reversing motor with an electrical interlocking circuit formed through the normally closed contacts of the auxiliary contact block.1013×759 86.7 KB

Figure 1: The reversing motor with an electrical interlocking circuit formed through the normally closed contacts of the auxiliary contact block. A redundant mechanical interlock is embedded within the mechanism. See this motor starter introduction for additional information about the Schneider Electric DPE09BL motor starter.

Tech Tip: The electrical interlocking circuit concept is directly applicable to Programmable Logic Controllers (PLCs). The ladder logic implementation has a similar appearance.

Tech Tip: The relay-based electrical interlocking circuit is susceptible to operator error especially when the relays are mechanically forced. There is a real hazard of violating the mutually exclusive functionality. The reversing motor starter features a mechanical interlocking mechanism to prevent the phase-to-phase short.

Electrical interlocking circuit using pushbuttons

Figures 2 and 3 present a simple example of an electrical interlock. This circuit features two pushbuttons, labeled A and B. They are used to activate a dual color industrial indicator (A for green and B for red).

Each pushbutton includes a normally open and a normally closed contact. Figure 2 shows a crossover condition with the normally open contact in the upper rung and a normally closed contact in the lower rung. Each pushbutton interferes with the operation of the other pushbutton. For example, if A is pressed, the green lamp is lit. However, if B is simultaneously pressed the lamp is extinguished as the pushbuttons have disabled each other.

Figure 2: Pushbutton implementation of an electrical interlocking circuit. Observe the crossover where the normally closed contact negates the normally open contact of the adjacent pushbutton.
Figure 2: Pushbutton implementation of an electrical interlocking circuit. Observe the crossover where the normally closed contact negates the normally open contact of the adjacent pushbutton.975×491 43.4 KB

Figure 2: Pushbutton implementation of an electrical interlocking circuit. Observe the crossover where the normally closed contact negates the normally open contact of the adjacent pushbutton.

Tech Tip: Industrial switches such as the 22mm devices featured in this article are best categorized in terms of family membership. For any given family, there are hundreds and sometimes tens of thousands of combinations and permutations. Click here to learn more about building switches for your project.

Figure 3: Physical implementation of the pushbutton electrical interlocking circuit. Note that each normally open switch block (green) is connected to the corresponding switch’s normally closed (red) block.
Figure 3: Physical implementation of the pushbutton electrical interlocking circuit. Note that each normally open switch block (green) is connected to the corresponding switch’s normally closed (red) block.1035×776 119 KB

Figure 3: Physical implementation of the pushbutton electrical interlocking circuit. Note that each normally open switch block (green) is connected to the corresponding switch’s normally closed (red) block. Additional information about wire identification may be found in this article.

Electrical interlocking circuit using relays

Figures 4 and 5 along with Video 1 show the operation of an electrical interlocking circuit based on relays. The fundamental idea is the same as with the pushbutton implementation. In each case, we have a crossover where one circuit prevents the other from activating.

Observe that the circuit in Figure 4 incorporates the pushbutton circuitry from Figure 2 for a measure of redundancy/safety. To this, it adds the K1 and K2 relays with interlock. Close inspection reveals that each relay is fed by a normally closed contact from the opposite relay. Consequently, only one relay may be activated at a time. However, it is possible to encounter a race condition if both pushbuttons are pressed at precisely the same time.

Tech Tip: The redundant electrical interlocking circuit featured in Figure 4 reduces, but does not eliminate, the possibility of a race condition. Here, the term race condition implies that both K1 and K2 are simultaneously activated before the self-canceling interlock can activate. Under this condition, it is possible to experience a shoot-through event resulting in a momentary short circuit of the power supply. This can be further mitigated by using a PLC with smart monitoring of the pushbuttons.

Figure 4: This schematic shows a reversing DC motor controller with dual electrical interlocks to prevent simultaneous activation of K1 and K2.
Figure 4: This schematic shows a reversing DC motor controller with dual electrical interlocks to prevent simultaneous activation of K1 and K2.977×679 72.7 KB

Figure 4: This schematic shows a reversing DC motor controller with dual electrical interlocks to prevent simultaneous activation of K1 and K2. Observe that the DC motor is driven by an H-bridge composed of K1 and K2 contacts. Simultaneous activation of K1 and K2 would short out the power supply.

Shoot through hazard of the bridge

Figure 4 features an H-bridge to control the rotational direction of the DC motor. In this example, the H is placed sideways. Recall that an H-bridge consists of 4 switches with the load in the middle. Activation of K1 will run the motor in clockwise direction, while K2 will operate the motor in a counterclockwise direction.

Relays K1 and K2 are mutually exclusive. They will short out the power supply if they activate at the same time. As stated in the tech tip, this is unlikely given the redundant interlocks. However, in Figure 5 we see that each 4PDT Finder relay features a lever to mechanically force the relay. This is a hazard as inadvertently forcing both relays will short out the power supply tripping the circuit breaker.

Figure 5: Picture of the DC motor controller as described in Figure 4.
Figure 5: Picture of the DC motor controller as described in Figure 4.1029×772 102 KB

Figure 5: Picture of the DC motor controller as described in Figure 4.

Video1: Demonstration of the DC motor controller described in Figures 4 and 5.

Parting thoughts

The electrical interlocking circuit is a fundamental building block for industrial control and automation systems. This mutually exclusive application of logic will follow you throughout your technical career as there are many “either-or” or “never-both” circuits. You can look forward to those corner case conditions where A and B must not occur at the same time.
Stay tuned as we will explore the complete operation of the three-phase reversing motor contactor. This will include both the electrical interlocking circuit and the physical interlock mechanism that prevents the semi-independent contactors from closing at the same time. Until then, please review the questions and critical thinking questions located at the end of this note.

Your questions and suggestions are welcomed. Please leave your comments in the space below.

Best wishes,

APDahlen

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About this 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 (partially interwoven with military experience). 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 educational articles about electronics and automation.

Highlighted experience

Dahlen is an active contributor to the DigiKey TechForum. At the time of this writing, he has created over 180 original posts and provided an additional 580 forum posts. Dahlen shares his insights on a wide variety of topics including microcontrollers, FPGA programming in Verilog, and a large body of work on industrial controls.

Questions

The following questions will help reinforce the content of the article.

  1. What is the difference between an electrical interlocking circuit and a mechanical interlocking circuit?

  2. From memory, sketch the schematic of a pushbutton-based and a relay-based electrical interlocking circuit.

  3. How can you identify an electrical interlocking circuit? What patterns are you looking for?

  4. The circuits in this article feature a dual red/green 22mm indicator. Describe how the device is wired into the circuit. Also, use the DigiKey parametric search tool to determine the availability of a 120 VAC substitute.

  5. What are the advantages and disadvantages of using redundant electrical interlocking circuits as featured in Figure 4?

  6. What is an H-bridge?

  7. What is a race condition and how might it impact the circuit shown in Figure 4? Hint: Identify the worst-case situation.

  8. What is meant by the expression of physically forcing a relay? How does that impact a circuit such as the schematic shown in Figure 4.

  9. Research the Altech switches. What are the part number(s) required to complete combination normally open / normally closed assembly featured in this article.

  10. Research the method to reverse the rotational direction of a three-phase motor. Describe the three-phase motor starter and how it is related to an H-bridge. Hint: Shoot through.

  11. Given a reversing motor starter, we could argue that it’s too late if the electronics are attempting to activate both contactors. Identify at least two purposes of the mechanical interlock. Hint: Mistakes happen during those 3 AM troubleshooting events.

Critical thinking questions

These critical thinking questions expand the article’s content allowing you to develop a big picture understanding of the material and its relationship to adjacent topics. They are often open ended, require research, and are best answered in essay form.

  1. Research application of the three-phase reversing motor starter. Examine how the electrical interlocking circuit was implemented in each application.

  2. The circuit shown in Figure 4 has undesirable features from the perspective of a DC motor. Redesign the circuit to include a time delay relay to prevent the operator from quickly changing motor direction. The delay relay shall prevent motor activation for 2 seconds after a pushbutton is released.

  3. Present a ladder logic implementation of Figure 4 that includes an electrical interlocking circuit and time delays to prevent motor damage. Be sure to include a wire diagram for your circuit. Hint: Trust but verify! There is value to distributing the interlocks between the PLC software and external relays.

  4. The individual switch contacts within an industrial pushbutton stack do not always open and close at the same time, especially if the operator is slow to push/release the button or pushes at an angle. How could this negatively impact the operation of Figures 2 and 4?

  5. An Automatic Transfer Switch (ATS) or Automatic Bus Transfer (ABT) is often used to switch a critical load from primary to emergency power. Provide an essay exploring the application of the electrical and mechanical interlock.