Safety relays versus safety PLC

Article#: 00076

Date: 2022-04-15

Author: Radim

When designing a system, it is good to consider whether we want to use a traditional safety solution, implemented with safety relays, or whether we want to use so-called integrated safety, implemented with a safety PLC.

With the help of a safety PLC we can implement safety requirements that were previously implemented with the help of safety relays, and we can do even more.
Safety relays still have the advantage of their simplicity and for systems up to a certain level of complexity and range of safety functions it is increasingly appropriate to use a safety relay instead of a safety PLC.

What are the advantages of safety relays?

• The functionality of the safety solution can be understood based on the connection of the safety circuit.
• The functionality of the safety solution can be measured and tested by maintenance personnel (only with their multimeters).
• Defective safety components can be replaced without having to upload a new safety program.

What are the disadvantages of the safety relays?

• The safety circuits must be wired separately next to the PLC bus wiring. This is a disadvantage especially when we need to wire between safety components over long distances.
• With the help of safety relays, we cannot meet some complex safety requirements.
• The safety solution using safety relays is rather rigid and it is not easy to create alternative options for machines with different configurations.

Above a certain level of safety complexity, it is advisable to choose a safety PLC.

What are the advantages of the safety PLC?

• We do not need to make additional safety circuit wiring and can make good use of safety via the PLC bus.
• In the software, we are able to implement requirements for complex safety functionality. (analog value comparison, safe limited speed, safe limited position, ...)
• In the safety project we can design optional safety components for different machine configurations.
• Extending and modifying safety functions in a safety project is easier than in a traditional wired safety circuit.
• The values of parameters and variables from the safety project can be easily transferred to the machine program and visualized in the human-machine interface.
• Once a safety project has been programmed, it can be reused in whole or in part on another machine.

What are the disadvantages of the safety PLC?

• The functionality of the safety solution cannot be understood from the wiring of the safety circuit on the machine.
• The functionality of the safety solution cannot be measured and tested on the machine without special software tools.
• Programming of the safety project requires human resources.
• Uploading a project to a safety PLC takes some time. That can bother you. The question is how often the project has to be reloaded onto the safety PLC or under what conditions the existing project becomes deleted.
• Booting the safety PLC takes longer. Therefore, it takes longer to start the machine after it is turned on.

Depending on the complexity of the safety solution and the distances between the safety components, it is good to consider whether we want to integrate simpler but more limited safety relays or whether we want to integrate a safety PLC.
Despite their limitations, safety relays could be the right choice in simple applications.

© Radim-Automation, 2020–2025. All rights reserved.
Sharing of this article is permitted with proper attribution (link to the original page).

Related previous articles:

• Knowledge gained from a completed project is priceless
• Machine modularization
• Modular software architecture
• Hardware concept
• Are there also any disadvantages?
• Think critically!
• Design machines with a focus on user's abilities and needs
• Why do automation projects fail?
• Collect all the requirements and sort them!
• Who will operate?
• Write down your best practices and processes!
• HMI and PLC applications should grow together
• Is the way really the goal?
• Do you really want to start with this?
• I can see what you can not see
• More than just start and stop
• Safety first!
• Do the right things and do them right!
• Define test scenarios and test cases continuously!
• Don't make it worse!
• Keep the answers to the question "Why"
• Everything is difficult until it becomes easy
• Get and stay in the flow!
• Industrial evolution - Listen to your customers and follow new trends!
• Stay alert!
• Have an expert on hand!
• What does the customer need?
• Don't disappoint your customer!
• Ask questions!
• Focus on target!
• Check the result!
• Keep it simple!
• Success is not a matter of coincidence
• Choose the right tool!
• Development is evolution
• Transparency and reliability

Related next articles:

• Software update
• Design considerations for effective HMIS
• Clear signals
• It doesn't work without sensors
• Diagnostic tools
• PLC network diagram
• Dream of a platform-independent PLC program
• Hidden dangers during commissioning

Comment#: 00001

Date: 2022-10-30

User: Radim

After we know the requirements for the functionality and safety of the machine, it is good to choose those electrical components that can be connected via one type of bus. This reduces the effort and costs for wiring components on the machine if, for example, the same bus can be used both for data transmission and for safety-related data.
What devices do we usually need to be connected to the bus? E.g. PLC, decentralized IOs, frequency converters, safety PLC and safety IOs.

Comment#: 00002

Date: 2023-01-05

User: Radim

My best practice is to define a constant with the safety project version ("YYMMDDVV") in the safety project. I pass this value to a variable in the "grey" application program, from where I can react to the value of the variable and display the version of the safety project in the visualization.
It has at least these advantages:
1. I know exactly which version of the safety project is installed on the safety PLC. This is particularly important when it is unclear whether the project upload to the safety PLC was successful.
2. If there is no safety project installed on the safety PLC (for whatever reason), we can see that no value (value "0") is passed to the "grey" application program and we can trigger and display an alarm in the visualization.
3. Since the safety ("yellow") PLC usually takes a little longer to boot than the process ("grey") PLC, we can use the passed value to know whether the safety PLC has already booted and whether the safety application is already running (i.e. when the safety project version value is available). Then we can also run the application program.

Comment#: 00003

Date: 2023-04-10

User: Radim

Special care should be taken before setting a breakpoint in a program on a real machine. Especially if the program is responsible for controlling and stopping drives and processes.
Hitting a breakpoint stops the entire application running on the target system. However, the states of the outputs of I/O modules remain unchanged!
Running drives and started processes are not stopped and keep running when the application stops at a breakpoint. And no one has control over them anymore!
In a properly designed system, an independent safety function must stop running drives and processes at the latest before a collision occurs.

Comment#: 00004

Date: 2023-07-21

User: Radim

"The essential function of the PLC is to receive data from sensors or other field devices; make a decision on what to do with that information based on its programming instructions, for example, to open or close a valve based on fluid levels or direct a robot to pick up an object based on its presence in the work cell; and send the correct signal to the actuator (such as a valve or robot gripper) to perform the task."
- David Greenfield. AutomationWorld (2023). Peer-to-Peer FAQ: Controllers.
- https://www.automationworld.com/control/article/22724233/industrial-controller-technologies

Comment#: 00005

Date: 2023-11-28

User: Radim

If you choose the path to the left at the crossroads, you will not be able to pick the fruit that grows on the path to the right.
So collect all the requirements and decide which ones you want to choose and which ones you want to eliminate by weighing up the pros and cons.

Comment#: 00006

Date: 2024-12-31

User:

When deciding between safety relays and safety PLCs for a system's safety implementation, several factors should guide the choice:

System Complexity and Safety Requirements:

For simple systems with straightforward safety functions, safety relays are often the optimal choice due to their simplicity and ease of maintenance.
For complex systems requiring advanced safety functionalities (e.g., safe speed monitoring, safe limited positions), a safety PLC provides the flexibility and capability needed.
Flexibility and Scalability:

Safety relays offer limited adaptability. Any changes in machine configuration typically require rewiring, which can be cumbersome.
Safety PLCs, on the other hand, allow for easier modifications and extensions through software, making them more scalable for systems that might evolve over time.
Wiring and Maintenance:

Systems with long wiring distances or requiring integration into existing PLC networks benefit from safety PLCs, as they reduce the need for extensive safety circuit wiring.
Safety relays, however, excel in environments where ease of maintenance and rapid replacement of components are priorities, as they don’t depend on software reprogramming.
Human Resources and Downtime:

Safety PLCs demand skilled personnel for programming and maintenance, which may not be feasible for all organizations.
The boot time and the need to reload safety projects into PLCs can extend machine downtime, making them less suitable in applications requiring frequent adjustments.
Recommendation:

Choose safety relays for simple systems with minimal safety requirements, limited scalability needs, and a preference for straightforward maintenance. Opt for safety PLCs in complex systems, where advanced functionality, configurability, and integration with other control systems are priorities. Evaluate your specific system requirements, available expertise, and long-term needs to make an informed decision.