SIL and PL functionality may be implemented in a stand-alone safety relay or in a PLC-based system. This brief challenges the assumption that the PLC-based implementation is the superior solution. It contrasts the self-contained nature of the safety relay (Figure 1) with the distributed functionality found in PLC-based systems.
Key Takeaways:
- A hybrid solution to safety and control can be used. Safety functionality may be handled by a stand-alone safety relay while machine control is handled by a conventional (non-safe) PLC.
- There are advantages to the simplicity of the dedicated hardware-based safety relay. It reduces the chance of error during the initial installation and over the life of the machine.
- The PLC-based solution distributes safety across multiple hardware components and the safety program. This requires careful component selection and system-level integration across F-rated hardware, hardware configuration, safety program(s), and even networked components.
- In the safety relay-based system, limited device configuration is done with DIP switches. In the PLC system, it is done with F-module parameterization and a safety program.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Integrate It → Safety Devices
Difficulty: 
System Integrator — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 14 Apr 2026
Disclaimer: This material is for education only. You are responsible for implementing the Safety Integrity Level (SIL) and the Required Performance Level (PL) for the equipment in your facility. Follow all applicable local, state, federal, and international regulations.
While the material has been prepared with care, it may contain unintentional errors or misinterpretation of the standards. Refer to DigiKey’s Terms of Service for official terms and guidance. We appreciate any feedback you can provide to improve the narrative and factual content.
Figure 1: Image of the Siemens 3SK1 safety relay showing front panel DIP switches.
Hardware-Based Safety Relays
The Siemens 3SK1 is representative of a purpose-built safety relay.
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Each module performs one function. For example, consider the setup shown in Figure 1.
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The 3SK1121-1CB42 is used for light curtain monitoring.
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The 3SK1220-1AB40 is used for the E-stop.
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Multiple modules are connected together via the backplane socket wiring.
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The output consists of a three-pole relay.
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The assembly is designed to protect a single hazardous zone.
DIP switches (Figure 1) are used to configure the individual modules to match the properties of the associated safety device. The light curtain module is set to detect the OSSD safety signal while the emergency stop module is set to generate and then monitor the 3SK1 generated T1 and T2 drive signals.
Tech Tip: Redundancy is a common pattern in industrial safety equipment. As an example, consider the physical output relay within the 3SK1. Here we find redundant (series connected) contacts. They are monitored by the control system. Should one set become stuck (welded) the system will enter a fault state.
Figure 2: Datasheet representative wire diagram for the 3SK1, showing E-stop, series redundant motor starter contactors (Q1 and Q2), and monitored normally closed contacts.
Implementation of PLC-Based Safety
The PLC-based safety is distributed across both hardware and software. We begin with the understanding that many “safety PLCs” are not equipped with built-in F-rated inputs or outputs.
PLC-Based Safety Using Local Modules
The simplest S7-1200 safety configuration is presented in Figure 3. Here, a 6ES7 226-6BA32-0XB0 F-Digital Input module and a 6ES7 226-6RA32-0XB0 relay based F-rated digital output module are physically attached to a S7-1200 F-rated PLC. All safety-critical signals are routed through dedicated safety I/O modules.
Figure 3: Siemens S7-1200 safety PLC with attached F-rated input and output modules.
Distributed I/O for a PLC-Based Safety System
The safety may also be implemented using the ET 200SP distributed I/O as shown in Figure 4.
The F-rated functions of the Siemens PLC are brought to fruition via the ET 200SP. A high-level idea is presented in Figure 4:
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Emergency stop is read by a 6ES7136-6BA00-0CA0 F-rated safety digital input module.
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The motor starter is controlled via a 6ES7136-6DB00-0CA0 F-rated digital output module.
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The safety modules are physically installed and logically addressed as slots in an ET 200SP distributed I/O assembly.
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The ET 200SP communicates with the safety PLC using the PROFIsafe protocol over PROFINET.
Figure 4: Siemens ET 200SP with F-rated input and output modules.
Tech Tip: The architecture implied in Figure 4 is not built to the highest SIL / PL standards. For added protection, an additional (redundant) series contactor is required.
Note that the safety system monitors the motor starter contactors via feedback from each contactor’s normally closed auxiliary contacts. This effectively checks for “welded contacts”. Should this occur, the machine enters a fault condition, thereby preventing the remaining, and presumably healthy, contactor from closing.
Hardware Configuration
In all cases, the hardware must be configured. In the relay-based system, device matching is done with DIP switches. In the PLC-based system, it is done with F-module parameterization.
As an example, consider a rudimentary system where an emergency stop is implemented using a one-out-of-two (1oo2) configuration where the two physical OSSD signals are combined into a single logical control signal. For example, the device is monitored via a pair of wires. Internally, this binary value represents the state of the emergency stop.
Compare DIP Switch Setting to TIA Portal Configuration
The DIP switches in Figure 1 are set for 1oo2 (one double channel sensor). In the PLC, the 1oo2 hardware configuration is conducted graphically as shown in Figure 5. This configuration data is then compiled and downloaded to the safety module. Similar operations are required for the output modules.
Figure 5: Configuration for a 1oo2 safety input for an ET 200SP safety input module.
Software Configuration
There is no software configuration for the 3SK1 safety relay. The functionality is built into the hardware with minimal control via the DIP switches.
By contrast, the PLC is much more flexible. An example is included in Figure 6. Observe that the ladder logic includes two external non-safety tags: including a request to reset the module and a request to activate the motor. Not shown are the monitoring capabilities allowing the operator (remote or local) to see the state of the PLC and associated safety hardware.
Figure 6: Configuration for an ET 200SP safety output module.
Advantages and Disadvantages of the Stand-Alone Safety Relay
A stand-alone safety relay embodies purpose-built safety hardware.
Advantages
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The safety relay is relatively simple to install, troubleshoot, and maintain.
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Easy to troubleshoot at 3 AM using only a multimeter—no need to pull out a laptop and initiate the PLC.
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For simple systems (single zone with one or two monitored interlocks), cost is not a clear differentiator.
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The safety relay is not deeply integrated, allowing a replacement over the 20- to 50-year lifecycle of the machine. This is like comparing an old car radio to a modern infotainment and telemetry console. One is trivial to change, while the other is deeply integrated.
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Hybrid solutions are viable where a stand-alone safety relay provides the appropriate SIL/PL protection and the non-safe PLC handles the machine control.
Disadvantages
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There is no safety auditing mechanism for the smaller safety relays. For example, a DIP switch setting could go unnoticed. Note that advanced stand-alone safety relays (not covered here) are often programmable. These are more like a programmable PLC with the ability to identify changes to the hardware configuration.
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Lack of monitoring and timestamp capability can leaving the technician scrambling to piece together the sequence of events leading to the shutdown.
Advantages and Disadvantages of PLC-Based Safety
Safety functionality may be implemented in a safety PLC along with the associated F-rated local or distributed I/O modules.
Advantages
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Safety trips may be monitored and logged.
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The safety configuration is tracked inside the PLC using a checksum providing a documented snapshot of the current safety configuration. This is an important aspect of post-incident investigation as the checksum documents that an unmodified safety program was or was not present.
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Advanced PLC safety programs may establish a hierarchical structure to accommodate machines with multiple safety zones. This is further enhanced with the ability to use both local (to the PLC) and distributed I/O.
Disadvantages
- Complexity. This is mostly felt by the repair technicians. However, with proper integration this is largely mitigated by monitoring and alarm logging.
Parting Thoughts
There are many ways to implement machine safety.
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The best, and only acceptable solution, is to properly implement the machine-appropriate SIL/PL.
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The optimal solution begins with an understanding of the hardware and, more importantly, how that hardware is integrated at a system level.
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About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.