In 2026, PLCs used in safety-critical applications must comply with IEC 61508 (functional safety of electrical and programmable electronic systems) and sector-specific standards such as IEC 62061 for machinery and IEC 61511 for process industries. These standards define how programmable logic controllers must be designed, validated, and maintained to reduce the risk of dangerous failures to an acceptable level.
The requirements apply to any facility where a PLC controls a process that could cause harm to people, equipment, or the environment if it fails. This includes chemical plants, oil and gas installations, food processing facilities, and energy infrastructure. The sections below unpack each key question around PLC safety standards so you can assess exactly where your systems stand.
Which international standards define PLC safety requirements?
The primary international standards that define PLC safety requirements are IEC 61508, IEC 62061, and IEC 61511. IEC 61508 is the foundational standard covering functional safety across all industries. IEC 62061 applies specifically to machinery, while IEC 61511 covers safety instrumented systems in the process industry. Together, these form the backbone of programmable logic controller safety compliance worldwide.
IEC 61508 is often described as the “umbrella” standard because sector-specific standards are derived from it. When a PLC manufacturer certifies hardware and software to IEC 61508, that certification provides the foundation for applying the product within systems governed by IEC 62061 or IEC 61511. For most industrial facilities, the relevant standard depends on the sector:
- Process industry (chemicals, oil and gas, water): IEC 61511
- Machinery and manufacturing: IEC 62061 (which aligns with ISO 13849 for mechanical systems)
- General electrical and programmable systems: IEC 61508
In the European Union, compliance with these standards also connects to the Machinery Directive and, for certain applications, the ATEX Directive. Meeting the relevant IEC standard is therefore not just a technical best practice but often a legal obligation.
What is a SIL rating and how does it apply to PLCs?
A SIL (Safety Integrity Level) rating is a discrete measure of the risk reduction capability of a safety function, defined in IEC 61508. SIL ratings run from SIL 1 (lowest) to SIL 4 (highest), where each level represents roughly an order of magnitude reduction in the probability of a dangerous failure on demand. For PLCs, a SIL rating indicates the maximum safety integrity level at which the device can be used within a safety instrumented function.
When engineers design a safety instrumented system, they perform a risk assessment to determine the required SIL for each safety function. The PLC or safety controller used in that function must be certified to at least that SIL level. A SIL 2 certified PLC, for example, can be used in SIL 1 or SIL 2 applications but not in a SIL 3 application without additional architectural measures.
SIL ratings are determined by two key metrics:
- PFD (Probability of Failure on Demand): Used for low-demand safety functions that activate only when a hazard occurs
- PFH (Probability of Failure per Hour): Used for high-demand or continuous-mode safety functions
It is important to understand that SIL is a system-level property, not just a device property. A SIL 3 certified PLC does not automatically deliver SIL 3 performance in a system. The entire safety instrumented function, including sensors, logic solvers, and final elements, must collectively achieve the required probability of failure.
How does IEC 61508 certification affect PLC hardware and software design?
IEC 61508 certification imposes strict requirements on both the hardware architecture and the software development lifecycle of a PLC. On the hardware side, it mandates redundancy, diagnostic coverage, and fail-safe behaviour. On the software side, it requires structured development methods, formal verification, and documented testing. These requirements fundamentally distinguish safety-certified PLCs from standard industrial controllers.
Hardware design requirements
IEC 61508 defines hardware fault tolerance requirements based on the target SIL level. Higher SIL levels require redundant architectures such as 1oo2 (one-out-of-two) or 2oo3 (two-out-of-three) voting systems. The standard also sets maximum limits on the safe failure fraction, which is the proportion of failures that are either safe or detected by diagnostics. PLC manufacturers must demonstrate through quantitative analysis that their hardware meets these limits.
Software development requirements
The software requirements in IEC 61508 are equally demanding. Safety software must be developed following a defined safety lifecycle, with requirements traceability from specification through to testing. The standard restricts which programming languages and constructs can be used, favouring deterministic, verifiable code. For higher SIL levels, formal methods and independent verification become mandatory. This is why safety PLC firmware and application software require far more rigorous documentation than standard automation software.
What’s the difference between a standard PLC and a safety PLC?
A safety PLC is specifically designed and certified to meet IEC 61508 requirements, with built-in hardware redundancy, comprehensive self-diagnostics, and a fail-safe operating mode. A standard PLC is optimised for reliable control and automation but lacks the certified fault detection, redundant processing, and systematic failure analysis required for safety-critical applications. The two are not interchangeable in safety instrumented systems.
The practical differences show up in several areas:
- Diagnostics: Safety PLCs continuously test their own hardware and software, detecting faults before they can cause a dangerous failure. Standard PLCs have diagnostics, but not to the depth or coverage required by IEC 61508.
- Fail-safe behaviour: When a safety PLC detects a fault, it moves to a defined safe state. Standard PLCs are not designed with this mandatory safe-state behaviour.
- Certification documentation: Safety PLCs come with a full safety manual and third-party certification (typically from TÜV). This documentation is required to use the device in a certified safety function.
- Separation of safety and standard logic: Modern safety PLCs allow safety and standard control logic to run on the same hardware platform while maintaining strict separation, which simplifies engineering without compromising integrity.
In practice, Siemens SIMATIC S7-300F, S7-400F, and the newer S7-1500F series are examples of safety PLCs that integrate with standard automation environments while meeting the certification requirements for use in safety instrumented systems.
Are there new or updated PLC safety requirements coming in 2026?
In 2026, the most significant development affecting PLC functional safety is the continued transition to IEC 61508 Edition 3, which is currently in advanced development, and the growing regulatory emphasis on cybersecurity as a component of functional safety. IEC 62443, the standard for industrial cybersecurity, is increasingly being referenced alongside IEC 61508 because a cyberattack that compromises a safety PLC is treated as a functional safety failure.
Several developments are shaping the 2026 compliance landscape:
- IEC 61508 Edition 3: The upcoming revision is expected to address software-intensive systems more explicitly, including the use of model-based development and artificial intelligence in safety applications. Facilities planning new safety system investments should track this revision, as it will affect future certification requirements.
- EU Machinery Regulation: The EU Machinery Regulation (2023/1230), which replaces the Machinery Directive 2006/42/EC, began applying in January 2027 but requires preparatory compliance work in 2026. It strengthens requirements for safety-related control systems.
- Cybersecurity integration: Regulators and certification bodies are increasingly requiring that safety PLC suppliers and integrators demonstrate that cybersecurity risks have been assessed as part of the functional safety case.
For existing installations, the immediate priority is ensuring that validation and proof testing records are current and that any ageing safety PLCs are assessed against the evolving requirements before the next scheduled safety lifecycle review.
How can industrial facilities verify their PLCs meet 2026 safety standards?
Industrial facilities can verify PLC safety standards compliance through a structured process that includes reviewing device certifications, conducting a functional safety assessment, and confirming that safety management procedures align with the applicable IEC standard. Verification is not a one-time activity but part of an ongoing safety lifecycle that includes periodic proof testing, management of change procedures, and documented audits.
A practical verification process covers the following steps:
- Confirm device certification: Check that each safety PLC has a valid third-party certificate (TÜV or equivalent) matching the required SIL level, and that the certificate covers the firmware version currently installed.
- Review the safety requirements specification: Ensure that the safety functions implemented in the PLC are traceable back to a documented hazard and risk assessment.
- Validate the safety application software: Confirm that the safety logic has been tested and validated in accordance with the PLC manufacturer’s safety manual and the applicable IEC standard.
- Check proof test intervals: Verify that proof tests are being performed at the intervals required to maintain the claimed SIL level over the system’s operational life.
- Assess cybersecurity measures: Review whether access controls, network segmentation, and patch management procedures are in place for safety PLC systems.
- Engage a competent functional safety engineer: For complex systems or where internal expertise is limited, an independent functional safety assessment by a certified engineer (TÜV FS Engineer or equivalent) provides authoritative verification.
Facilities that have not reviewed their safety PLC installations within the last three to five years should treat 2026 as a prompt to schedule a formal safety lifecycle review, particularly in light of the evolving cybersecurity requirements and the approaching EU Machinery Regulation transition.
How CoNet helps with PLC functional safety and compliance
We are CoNet, and functional safety in industrial automation is at the core of what we do. As the only company in the Netherlands certified as both a Siemens PCS 7 Process Safety Specialist and a Siemens COMOS partner, we bring a depth of expertise that covers the full safety lifecycle, from initial risk assessment through engineering, validation, and ongoing maintenance.
When it comes to PLC safety standards compliance in 2026, we support our clients in concrete ways:
- Safety PLC engineering and integration: We design and implement Siemens safety PLC systems (including SIMATIC S7-1500F and PCS 7 F-systems) that are certified to the required SIL level for your specific application, whether in chemicals, oil and gas, food and beverage, or energy.
- Functional safety assessments: We review existing installations against IEC 61508, IEC 61511, and IEC 62061 requirements, identifying gaps and providing a clear remediation roadmap.
- Safety lifecycle documentation: We produce and maintain the safety requirements specifications, validation reports, and proof test procedures that regulators and certification bodies require.
- Cybersecurity integration: We address the growing overlap between functional safety and IEC 62443 cybersecurity requirements, ensuring your safety PLCs are protected against both process failures and cyber threats.
- Ongoing support and proof testing: Through our plant automation services, we provide scheduled proof testing and management of change support to keep your safety systems compliant throughout their operational life.
If you want to confirm that your PLCs meet the safety standards required in 2026 or are planning a new safety instrumented system, we are ready to help. Contact us to discuss your situation with one of our functional safety specialists.
