What is functional safety and why is it important?

Functional safety is the engineering discipline that aims to ensure that protective systems in a plant or machine operate correctly and reliably when needed, protecting it from hazardous conditions. When a process variable such as pressure, temperature, level, or gas reaches a dangerous limit, a safety system is expected to activate and restore the process to a safe state. Whether this protection is 'reliable enough' is defined not by sentimental evaluation, but by measurable probability targets.

The international umbrella standard for functional safety is IEC/EN 61508. This standard covers the entire lifecycle of electrical, electronic, and programmable electronic (E/E/PE) safety systems, from design to commissioning and maintenance. The required level of reliability of a safety system is determined in proportion to the risk it carries, and this reliability level is expressed as SIL (Safety Integrity Level).

SIL (Security Integrity Level) and PFD concepts

SIL is the target reliability level assigned to a safety function and is defined in four levels from SIL 1 to SIL 4. As the number increases, the expected reliability of the function, and therefore the design rigor, increases. SIL is not a 'brand' of a device; it is a performance target assigned to the entirety of a specific safety function (sensor + decoder + end element).

In on-demand (low-demand mode) safety functions, SIL is correlated with PFD (Probability of Failure on Demand). Roughly speaking, the higher the SIL level, the lower the probability that the function will fail when needed. Therefore, the SIL assessment is based on engineering calculations using data such as equipment failure rates, test intervals, diagnostic coverage, and architecture (redundancy).

• SIL 1–SIL 4: four levels of safety integrity; the higher the level, the greater the reliability target and design rigor.
• PFD: Probability of Failure on Demand — SIL metric for functions in low demand mode.
• SIL is assigned to a security function; it belongs to the entire detection-decision-response chain, not a single component.
• The SIL level is determined in proportion to the magnitude of the risk involved (risk reduction objective).

SIF, SIS, and security lifecycle

A safety instrumented function (SIF) is a single protective function designed to prevent or mitigate a specific hazard; for example, shutting off a feed valve when excessive pressure builds up in a reactor. The hardware and software system that implements these functions is called a safety instrumented system (SIS).

Functional safety is not a one-time calculation; it is a safety lifecycle that begins with hazard and risk analysis, continues with SIL assignment, and encompasses design, verification, commissioning, operation, periodic testing, and change management. Standards expect traceable documentation of evidence generated at each stage (SRS – safety requirement specification, SIL calculations, verification records, proof-test plans). AES's assessment is also based on this lifecycle evidence.

IEC 61508, IEC 61511 and machine side (ISO 13849-1 / IEC 62061)

IEC/EN 61508 is the general (umbrella) standard for functional safety and primarily targets system/equipment developers. The sector-specific application standard for the process sector (petrochemical, chemical, refinery, energy, boiler/burner plants) is IEC 61511; this standard targets users and integrators who design, install, and operate SIS with existing products.

In terms of machine safety, functional safety is generally addressed by ISO 13849-1 (PLr – required performance level) and IEC 62061 standards. Which standard is used depends on whether the application is process or machine safety, the type of equipment, and the customer's contractual/design requirements. AES will clarify the appropriate standard framework for your application together with you before the assessment.

• IEC/EN 61508: General framework standard for functional safety (E/E/PE systems).
• IEC 61511: SIS application standard for the process sector.
• ISO 13849-1 (PLr) and IEC 62061: related standards for machine safety functions.
• The selection of the appropriate framework is application-specific and clarified when defining the scope of assessment.

AES's role: independent verification and evaluation (not certification)

AES Innovation's role in this service is to provide an independent engineering assessment and verification service. Specifically, this includes checking SIF and SIL calculations, reviewing safety requirement specifications and architectural assumptions, reviewing safety lifecycle documentation for compliance with standards, and evaluating the integrity of the evidence file. The output is a technical assessment/verification report signed by a qualified engineer.

It is important to clearly understand the limitations of this service: AES does not perform SIL product certification, issue 'SIL certificates', or certify any equipment for SIL compliance. Product certification of equipment for SIL compliance is a separate and accredited certification activity. What AES offers is an impartial, standards-based engineering review and consultation. This assessment helps you identify gaps in your design early and strengthen your evidence file.

• Included in the scope: SIF/SIL account verification, security cycle review, evidence file evaluation, independent verification report.
• Excluded from scope: SIL product/equipment certification, issuing 'SIL certificates', issuing any conformity certificates.
• Output: Technical evaluation/verification report prepared by an authorized engineer.
• Approach: an unbiased, educational, and standards-based engineering assessment.

In which facilities and applications is it needed?

Functional safety and SIL assessments are relevant wherever the safety of a hazardous process or machine depends on instrumented systems. The assessment is tailored to the facility's risk profile and the safety functions employed.

• Process plants, petrochemical, chemical and refinery applications.
• Boiler and burner safety systems
• Emergency Shutdown (ESD) systems
• Flammable and toxic gas detection systems.
• Pressure, temperature, and level-based protective functions.
• Machine safety functions (within the framework of relevant standards)

Is it legally mandatory? Why is it required?

To be frank: there is no single, universally applicable regulation in Türkiye that directly mandates SIL verification. SIL is not a legally enforced type of periodic inspection; it is primarily carried out for risk management, safe design, and compliance with international standards—as a requirement of good engineering practice.

However, regulatory contexts exist that require addressing process safety. For example, organizations covered by the Regulation on Prevention and Mitigation of Major Industrial Accidents (SEVESO) are expected to manage process safety systematically; in such contexts, functional safety and SIL (Sequential Identification) studies can be a supporting tool. However, this is not a definitive legal claim that 'SEVESO mandates SIL verification'; each facility's obligation should be determined separately according to its own regulatory situation, operations, and risk assessment. At this point, it is recommended that the company confirm its own legal obligations with the relevant legislation and competent authority.

• It is not a legally mandated type of periodic examination.
• Main objectives: risk reduction, safe design, and compliance with international standards.
• It can be a supporting tool in regulatory contexts like SEVESO (not a strict mandatory claim).
• The obligations are specific to the facility; they must be confirmed with the relevant legislation and the competent authority.

Independent SIL verification process with AES

AES Innovation will independently evaluate your existing proof file and SIF/SIL calculations after jointly determining the appropriate standard framework for your application (IEC 61508 / IEC 61511 and machine-side standards as required). Our goal is not to simply issue a document; it is to examine whether your design meets safety objectives from an unbiased engineering perspective and share the findings in a clear, traceable report.

At the end of the process, you will receive a technical evaluation/verification report prepared by a qualified engineer; this report includes strengths, areas for improvement, and suggestions for improvement. The scope and methodology are clarified separately for each project.

This content is for informational purposes only and does not constitute legal opinion, conformity assessment, or certification commitment. Functional safety/SIL verification is offered as an independent engineering assessment service outside the scope of AES Innovation's TÜRKAK accreditation (6.1 Electrical Installation only); AES is not a SIL product certification body. For precise obligations, the relevant current legislation and standards should be followed.

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Frequently Asked Questions

Does AES provide SIL certification?

No. AES does not issue 'SIL certificates' and does not perform equipment/product SIL certification. SIL certification of a product is a separate and accredited certification activity. AES's service is independent engineering assessment and verification and consultancy within the framework of IEC 61508 / IEC 61511. The output is a technical assessment report signed by an authorized engineer — it is not a certificate of conformity.

What is the difference between SIL verification and SIL certification?

SIL certification is typically the confirmation of a piece of equipment or a system's SIL compliance by an accredited certification body. SIL verification, on the other hand, is an independent engineering review of SIF/SIL calculations, architectural assumptions, and safety cycle proofs against the standard. AES offers only the latter, namely independent verification and evaluation.

What do SIL levels 1–4 represent?

SIL is the reliability target assigned to a safety function, increasing from SIL 1 to SIL 4. In functions operating in low-demand mode, this target is measured by the probability of failure at demand (PFD): as the level increases, the probability of the function failing to perform its task when needed decreases, and the design is expected to be more rigorous. SIL is determined according to the magnitude of the risk involved.

Is this a periodic inspection/examination service?

No. This service is not a legally mandated periodic inspection, but rather an engineering assessment and verification service. The aim is to independently review the compliance of your safety instrument functions and evidence file with the standard. Periodic tests conducted as part of the safety lifecycle are the responsibility of the company's own safety management.

Is SIL assessment legally mandatory in Türkiye?

In Türkiye, there is no single regulation that directly mandates SIL verification and applies to everyone. SIL studies are primarily conducted for risk management, safe design, and compliance with international standards. They can be supportive in regulatory contexts requiring process safety, such as SEVESO; however, each facility's obligation must be determined separately according to its own regulatory situation and confirmed with the competent authority.

Which standards and practices are covered?

The core standards are IEC/EN 61508 (general framework) and IEC 61511 (process sector); on the machine side, ISO 13849-1 (PLr) and IEC 62061 are related. Application areas include process/petrochemical plants, boiler-burner safety, emergency shutdown (ESD) systems, flammable/toxic gas detection, and machine safety functions. The appropriate standard framework for your application will be clarified together before the assessment.