SIL and PL Certification — Functional Safety for Machinery and Systems

EQS supports companies in the evaluation, design, and certification of functional safety systems according to international standards IEC 62061 (SIL) and ISO 13849 (PL), ensuring regulatory compliance and the protection of operators and plants.

Safety Integrity Level (SIL)

SIL (Safety Integrity Level) is a discrete level (from SIL 1 to SIL 4) that defines the safety integrity requirements of Safety Instrumented Functions (SIF) within a Safety Instrumented System (SIS). The concept is defined by the IEC 61508 standard and applied in the certification of safety-related control systems for machinery via the IEC 62061 standard.

SIL expresses the average probability of a safety function failing to perform its design function on demand (PFD – Probability of Failure on Demand) or the frequency of a dangerous failure per hour (PFH – Probability of dangerous Failure per Hour) in continuous mode.

Reference standard for machinery: IEC 62061:2021 specifies requirements and provides recommendations for the design, integration, and validation of safety-related electrical, electronic, and programmable electronic control systems (SRECS) for machinery.

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The Four SIL Levels
  • SIL 1 — Low risk. PFD: 10⁻² to 10⁻¹. Safety functions with moderate risk reduction. Typical applications: controlled starting, emergency stop in low-hazard contexts.
  • SIL 2 — Medium risk. PFD: 10⁻³ to 10⁻². Required in many industrial plants, supervision, and protection systems in contexts with potential serious reversible damage.
  • SIL 3 — High risk. PFD: 10⁻⁴ to 10⁻³. High risk reduction. Typical in process industry, oil & gas, chemical plants, and systems with possible irreversible damage to persons.
  • SIL 4 — Critical risk. PFD: 10⁻⁵ to 10⁻⁴. Maximum integrity. Reserved for sectors such as nuclear, rail, and aerospace. Not covered by IEC 62061 for standard machinery.
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Performance Level (PL)

PL (Performance Level) is a discrete level (from PL a to PL e) used to specify the ability of safety-related parts of control systems (SRP/CS) to perform a safety function under foreseeable conditions. It is defined by the ISO 13849-1:2023 standard, the main reference standard for the design of safety functions in machinery.

The PL takes into account the system structure (Category), Diagnostic Coverage (DC), Mean Time to Dangerous Failure (MTTFd), and Common Cause Failure (CCF).

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TÜV SIL certificate pdf
The Five PL Levels
  • PL a — Minimum. PFHd ≥ 10⁻⁵
  • PL b — Low. PFHd between 10⁻⁶ and 10⁻⁵
  • PL c — Medium. PFHd between 10⁻⁷ and 10⁻⁶
  • PL d — High. PFHd between 10⁻⁸ and 10⁻⁷
  • PL e — Maximum. PFHd < 10⁻⁸
ISO 13849 Structural Categories

The ISO 13849 standard defines 5 categories that describe the structure of the safety-related control system:

  • Category B: Basic structure, no specific diagnostic requirements.
  • Category 1: Well-tried components, without self-diagnosis.
  • Category 2: Periodic testing of the safety function via a test circuit.
  • Category 3: A single fault does not lead to the loss of the safety function (redundancy without continuous self-diagnosis).
  • Category 4: Single fault detected automatically, maximum reliability (redundancy with self-diagnosis).

SIL vs PL: Comparison and Correlation

SIL and PL are two parallel scales that both express the ability of a safety function to reduce risk. The IEC 62061 standard uses SIL, while the ISO 13849 standard uses PL. Both are applicable to the Machinery Directive.

PL (ISO 13849)

SIL (IEC 62061)

PFHd (failures/hour)

Risk Reduction

PL a

—

≥ 10⁻⁵

Low

PL b

SIL 1

3×10⁻⁶ to 10⁻⁵

Moderate

PL c

SIL 1

10⁻⁶ to 3×10⁻⁶

Medium

PL d

SIL 2

10⁻⁷ to 10⁻⁶

High

PL e

SIL 3

10⁻⁸ to 10⁻⁷

Very high

The EQS Service for SIL and PL

EQS accompanies machine manufacturers and system integrators throughout the entire functional safety assessment and certification process.

  1. Risk analysis and determination of target PLr / SIL. Application of the graph method (ISO 13849) or the risk matrix (IEC 62061) to determine the required safety level for each safety function.
  2. Safety architecture design. Selection of the structural category (ISO 13849), system topology, and suitable components. CCF, MTTFd, and DC analysis.
  3. Calculation and verification of achieved PL / SIL. Calculation of the system’s aggregate PFHd using dedicated software tools (SISTEMA, SAFEXPERT) and verification that the achieved PL ≥ PLr.
  4. Validation and functional testing. Experimental verification of the correct operation of safety functions through functional tests, fault tests, and documented procedures.
  5. Technical documentation and CE certification support. Drafting of the Technical File, SRS specifications, and support for the Notified Body for CE certification (EU Machinery Regulation 2023/1230).

Typical Sectors and Applications

  • Machine tools and machining centers: motorized guards, safety stops, safe speed control.
  • Robotics and automation: emergency stops, safe space control, safe-speed, and safe-torque-off.
  • Process industry (oil & gas, chemical, pharmaceutical): SIS systems, safety valves, ESD.
  • Food and packaging industry: protection on high-speed lines, safety doors, and gates.
  • Lifts, lifting platforms, and cranes: overload protection, position control, limit stops.
  • Energy and infrastructure: protection systems for turbines, generators, and large-scale photovoltaic plants.

Reference Standards

  • IEC 61508 — Functional safety of E/E/PE systems
  • IEC 62061:2021 — SIL for machinery
  • ISO 13849-1:2023 — Performance Level
  • ISO 13849-2 — Validation
  • Machinery Directive 2006/42/EC
  • Machinery Regulation (EU) 2023/1230