What do PL and SIL mean in safety standards

In the field of machine safety, Performance Level (PL) and Safety Integrity Level (SIL) are critical standards used to assess the reliability of safety-related control systems. PL classifies safety levels based on the Probability of a Dangerous Failure per Hour (PFHD), whereas SIL evaluates both the frequency and severity of hazardous events.
Both methodologies contribute to functional safety, but PL, defined in ISO 13849-1, provides a broader framework that also covers pneumatic and hydraulic systems, while SIL, defined in IEC 62061, applies specifically to electrical and electronic safety-related control systems.
Products such as tGard and mGard comply with PLd / SIL2 and PLe / SIL3, demonstrating their suitability for sector-specific safety requirements.

Performance Level (PL)

Definition of PL:
Performance Level (PL) is a classification system used to assess the capability of electrical, mechanical, pneumatic and hydraulic safety-related control systems to perform their intended safety function.
PL defines five levels, from PL a to PL e, based on PFHD values that express the probability of a dangerous failure occurring within one hour.

Factors influencing PL:
Several parameters determine the required or achieved PL, including system diagnostic coverage (DC), the Mean Time to Dangerous Failure (MTTFd) of components, and the overall system architecture (Categories B, 1-4).

A simplified approach to determining PFHD considers he severity of potential harm (S),
the frequency or duration of exposure to the hazard (F), and the possibility of avoiding or limiting the hazardous event (P). These factors together support the assignment of the appropriate PL

Safety Integrity Level (SIL)

Definition of SIL:
Safety Integrity Level (SIL) is applied specifically to the functional safety of electrical and electronic systems.
In contrast to PL, SIL assesses not only the frequency of hazardous events but also their potential severity.
SIL determination typically uses a risk matrix, often referred to as a Layer of Protection Analysis (LOPA) matrix.

Steps in determining SIL:

1. Identify the loss-of-containment scenario, often derived from a Hazard and Operability Study (HAZOP).
2. Define the initiating cause and the consequence of the scenario.
3. Establish the frequency of the initiating event.
4. Determine the tolerable incident frequency (acceptable risk), commonly using a risk matrix.
5. Evaluate the effectiveness of existing protection layers (LOPs or Independent Protection Layers, IPLs).
6. Consider conditional modifiers, variables that affect the likelihood of specific outcomes.
7. Calculate the scenario frequency and verify that it remains below the maximum allowable incident frequency.

SIL classification: The result of these steps lead to the classification of the SIL-level.

• SIL 4: 10⁻⁵ ≥ Probability of Failure on Demand (PFD) < 10⁻⁴
• SIL 3: 10⁻⁴ ≥ PFD < 10⁻³
• SIL 2: 10⁻³ ≥ PFD < 10⁻²
• SIL 1: 10⁻² ≥ PFD < 10⁻¹

Distinction between SIL and PL:

Although both systems aim to enhance functional safety, they differ in methodology and application.
ISO 13849-1 (PL) provides a broad framework that includes pneumatic and hydraulic control systems and is commonly used in machinery applications.
IEC 62061 (SIL) focuses on electrical and electronic safety-related control systems and is typically applied to more complex architectures.

Examples of products complying with SIL and PL:

• The tGard platform from Fortress meets PLd and SIL2, offering a compact access and control system integrating trapped-key systems, safety interlocks, pushbuttons and selector switches.
• mGard, amGard Pro, amGard Network, amGard S40, Alfred Ex, Atom, Louis, Steute Ex and Proton safety switches meet PLe and SIL3 requirements.
• Fluidsentry components designed for pneumatic and hydraulic applications are certified to PLe.

In summary, PL and SIL are essential tools for ensuring the safety and reliability of safety-related control systems.
Understanding their specific scope and application enables engineers and safety professionals to implement comprehensive safety measures tailored to the requirements of their industry.