· Frequency and Duration of Exposure (F): Refers to how often and how long an operator is exposed to a hazard. Occasional access is classified as F1, whereas frequent or constant exposure is F2.

· Possibility of Avoiding the Hazard (P): Evaluates whether the operator can reasonably avoid or limit harm once the hazard occurs. If avoidance is feasible, it is rated P1; if avoidance is nearly impossible, it is P2.

By combining these three parameters, the required Performance Level can be determined. For instance, a scenario involving minor injury potential (S1), frequent exposure (F2), and low avoidance possibility (P2) would lead to a Required Performance Level of PLr = C. In this case, the safety-related control system must meet PL C or higher (PL C–PL E) to ensure adequate protection. This systematic approach ensures that each safety function is designed with the appropriate reliability, aligning both with regulatory compliance and the operational safety goals of the machine.

ISO 13849-1 defines four essential technical indicators:

· Category (System Architecture)

Describes redundancy, fault tolerance, and behaviour during failure.

Higher categories (2, 3, 4) provide more robust protection.

· MTTF_d — Mean Time to Dangerous Failure

Indicates how reliable the internal components are over time.

Longer MTTFd = more stable and dependable safety system.

· DC — Diagnostic Coverage

Measures how effectively the system detects internal faults.

Higher DC = higher achievable PL.

· CCF — Common Cause Failure Resistance

Ensures redundant channels won't fail simultaneously due to the same external influence (vibration, wiring, EMC interference, etc.).

PLr tells you what level of safety is required. These four indicators determine whether your design can reach that level. A machine is compliant only when PL ≥ PLr.

· Enhanced Safety Assurance: The system maintains a predictable and dependable safety response, minimizing the risk of injury or equipment damage.

· Regulatory Compliance: Meeting the appropriate PL fulfills international safety requirements, supporting conformity with ISO 13849-1 and global market standards.

· Operational Efficiency: Well-designed safety architectures reduce unnecessary interruptions, enabling continuous, safe, and efficient production.

· Long-Term Reliability: Verified PL levels help ensure the system's safety performance remains stable over its entire lifecycle.