What is PFHd?
PFHd (Probability of dangerous Failure per Hour) is a measure of the average probability per hour that a safety function fails in a dangerous way. It is a core concept within the ISO 13849-1 and IEC 62061 standards for determining the Performance Level (PL) or SIL class of a safety function.
In an OT context, PFHd is used in designing and verifying systems such as a Safety PLC, emergency stop, light curtain or safety relay.
🧠 Why is PFHd important?
| Application | Meaning |
|---|---|
| Machine safety under the Machinery Directive | Demonstrating that risks have been sufficiently reduced |
| Component selection | The PFHd value determines whether a component is suitable for the required PL/SIL |
| Risk assessment and validation | PFHd calculation supports FMEA and design verification |
| Accountability towards regulators | Objective justification of the safety level |
🔢 PFHd ranges per Performance Level
| Performance Level (PL) | PFHd range (1/h) | Equivalent to SIL |
|---|---|---|
| PL a | ≥ 10⁻⁵ to < 10⁻⁴ | — |
| PL b | ≥ 3 × 10⁻⁶ to < 10⁻⁵ | — |
| PL c | ≥ 10⁻⁶ to < 3 × 10⁻⁶ | ~ SIL 1 |
| PL d | ≥ 10⁻⁷ to < 10⁻⁶ | ~ SIL 2 |
| PL e | ≥ 10⁻⁸ to < 10⁻⁷ | ~ SIL 3 |
A lower PFHd means a safer system.
📐 How is PFHd calculated?
PFHd is typically calculated using:
- Component data (MTTFd, DC, CCF) according to ISO 13849-1
- Architectural model of the safety circuit (category B, 1–4)
- SISTEMA or other tools from TÜV or IFA for automatic calculation
- Or manually via formula (only for simple circuits)
📌 Example:
A safety function with a PFHd of
8.5 × 10⁻⁸meets PL e / SIL 3.
🔧 Where do you encounter PFHd in practice?
| Application | Typical PFHd requirement |
|---|---|
| Emergency stop on a packaging machine | PL c or d (PFHd ~ 10⁻⁶) |
| Safety PLC in a press installation | PL d or e (PFHd < 10⁻⁷) |
| Light curtain for a robot cell | PL d or e, depending on the risk assessment |
| Access door interlock | PL c at low speed, PL e for robotics |
✅ Best practices
- Request PFHd data from component suppliers (or download SISTEMA libraries)
- Document the PFHd calculation as part of the safety dossier
- Take into account the age, contamination and frequency of use of components
- Validate PFHd whenever the system changes via MOC or a re-run of the FMEA
- Use PFHd values to substantiate SIL/PL claims in the CE marking
📌 In summary
PFHd is a technical measure that helps to quantitatively determine how safe a system is. It bridges risk assessment and standard-compliant design under ISO 13849-1 or IEC 62061.