Glossary · Functional safety
Probability of dangerous failure per hour (PFHd)
Also known as: PFHd, PFHD, PFH, average frequency of dangerous failure per hour
German: Wahrscheinlichkeit eines gefährlichen Ausfalls pro Stunde (PFHd)
In functional safety, the probability of dangerous failure per hour (PFHd) is the average probability per hour that a safety-related system fails dangerously. It is the quantitative measure behind the Performance Level and, for high-demand or continuous operation, the SIL.
- Functional safety
- Verification
In one sentence
PFHd is the average hourly probability of dangerous failure of a safety function, the number behind PL and SIL for high-demand operation.
Example
The integrator of a conveyor line adds up the PFHd values of the emergency stop button, the safety relay and the contactors to obtain the value for the complete emergency stop function.
How it applies
- Functional safety: PL e and SIL 3 both correspond to a PFHd from 10^-8 to below 10^-7 per hour. PL d corresponds to 10^-7 to below 10^-6 per hour. The value of a safety function is roughly the sum of the values of its subsystems (sensor, logic, actuator).
- Component data: Manufacturers of safety components publish PFHd values under stated conditions such as mission time and demand rate. Check that those conditions match the application.
- Technical documentation: Keep the calculation and the data sources with the technical file. Do not quote a PFHd of a single component as if it were the value of the whole function.
- Terminology: IEC 61508 and IEC 62061 speak of PFH (average frequency of dangerous failure); low-demand functions in IEC 61508 use the average probability of failure on demand instead.
PFHd vs. MTTFd
MTTFd is a reliability input for one channel. PFHd is the result for the safety function, after architecture, diagnostics and common-cause failures have been considered.