Glossary · Functional safety
Mean time to dangerous failure (MTTFd)
Also known as: MTTFd, MTTFD
German: Mittlere Zeit bis zum gefahrbringenden Ausfall (MTTFd)
In functional safety of machinery, the mean time to dangerous failure (MTTFd) is an estimate of the average operating time before a dangerous failure occurs. ISO 13849-1 uses it per channel as one input for determining the Performance Level.
- Functional safety
- Verification
In one sentence
MTTFd estimates how long a channel runs on average before failing dangerously, one input for the Performance Level under ISO 13849-1.
Example
For a door interlock switch, the designer derives the MTTFd from the manufacturer's B10d value and the expected number of operations per year on the packaging line.
How it applies
- Functional safety: ISO 13849-1 groups MTTFd per channel into low, medium and high. Together with the Category and diagnostic coverage, it determines the achievable Performance Level.
- Wear parts: For electromechanical and pneumatic components, MTTFd depends on the number of operations. It is derived from B10d values and the assumed operating cycles, which are therefore assumptions to document.
- Technical documentation: Record the data source for each value (manufacturer data, standard tables) and the assumed demand rate. If the machine is used more intensively than assumed, the value no longer holds.
- Maintenance: Components that reach their assumed service life must be replaced. The maintenance manual must state this.
MTTFd vs. PFHd
MTTFd describes the reliability of a channel as an average time. PFHd describes the probability of dangerous failure of the whole safety function per hour, taking architecture and diagnostics into account.