MTTFd
Mean Time to Dangerous Failure (MTTFD) is the average time before a failure mode could cause hazards to people, the environment, or equipment. It is a key factor in assessing how safe a system is and how well its safety features perform.
ISO 13849 describes three ways to estimate MTTFD for a safety channel, reflecting different approaches to understanding how often dangerous failures can occur.
MTTF stands for mean time to failure: the expected time until a component first fails, assuming a constant failure rate. The system’s overall mean time between failures (MTBF) is roughly the sum of MTTF and the mean time to repair (MTTR). The MTTF of a system can be split into two parts: time to safe failure (MTTFS) and time to dangerous failure (MTTFD).
Consider a switch that turns a motor on or off. It can fail by sticking open (safe, the motor won’t start) or sticking closed (dangerous, the motor may not stop). The likelihood of dangerous vs. safe failures depends on design. Poorly designed parts may have more dangerous failures and a lower MTTFD, while safety-rated components aim to minimize dangerous failures, sometimes making MTTFD very high or effectively infinite.
To judge a safety system’s performance, you need to know how its components fail in dangerous vs. safe ways and the resulting MTTFD.
This page was last edited on 2 February 2026, at 13:50 (CET).