Failures In Time or Failure UnIT
FIT is the number of failures per billion hours for a piece of equipment.
It is mentioned in both IEC 61508 and IEC 61511 standards as a preferred unit of measurement expressed by 109 hours.
Example: 5 FIT is expressed as 5 failures within 109 hours .
When you hear people talk about failure rates from certificates or FMEDAs, they are generally speaking in terms of FIT. These failure rates can be broken down into safe, detected, and undetected, as well as dangerous, detected, and undetected failures. They are very important pieces of information for end users.
Because failure rates are entered into significant end user equations such as PFDavg and other SIL Verification tasks, it is imperative that they are not only applied to the right application, operation, and demand, but also that they are realistic and correct.
To check if your failure rates are too optimistic, go to SILsafeData.com.