Automotive Semiconductors and Functional Safety
Which correctly describes the fault tolerant time interval (FTTI) in ISO 26262?
- AThe average time a component can be used without failing; it has the same meaning as MTTF, and longer means safer
- BThe time from a fault occurring until repair is completed; shorter is considered safer
- CThe minimum time from a fault occurring until a hazardous event could occur if no countermeasure were taken; fault detection and transition to a safe state must be completed within this time
Show answer and explanation
Answer: C. The minimum time from a fault occurring until a hazardous event could occur if no countermeasure were taken; fault detection and transition to a safe state must be completed within this time
FTTI is the minimum time from the occurrence of a fault until a hazardous event would occur if safety mechanisms did not act. A safety mechanism must keep the sum of the time to detect the fault and the time from detection to reaching a safe state, for example by shutting off outputs, within the FTTI. For example, for a function with an FTTI of several tens of ms, a diagnosis once every few seconds is too slow. It is a different concept from MTTF, which represents component life, or from repair time.
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