FTTI - Fault Tolerant Time Interval

 What is FTTI?

• FTTI - Fault Tolerant Time Interval

• FTTI time is one of the several Safety Relevant time intervals (others being FHTI, FRTI, FDTI)
• FTTI time is minimum time-span from the occurrence of a fault in an item to a possible occurrence of a hazardous event, if the safety mechanisms are not activated.

• FTTI is a relevant attribute for Safety Goals derived from hazards caused by malfunctioning behavior of an item. 
• FTTI time is only defined at the Item level

• Example

         Safety Goal : Avoid unintended vehicle longitudinal acceleration

        ASIL            : B

        FTTI            : 500 ms

• Vocabulary

Fault                                : an abnormal condition that can cause an element or item to fail.

Item                                 : system or part of a system to which ISO 26262 is applied, that implements a function or part of a function at vehicle level.

Hazard                            : potential source of harm caused by malfunctioning behavior of  an item

Operational Situation    : scenario that can occur during a vehicles life (driving at high speed,  maintenance)

Hazardous event           : combination of hazard and operational situation. The occurrence of a hazardous event is dependent on fault being present and a vehicle being in a scenario that allows to fault to affect vehicle behavior. E.g a failure in the brake system may not result in a hazardous event until the brakes are applied.