Definitions
The definitions for those reliability terms most often used within this guide are as follows.
• Reliability—
◦ The ability of an item to perform a required function without failure under stated conditions for a stated period of time.
◦ Or, as more commonly used in engineering applications:
The probability that an item can perform a required function under given conditions for a given time interval, (t1, t2). This is normally denoted either by the letter R or by R(t), with t denoting the interval (t1, t2.
• Failure—The state of the item when it is unable to perform a required function. In the case of non-repairable items, it is the termination of the ability of an item to perform a required function.
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1. After the occurrence of a failure, the item is in a faulty condition.
2. An occurrence of a failure is an event (as distinguished from a fault, which is a state.
3. This concept as defined does not apply to items consisting of software only.
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• (Instantaneous) Failure Rate—The limit, if this exists, of the ratio of the conditional probability that the instant of time, t, of a failure of an item falls within a given time interval, (t , t+∆τ), to the length of this interval, ∆τ, when ∆τ tends to zero, given that the item is in an up state at the beginning of the time interval. This limit is normally denoted by λ(t). Failure rates are often given in terms of failures per million hours (fpmh); however, some industries use an alternative measure of failures per 109 known as FITs (Failures in Time). Such failure rates are given in terms of failures per billion hours.
• Mean Time To Restore, Mean Time To Recovery or Mean Time to Repair (MTTR)—The expectation of the time to restore.
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In this document, the term MTTR is frequently used. This is to maintain a measure of consistency with other work. The term Mean Active Corrective Maintenance Time (MACMT) may often be interchanged with Mean Active Repair Time (MART).
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