may be considered as an extension of quality into the time domain.
1.3.5 Dependability
Dependability is a more recent concept that embraces the concepts of reliability, maintainability, and availability, and in some cases also safety and security. Dependability has, especially, become known through the important series of standards IEC 60300 “Dependability management.” The IEV defines dependability as follows:
Definition 1.8 (Dependability)
The ability (of an item) to perform as and when required (IEV 192‐01‐01).
Another commonly used definition is “Trustworthiness of a system such that reliance can justifiably be placed on the service it delivers” (Laprie 1992).
Remark 1.1 (Translating the word “dependability”)
Many languages, such as Norwegian and Chinese, do not have words that can distinguish reliability and dependability, and reliability and dependability are therefore translated to the same word.
1.3.6 Safety and Security
General safety is outside the scope of this book, and we deal only with the safety aspects of a specified technical item and define safety as follows:
Definition 1.9 (Safety)
Freedom from unacceptable risk caused by the technical item.
This definition is a rephrasing of definition IEV 351‐57‐05. The concept safety is mainly used related to random hazards, whereas the concept security is used related to deliberate hostile actions. We define security as:
Definition 1.10 (Security)
Dependability with respect to prevention of deliberate hostile actions.
The deliberate hostile action can be a physical attack (e.g. arson, sabotage, and theft) or a cyberattack. The generic categories of attacks are called threats and the entity using a threat is called a threat actor, a threat agent, or an adversary. Arson is therefore a threat, and an arsonist is a threat actor. The threat actor may be a disgruntled employee, a single criminal, a competitor, a group, or even a country. When a threat actor attacks, he seeks to exploit some weaknesses of the item. Such a weakness is called a vulnerability of the item.
Remark 1.2 (Natural threats)
The word “threat” is also used for natural events, such as avalanche, earthquake, flooding, landslide, lightning, tsunami, and volcano eruption. We may, for example, say that earthquake is a threat to our item. Threat actors are not involved for this type of threats.
1.3.7 RAM and RAMS
RAM, as an acronym for reliability, availability, and maintainability, is often used, for example, in the annual RAM Symposium.1 RAM is sometimes extended to RAMS where S is added to denote safety and/or security. The RAMS acronym is, for example, used in the railway standard IEC 62278.
Remark 1.3 (Broad interpretation of reliability)
In this book, the term “reliability” is used quite broadly, rather similar to RAM as defined above. The same interpretation is used by Birolini (2014).
1.4 Reliability Metrics
Throughout this book, it is assumed that the time‐to‐failure and the repair time of an item are random variables with probability distributions that describe the future behavior of the item. The future behavior may be evaluated based on one or more reliability metrics. A reliability metric is a “quantity” that is derived from the reliability model and is, as such, not directly measurable. When performance data become available, we may estimate or predict quantitative values for each reliability metric.
A single reliability metric is not able to tell the whole truth. Sometimes, we need to use several reliability metrics to get a sufficiently clear picture of how reliable an item is.
1.4.1 Reliability Metrics for a Technical Item
Common reliability metrics for an item include
1 The mean time‐to‐failure (MTTF)
2 The number of failures per time unit (failure frequency)
3 The probability that the item does not fail in a time interval (survivor probability)
4 The probability that the item is able to function at time (availability at time )
These and several other reliability metrics are given a mathematical precise definition in Chapter 5, and are discussed and exemplified in all the subsequent chapters.
Example 1.1 (Average availability and downtime)
Consider the electricity supply, which is supposed to be available at any time. The achieved average availability
If we consider a period of one year, the total time is approximately 8760 hours. The downtime is the time, during the specified time period, the service is not available. The relationship between the average availability and the length of the downtime is illustrated in Table 1.1.
Table 1.1 Availability and downtime.
| 90 | 36.5 d |
| 99 | 3.65 d |
| 99.9 | 8.76 h |
| 99.99 | 52 min |
| 99.999 | 5 min |
1.4.2 Reliability Metrics for a Service
A wide range of service reliability metrics have been defined, but these vary significantly between the application areas. The most detailed metrics are available for electric power supply (e.g. see IEEE Std. 1366 2012).
Example 1.2 (Airline reliability and availability)
Airline
