study object may be a closed or an open system. A closed system may be defined as follows:
Definition 2.4 (Closed system)
A system where the interface to the environment is static and always according to the assumptions specified.
In a closed system, the required inputs are always available, and random disturbances in the environment that may influence the study object are nonexisting. Most of the study objects considered in this book are closed systems. An open system is defined as follows:
Definition 2.5 (Open system)
A system where disturbances in the environment may influence the study object and where required system inputs and outputs may fluctuate or even be blocked.
Open system are generally much more difficult to analyze than closed systems. Some open systems allow users to manipulate the system structure.
2.4 Operating Context
Items are generally designed and built for an intended operating context that should be clearly stated in the item specification and in the user documentation. The operating context specifies how the item is to be operated and maintained, limits to inputs, usage, and loads, and also which environmental conditions the item is supposed to work in and to tolerate. The user manual of a washing machine may, for example, specify intervals for the voltage and frequency of the power supply, the pressure and temperature of the water supply, the type and weight of laundry (e.g. clothes, carpets) put into the machine, the temperature in the room where the machine is located, and the surface on which the machine is placed. The operating context of the item is defined as follows:
Definition 2.6 (Operating context)
The environmental and operating conditions under which the item is (or is expected to be) operating.
In some applications, the concept of operations (CONOPS) document describes the operating context of the item.
2.5 Functions and Performance Requirements
To be able to identify all potential item failures, the reliability engineer needs to have a thorough understanding of the various functions of the item and the performance criteria related to each function.
2.5.1 Functions
A function is a duty or an action the item has been designed to perform. A function requires one or more inputs to provide an output. The function is performed by technical and other resources and will usually also require some control (e.g. start signals). A function and its inputs and outputs are shown in Figure 2.3. The function and its elements are illustrated in Example 2.1.
Figure 2.3 A function illustrated as a functional block.
Consider a simple flashlight. The main function of the flashlight is to produce light. The required input is electric power coming from a battery. The resource is the flashlight with battery. The function is controlled by switching on/off the flashlight.
A function may be defined as follows:
Definition 2.7 (Function)
An activity, process, or transformation stated by a verb and a noun that describes what must be accomplished.
A function is an intended effect of an item and should be described such that each function has a single definite purpose. It is recommended to give the functions names that have a declarative structure, and say “what” is to be done rather than “how.” The functions should preferably be expressed as a statement comprising a verb plus a noun; for example, provide light, close flow, contain fluid, pump fluid, and transmit signal. In practice, it is often difficult to specify a function with only two words, and additional words may need to be added.
2.5.2 Performance Requirements
New products and systems are developed to fulfill a set of requirements. These requirements are usually written into a requirement document. The requirements may be based on (i) identified customer needs, (ii) manufacturer's ideas to make the product more competitive, and (iii) requirements in standards, laws, and regulations. The IEV defines the term “requirement” as follows:
Definition 2.8 (Requirement)
Need or expectation that is stated, generally implied or obligatory (IEV 192‐01‐13).
A performance requirement is a specification of the performance criteria related to a function. If, for example, the function is “pump water,” a performance requirement may be that the output of water must be between 100 and 110 l/min. Some functions may have several performance requirements. Performance requirements are also referred to as functional requirements or performance standards.
2.5.3 Classification of Functions
A complicated item may have a high number of required functions. All functions are not equally important, and a classification may therefore be an aid for identification and analysis purposes. One way of classifying functions is as follows:
Essential functions. These are the functions required to fulfill the intended purpose of the item. The essential functions are simply the reasons for installing or using the item. The essential function is sometimes reflected in the name of the item. An essential function of a pump is, for example, to “pump fluid.”
Auxiliary functions. These are the functions that are required to support the essential functions. The auxiliary functions are usually less obvious than the essential functions, but may in many cases be as important as the essential functions. Failure of an auxiliary function may in many cases be more safety‐critical than a failure of an essential function. An auxiliary function of a pump is, for example, to “contain fluid.”
Protective functions. These functions are intended to protect people, equipment, and the environment from damage and injury. The protective functions may be classified as follows:Safety functions (i.e. to prevent hazardous events and/or to reduce consequences to people, material assets, and the environment)Security functions (i.e. to prevent vulnerabilities, physical attacks, and cyberattacks)Environment functions (e.g. anti‐pollution functions)Hygiene functions (e.g. for items used in food production or in hospitals).
Information functions. These functions cover condition monitoring, various gauges and alarms, communication monitoring, and so forth.
Interface
