What is dependability?
Dependability is defined as the "ability to perform as and when required". It applies to any system, product, process or service and may involve hardware, software and human aspects. Dependability is a collective set of time-related performance characteristics that coexist with other requirements such as output, efficiency, quality, safety, security and integrity.
Dependability does not have a single measure that can be attributed to it but is instead a combination of relevant measures that vary with each application. In a broad sense, dependability is the trust that can be placed in a system to provide the required functionality and deliver its expected value and benefits.
Dependability is the term that has been adopted internationally to cover a range of attributes of which the main ones are availability, reliability, maintainability and supportability.
There are, however, a number of different terms that are used today instead of dependability.
During design and development, reliability and maintainability are most relevant so the term R&M is common.
During operation, availability comes into play and the acronym RAM (reliability, availability and maintainability) is prevalent. Sometime this becomes RAMS, where the “S” can stand for either safety or supportability.
It is still common for reliability to be used as an overall term that includes all of these attributes, as well as its more exact measure as the probability that something may fail within a certain time period. This proliferation of terms has resulted in considerable misunderstanding of this important engineering discipline and thus standardization is much needed.
From reliability to dependability
In 1965, responding to a German proposal from 1962, approved by the IEC Committee of Action in 1964, the IEC established TC 56 to address reliability standardization. The initial title of IEC TC 56 was "Reliability of electronic components and equipment". In 1980 the title was amended to "Reliability and Maintainability" to address reliability and a number of associated characteristics applicable to products.
In 1989, the title was finally changed to "Dependability" to better reflect the technological evolution and business needs of a broader scope of applications based on the concept of dependability as an umbrella term for a number of attributes.
In 1990, following consultations with the International Organization for Standardization (ISO), it was agreed that the scope of TC 56 work should no longer be limited to the electrotechnical field, but should address generic dependability issues across all disciplines.
The scope of TC 56, according to its Strategic Business Plan, covers the generic aspects of dependability programme management, testing and analytical techniques, software and system dependability, life cycle costing and technical risk assessment. This includes International Standards related to:
- product issues ranging from component reliability to testing
- guidance for engineering dependability of systems
- process issues from technical risk assessment
- integrated logistics support
- management issues ranging from dependability management to managing for obsolescence.
Attributes of dependability
The main dependability characteristics of a system consist of:
- availability for readiness to operate
- reliability for continuity of operation
- maintainability for ease of preventative and corrective maintenance actions, and
- supportability for provision of maintenance support and logistics needed to perform maintenance.
Dependability is a general term that provides a framework for these attributes as well as others such as recoverability, durability, operability and serviceability.
As clarification, it should be mentioned that safety is not considered to be a direct attribute of dependability although it is very closely related.
Reliability can be defined as the "ability to perform as required, without failure, for a given time interval, under given conditions". Reliability is first of all an inherent result of the design of a system. The bases for high reliability in a system are components and equipment designed to resist applied forces and environmental factors such as temperature, pressure, and physical and chemical properties. Applied stresses may be static or dynamic. Reliability must then be further assured through sound manufacturing and installation techniques. Safety is enhanced in a major way when dependability is properly considered in a system, particularly in the case of reliability.
Finally, reliability is sustained by proper operation within prescribed conditions of use and appropriate maintenance.
Maintainability is the "ability to be retained in, or restored to a state to perform as required, under given conditions of use and maintenance". Maintainability is dependent on the design and architecture of a system and its technology and is guided by maintenance strategies, including equipment installation. It includes how easy it is to perform maintenance operations. It can be seen as the ease of performing maintenance.
Supportability is the "ability to be supported, to sustain the required availability with a defined operational profile and logistic and maintenance resources". It consists of two components: maintenance support and the associated logistics required to deliver that maintenance support. The starting point for supportability is how easy it is to maintain a system. Another element relates to the specific resources and logistics necessary for the use of the system. It is possible to plan completely, organize and deliver the necessary supportability prior to operation, and this is the case with many systems such as a commercial airliner or a railway.
Availability is defined as the "ability to be in a state to perform as required" and is the operational result of a combination of reliability, maintainability and supportability criteria. It is directly related to operational factors such as production capability and assurance in the energy industry, uptime in a manufacturing environment or availability of service for transportation applications.
For dependability to be successful, it must be managed using systems and approaches that are similar to those commonly applied to all management activities. In general, the value of dependability for systems can be expressed in these ways:
- Safety is addressed systematically;
- Customer or user satisfaction is achieved;
- Life cycle cost is minimized;
- Maximum system life can be attained;
- Environmental impact is minimized; and
- Reputation is maintained or enhanced.
The application of dependability tools and techniques to systems is called dependability engineering. Practically every engineering discipline focuses on these aspects as a key component of business success.
Dependability engineering encompasses a wide variety of statistical methods, analytical techniques, physics of failure, hardware, software and human reliability, probabilistic or quantitative risk assessment and reliability prediction.
The primary focus of dependability engineering has always been on design and development to estimate and improve system reliability and evaluate the areas where failure can occur, along with reliability testing and monitoring during the design and early phases of product introduction to enable reliability growth. During both the design and use phases of the life cycle, techniques such as RCM (reliability-centered maintenance), condition monitoring and maintenance optimization have become more important.
Extensive toolbox help meets wide market demand
To properly address the very wide range of dependability issues, IEC TC 56 has issued, as of March 2015, 57 publications, which include International Standards on dependability as well as International Standards for specific techniques and tools for reliability, maintainability and supportability.
TC 56 dependability Standards provide systematic methods and tools for dependability assessment and the management of equipment, services and systems throughout their life cycles. They meet a market demand that covers a broad range of industry sectors, such as computers and electronics, communication networks, process controls, transport and distribution, safety and security, and many more.
Dependability central to electrotechnology and other domains
The fundamental nature and scope of TC 56 work in electrotechnology is clearly illustrated by its liaison activities with the following IEC TCs and SCs:
- TC 1: Terminology
- TC 9: Electrical equipment and systems for railways
- TC 13: Equipment for electrical energy measurement and load control
- TC 44: Safety of machinery – Electrotechnical aspects
- SC 45A: Instrumentation and control of nuclear facilities
- TC 47: Semiconductor devices
- TC 65: Industrial-process measurement and control
- SC 65A: Industrial-process measurement and control – System aspects
- TC 104: Environmental conditions, classification and methods of test
- TC 107: Process management for avionics
- TC 111: Environmental standardization for electrical and electronic products and systems
- IEC/ISO Joint Technical Committee (JTC) 1/SC 7: Information technology – Software and system engineering
However, as it plays a major role in asset management, dependability extends beyond electrotechnology and, as a result, TC 56 also maintains liaison with nine ISO TCs and SCs.
*Thomas Van Hardeveld is Convenor of TC 56/Work Group 3: Management and Systems