IEC 60300-3-4:2022
(Main)Dependability management - Part 3-4: Application guide - Specification of dependability requirements
Dependability management - Part 3-4: Application guide - Specification of dependability requirements
IEC 60300-3-4:2022 gives guidance on specifying dependability requirements and collating these requirements in a specification, together with a list of the means of assuring the achievement of the dependability requirements.
The guidance provided includes:
• specifying quantitative and qualitative reliability, maintainability, supportability and availability requirements;
• advising acquirers on how to ensure that the requirements can be fulfilled by suppliers;
• advising suppliers to help them meet the acquirer's requirements.
Other obligations, such as legislation and governmental regulations, can also place requirements on items, in addition to any requirements derived in accordance with this document.
Gestion de la sûreté de fonctionnement - Partie 3-4: Guide d'application - Spécification d'exigences de sûreté de fonctionnement
L'IEC 60300-3-4:2022 fournit des recommandations relatives à la spécification des exigences de sûreté de fonctionnement et au rassemblement de ces exigences dans une spécification, ainsi qu'une liste des moyens permettant d’assurer la réalisation des exigences de sûreté de fonctionnement.
Les recommandations fournies comprennent:
• la spécification d’exigences quantitatives et qualitatives de fiabilité, de maintenabilité, de supportabilité et de disponibilité;
• des conseils aux acquéreurs portant sur la façon d’assurer que les exigences peuvent être satisfaites par les fournisseurs;
• des conseils aux fournisseurs afin de les aider à satisfaire aux exigences des acquéreurs.
D’autres obligations telles que des règlements législatifs et gouvernementaux peuvent aussi imposer des exigences relatives aux entités, en plus de toutes les exigences déduites conformément au présent document.
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IEC 60300-3-4 ®
Edition 3.0 2022-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Dependability management –
Part 3-4: Application guide – Specification of dependability requirements
Gestion de la sûreté de fonctionnement –
Partie 3-4: Guide d'application – Spécification d'exigences de sûreté de
fonctionnement
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IEC 60300-3-4 ®
Edition 3.0 2022-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Dependability management –
Part 3-4: Application guide – Specification of dependability requirements
Gestion de la sûreté de fonctionnement –
Partie 3-4: Guide d'application – Spécification d'exigences de sûreté de
fonctionnement
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 03.100.40; 03.120.01 ISBN 978-2-8322-1059-0
– 2 – IEC 60300-3-4:2022 © IEC 2022
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 8
4 Specifying dependability . 10
4.1 Description of dependability specification . 10
4.2 Principles . 13
4.3 Benefits . 15
5 Derivation of dependability requirements . 15
5.1 General . 15
5.2 Define stakeholder needs and expectations . 17
5.3 Develop supporting documentation . 18
5.4 Derive dependability requirements . 19
5.5 Justify the measures used for the dependability requirements . 33
5.6 Complete dependability specification . 34
5.7 Review dependability specification . 34
Annex A (informative) Discussion on useful life . 35
A.1 General . 35
A.2 Factors that determine useful life . 35
A.3 Specification of useful life of non-repairable items (components) . 36
Annex B (informative) Process for prioritizing dependability attributes . 38
Annex C (informative) Development of a dependability specification for a home
security system . 40
C.1 Define stakeholder needs and expectations . 40
C.2 Develop supporting documentation . 40
C.3 Derive the dependability requirements . 45
C.4 Complete dependability specification . 46
Annex D (informative) Influencing factors for dependability specification . 48
D.1 Examples of constraints on system dependability . 48
D.2 Type of system operation . 48
D.3 Criticality of operation . 49
D.4 Determining relevant influencing factors for the evaluation of system
functions . 52
Bibliography . 54
Figure 1 – High level process for derivation of dependability requirements in the
specification. 16
Figure 2 – What are we trying to achieve? . 18
Figure 3 – What do we need to manage? . 22
Figure 4 – What constraints are there? . 22
Figure 5 – Assurance considerations . 23
Figure 6 – Reliability requirements . 27
Figure 7 – Maintainability requirements . 28
Figure 8 – Supportability requirements . 31
Figure 9 – Availability requirements . 33
Figure B.1 – Process for prioritizing attributes . 39
Figure C.1 – System configuration for normal mode of operation . 44
Figure C.2 – System configuration for panic mode of operation . 44
Figure C.3 – System configuration for security service mode of operation . 45
Table B.1 – Questions for prioritizing dependability attributes . 38
Table D.1 – Examples of influencing factors under each influencing condition . 52
Table D.2 – Relationship of system properties with influencing conditions . 53
– 4 – IEC 60300-3-4:2022 © IEC 2022
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
DEPENDABILITY MANAGEMENT –
Part 3-4: Application guide –
Specification of dependability requirements
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their
preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
may participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
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6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
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Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
rights. IEC shall not be held responsible for identifying any or all such patent rights.
IEC 60300-3-4 has been prepared by IEC technical committee 56: Dependability. It is an
International Standard.
This third edition cancels and replaces the second edition published in 2007. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) consistency with the other of the six core IEC dependability standards;
b) a process for defining requirements has been included;
c) the definitions and language used have been made consistent with other system related
standards.
The text of this International Standard is based on the following documents:
Draft Report on voting
56/1932/FDIS 56/1939/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/standardsdev/publications.
A list of all parts in the IEC 60300 series, published under the general title Dependability
management, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates that it
contains colours which are considered to be useful for the correct understanding of its
contents. Users should therefore print this document using a colour printer.
– 6 – IEC 60300-3-4:2022 © IEC 2022
INTRODUCTION
Dependability is the ability to perform as and when required. A dependable item is one where
there is justified confidence that it operates as desired and satisfies agreed stakeholder
expectations.
Dependability has many attributes, but is usually characterized in terms of reliability,
maintainability, and supportability, and the derived characteristic of availability. Dependability
also includes the performance characteristics such as durability, testability and restorability as
well as security and integrity, particularly in relation to software-based systems.
Dependability is an important attribute that affects the value items generate. Consequently,
relevant dependability attributes should be defined and specified in addition to functional
performance requirements and physical attributes. Whilst mainly addressing system and
equipment level dependability, many of the techniques described in the various dependability
related IEC standards may also be applied to products or at the component level. The term
"item" is used throughout this document to mean an individual part, component, device,
functional unit, off-the-shelf (OTS) equipment, subsystem, or system. The item may consist of
hardware, software, people or any combination thereof (see IEC 60050-192). In order to refer
to a specific kind of "item", terms like component, OTS, product or large open system are used.
Dependability attributes may be specified for an individual system or product (for example, a
vehicle) and/or a group of similar systems or products (for example, a fleet of similar vehicles).
Dependability attributes may be specified using either quantitative and/or qualitative measures.
In order to assess the values of some of the dependability attributes achieved, statistical
methods may be necessary.
The levels of reliability, maintainability, supportability and availability achieved by an item
depend on the conditions under which it is realized, utilized, maintained and supported and also
on the life profile of the system. The requirements in the dependability specification, should
also define the following:
• conditions under which the item is stored, transported, realized and utilized;
• life profile and expected useful life;
• maintenance policies;
• available support.
Dependability attributes may be specified, along with other performance characteristics, in
various ways depending on the situation. In a basic project context where an acquirer obtains
an item from a supplier, three main types are:
1) specifications written by the supplier;
2) specifications written by the acquirer;
3) specifications mutually agreed or written by the supplier and the acquirer.
The guidance in this document is applicable to all three types of specifications and may be
adapted to other situations as needed.
This document provides guidance for writing dependability requirements in specifications,
together with a means of assuring the achievement of those requirements.
This document is one of the six "top level" interrelated dependability standards that provide
managers and technical personnel with guidance on how to effectively plan and implement
dependability activities. As such, this document should be used in conjunction with:
• IEC 60300-1 [1] , which highlights the importance and benefits of managing dependability.
It gives guidance on dependability activities and how to integrate them into an existing
management system and life cycle processes;
• IEC 60300-3-1 [2], IEC 60300-3-10 [3], IEC 60300-3-14 [4] which provide guidance on how
to identify and apply appropriate analysis and assurance techniques for reliability,
maintainability (and maintenance) and supportability (and support) respectively. A standard
to cover availability is planned.
____________
Numbers in square brackets refer to the Bibliography.
– 8 – IEC 60300-3-4:2022 © IEC 2022
DEPENDABILITY MANAGEMENT –
Part 3-4: Application guide –
Specification of dependability requirements
1 Scope
This part of IEC 60300 gives guidance on specifying dependability requirements and collating
these requirements in a specification, together with a list of the means of assuring the
achievement of the dependability requirements.
The guidance provided includes:
• specifying quantitative and qualitative reliability, maintainability, supportability and
availability requirements;
• advising acquirers on how to ensure that the requirements can be fulfilled by suppliers;
• advising suppliers to help them meet the acquirer's requirements.
Other obligations, such as legislation and governmental regulations, can also place
requirements on items, in addition to any requirements derived in accordance with this
document.
Whilst mainly addressing system and equipment level dependability, many of the techniques
described in the various dependability related IEC standards can also be applied to products or
at the component level. The term "item" is used throughout this document.
This guidance is given in a basic project context where an acquirer obtains an item from a
supplier. It can be modified and adapted to other situations as needed.
NOTE 1 This document does not directly consider safety and environment specifications although much of the
guidance in this document could also be applied to them.
NOTE 2 This document does not cover items with special multi-stakeholder long-term arrangements (e.g. services
provided through Public-Private Partnership procurements) and how dependability is specified in such arrangements.
NOTE 3 The guidance in this document can be applied to some aspects of the specification of requirements relating
to software but specific guidance can be found in IEC 62628 [5] and the different parts of the IEC 61508 series [6].
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60050-192, International Electrotechnical Vocabulary (IEV) – Part 192: Dependability
(available at http://www.electropedia.org)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60050-192 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
NOTE Definitions of "dependability", "availability", "reliability", "maintainability", "supportability”, "failure", "fault",
“time to failure", "operating time between failures", "verification" and "validation" are given in IEC 60050-192.
3.1
goal
statement which translates or expresses desires or aspirations and for which evidence of
fulfilment either need not or cannot be provided
3.2
item
subject being considered
Note 1 to entry: The item may be an individual part, component, device, functional unit, equipment, subsystem, or
system.
Note 2 to entry: The item may consist of hardware, software, people or any combination thereof.
Note 3 to entry: The item is often comprised of elements that may each be individually considered.
[SOURCE: IEC 60050-192:2015, 192-01-01, modified – Note 3 modified by omission of internal
references and Notes 4 and 5 deleted.]
3.3
off-the-shelf
OTS
non-developmental item of supply that is both commercial and sold in substantial quantities in
the commercial marketplace
Note 1 to entry: Sometimes referred to as COTS (commercial off-the-shelf) or MOTS (modified off-the-shelf).
3.4
requirement
statement which translates or expresses a need and its associated constraints and conditions
Note 1 to entry: Requirements exist at different levels in the system structure.
Note 2 to entry: A requirement is an expression of one or more particular needs in a very specific, precise and
unambiguous manner.
Note 3 to entry: A requirement always relates to a system, product or service, or other item of interest.
Note 4 to entry: A requirement is a statement where evidence or assurance of compliance can be provided.
[SOURCE: ISO/IEC/IEEE 29148:2018, 3.1.19 [7], modified – Note 4 added.]
3.5
specification
information item that identifies the dependability requirements and goals of
a system, product or service together with any supporting information
Note 1 to entry: Supporting information can include details of use, operating and environmental conditions, failure
criteria and the methods intended to be applied for assurance of compliance with the requirements, including
accept/reject criteria.
Note 2 to entry: ISO/IEC/IEEE 15289 [8] defines specification as an information item that identifies in a complete,
precise and verifiable manner the requirements, design, behaviour or other expected characteristics of the system,
service or process. The specification of dependability has a greater scope than that used in ISO/IEC/IEEE 15289.
– 10 – IEC 60300-3-4:2022 © IEC 2022
3.6
useful life
time interval from first use until user requirements are no longer met due to
performance, obsolescence and/or economic factors
Note 1 to entry: The applicable time interval is dependent on the nature and application of the item and can be
elapsed time, operating hours, number of cycles, etc.
Note 2 to entry: Performance factors to be considered for useful life include a decrease in item performance below
acceptable limits which cannot be rectified, a change in performance or service requirements, increase in failure
probability or decrease in availability.
Note 3 to entry: Economic factors to be considered for useful life include excessive maintenance costs, increases
in operating costs and emergence of more cost-effective solutions.
Note 4 to entry: In some cases, the useful life of an item is more than that required by an organization, in which
case it has residual value and could be repurposed.
Note 5 to entry: In this context, "first use" excludes testing activities prior to hand-over of the item to the end-user.
[SOURCE: IEC 60500-192:2015, 192-02-27, modified – "economics of operation and
maintenance, or obsolescence" replaced by "performance, obsolescence and/or economic
factors".]
4 Specifying dependability
4.1 Description of dependability specification
4.1.1 What is dependability?
Dependability is the ability to perform as and when required. A dependable item is one where
there is justified confidence that it operates as desired and satisfy agreed stakeholder
expectations.
Dependability has a strong impact on the user's perception of the value of an item developed
or provided by an organization and poor dependability affects an organization's profitability and
reputation.
Dependability is specified and verified using a set of measurable or demonstrable attributes,
which are quantified, where practicable. Dependability is usually characterized in terms of
reliability, maintainability, and supportability, and the derived characteristic of availability.
• Reliability relates to the ability to provide a required function for a given interval (time,
operating cycles, distance, etc.).
• Maintainability relates to the ease and speed with which an item can be retained in, or
restored to, a state to perform as required.
• Supportability is the ability to be supported to sustain the required operational capability
with a defined use profile and given logistic and maintenance resources.
• Availability is the ability to be in a state to perform when called upon to do so. It is often
quantified as ratio of uptime to total time. As reliability, maintainability and supportability are
major contributors of uptime and downtime, availability is impacted by the trade-offs
between these attributes.
Dependability is also related to other attributes of life cycle processes such as design,
manufacturing, installation, and ongoing operation and support activity. In addition,
dependability also affects other attributes such as safety and environmental protection, where
the inability to perform a function has safety or environmental protection consequences. The
dependability specification therefore should be part of the item specification with the interaction
between dependability requirements and functional requirements recognized and considered.
4.1.2 What is a requirement?
A dependability requirement is a statement of a single aspect of a dependability attribute which
expresses one or more stakeholder needs or expectations and for which evidence or assurance
of compliance can be provided. Multiple requirements may be necessary to fully define how the
stakeholder's needs and expectations are to be met for each dependability attribute. This
involves decomposing the needs and expectations in terms of individual aspects of
dependability attributes and setting the balance between the aspects.
Requirements are part of the specification that the acquirer requires the supplier to meet and
provide evidence or assurance for meeting the same. This evidence may be supplied before
the item is utilized as part of the deliverables or once the item is utilized through the application
of incentives (or penalties) for meeting (or not meeting) the requirements.
Dependability performance measures are the different ways in which each of the dependability
attributes can be expressed within a requirement. See 5.4.7 to 5.4.10 for further details.
4.1.3 What is assurance?
Assurance is grounds for justified confidence that a claim has been or will be achieved. It is
typically provided as a body of evidence, which aims to decrease the uncertainty around the
achievement of the dependability requirements. This evidence is the output of activities
developed to meet and demonstrate the requirements and manage the risks, such as analysis
and testing.
4.1.4 What is dependability specification?
The requirements for an item are collated into a specification. As the intended context in which
the item is used, such as expected demand and operating environment, also affect the
dependability performance, the context should also be provided in the specification as
supporting information (see 5.3 for further guidance). Keeping specifications accurate across
the item's life cycle is also important and the specification shall be updated in response to
changes in requirement or context.
4.1.5 Which attributes to specify?
The way that dependability requirements are specified depends on the type and nature of the
item, for example whether the item is repairable or non-repairable, and whether it is a single
use device. For example, only reliability requirements need to be specified for non-repairable
items that do not require maintenance. Examples of non-repairable items include sealed items
and items where the cost of repair outweighs the cost of replacement, such as many consumer
goods, and items at remote locations, such as deep-sea systems and satellites, where
deployable maintenance resources are not available when needed. Single use devices include
explosives, passenger airbags and emergency flares. For non-repairable items, replacement of
sub-items are sometimes a concern and maintainability requirements may be specified, if
applicable.
Many complex items, however, are repairable and maintainability is important for the acquirer
or user, for example, to increase the item's useful life. Maintainability requirements should be
specified if the maintenance costs contribute significantly to life cycle cost, if downtime could
lead to a significant reduction in value or if maintenance is important for the acquirer or user.
Preventive and corrective maintenance requirements may be specified, if applicable.
Supportability is the combined result of required support and the logistics required to provide
that level of support. The level of supportability is very often influenced by the conditions of use
and factors that change through the life cycle. Therefore, the supportability and/or support
requirements can be important in a specification.
– 12 – IEC 60300-3-4:2022 © IEC 2022
Availability requirements are generally specified for items where downtime could cause
economic or other loss through increased operating costs, personal injury or loss of service, for
example, communication networks, production plants, medical equipment, safety equipment
and military systems. Availability can be determined from the item configuration, its subsystems
and their reliability, maintainability and supportability in the context of the operational and
support arrangements. Availability may also be expressed as service levels to be provided to
an operator or end-users.
4.1.6 Contracting for dependability
The purpose of any specification is to provide a basis for development or purchase of an item.
It usually forms part of the contract between the acquirer and supplier and therefore it is
essential that the specification is written in such a way that it can be used for contracting.
Contracting for dependability can take many forms, from milestone payments dependent upon
the successful completion of a status review (see IEC 62960 [9]) or a demonstration test, to the
use of penalty clauses and incentives for in-service achieved dependability, such as
performance-based contracts.
The benefits and drawbacks of the different contract types are as follows.
• Penalties for poor performance encourage the supplier to give dependability its full attention
and can lead to higher levels of dependability than would otherwise be achieved.
• Incentives for passing demonstration testing focus the supplier on demonstrating the
dependability. However, demonstration testing can be costly and time-consuming and might
only reveal, just before delivery, that the item does not meet its dependability requirements.
Another important consideration when including dependability demonstrations into contracts
are the relevant supplier and acquirer risks associated with the design of these
demonstrations. For example, the design of the test determines the likelihood of incorrectly
accepting a bad item (acquirer risk) as well as the likelihood of incorrectly rejecting a good
item (supplier risk). The confidence levels to which a dependability measure shall be
demonstrated, and the amount of testing required (or available) should be balanced in order
to fairly manage these risks.
• Requiring the supplier to provide maintenance encourages the supplier to mitigate the risk
that the item achieves poor availability as the supplier may then experience consequential
losses as a result of item unreliability. However, this requires a much longer contract, for
example a fixed cost maintenance agreement, with its associated difficulties.
The way in which the specification is written depends on the circumstances. There are three
main types:
• specifications written by the supplier;
These specifications are mainly used for mass produced items that need to have certain
dependability characteristics, in order to be accepted by the marketplace. They are therefore
used between the management team of the supplier and the supplier's product development
team. The specifications are mainly used for standard items like industrial products,
consumer products or components.
• specifications written by the acquirer;
These specifications are mainly used when an acquirer seeks an item for a specific purpose
like components or OTS. They are also used when an acquirer issues an invitation for
tenders.
• specifications mutually agreed or written by the supplier and the acquirer;
These specifications are mainly used for acquirer specified items which are usually unique
items or items produced in small numbers. Often the process starts with an invitation for
tenders from the acquirer followed by an offer (tender) from a supplier. A contract is
negotiated including the final specification written by the supplier and the acquirer.
The content of the specification varies according to its type. Subclause 5.3 lists the different
types of supporting documentation which needs to be tailored to the exact nature of the
specification.
4.2 Principles
4.2.1 General
The development of dependability requirements and the creation of the dependability
specification are founded on a set of principles. These principles should influence an
organization's intent as well as its approach to design for, and delivery of, dependability of an
item. Maximum benefit is obtained from the dependability specification if all of these principles
are applied.
4.2.2 Systems approach
In general, a system for which dependability is to be specified can include equipment (both
hardware and software), the people who manage, operate and maintain it, data, processes and
procedures, facilities, materials and naturally occurring entities.
Requirements can be defined for:
• the system as a whole;
• any component or subsystem;
• a separate entity (e.g. a product or item);
• a collection of entities (e.g. a fleet of products or items);
• a collection of functions (e.g. a service or set of services).
In general, the dependability of an item is affected by its conditions of use, the people who use
it and by the environment in which it operates. In consequence, dependability requirements
should be specified considering not just the item itself but also the broader system in which it
is used as well as interfaces with other systems. This includes the people who use it and how
it is utilized. Conditions of use should be monitored, and changes managed as part of achieving
dependability through the item's life cycle.
4.2.3 Requirements and goals
It is important to clearly distinguish between requirements and stakeholder goals described in
a specification, as the method of assurance is different.
A requirement is an essential element of the acquirer's needs and expectations and it shall be
possible to provide evidence or assurance of compliance with the requirement. A goal is not a
requirement but is the acquirer's desires or aims and evidence of the degree to which the goal
has been fulfilled either need not or cannot be provided.
Both requirements and goals should be defined. For example, for high availability or reliability
systems, it might not be practicable or cost effective to provide justified evidence that the high
level of availability or reliability has been achieved. The acquirer needs to clearly identify both
the high availability and reliability goals, for which evidence cannot be provided, but also define
demonstrable requirements for which evidence can be provided.
– 14 – IEC 60300-3-4:2022 © IEC 2022
4.2.4 Assuring achievement of requirements
There are two elements to any specification; the dependability requirements and the means by
which the supplier shall demonstrate or assure the achievement of the requirements to the
acquirer. This means that the supplier shall provide sufficient evidence to the acquirer that the
item meets its requirements to give the acquirer the confidence needed to pay the agreed price.
IEC 60300-3-1 [2], IEC 60300-3-10 [3], IEC 60300-3-14 [4], IEC 60706-3 [10] and
IEC 62308 [11] discuss dependability evaluation for reliability, maintainability (and
maintenance) and supportability (and support). Availability measurement and assurance is
discussed in IEC 61070 [12].
The choice of which assurance activities to specify in a contract is dependent upon the level of
project risk that the acquirer is willing to accept. The acquirer might be willing to take the risk
that the item might fail, so long as it is maintained by the supplier. In such a case, assuring the
overall availability of the item by the use of penalties for poor performance and incentives for
exceeding the requirements might be preferable to assuring the reliability of the item on its own.
If however, the acquirer is not willing to risk unavailability of the item, then formal reliability or
availability demonstration testing may be necessary. Rare events can seldom be proven to be
absent by testing so assurance may need to be via a well-reasoned argument. A dependability
case (see IEC 62741 [13]) provides one method of presenting such an argument.
Generally, evidence required to provide assurance that the higher reliability items have met
their requirements is more costly, as greater testing is required to obtain the confidence
required. This is one factor in the higher cost of higher reliability items but, without these
activities, the supplier provides the acquirer with limited confidence that the item meets its
requirements.
4.2.5 Cost implications
All items exhibit some level of reliability, whatever the frequency of failure or the required level
of maintenance. At all times, a balance should be sought between the cost, risk and
performance of items. This includes any potential trade-offs between the cost of designing for,
and assuring, high dependability and the cost and other risks of resulting from lower
dependability over the entire life of the item. The nature of the item's function, associated risks
(performance, cost safety environmental and reputational) and other contextual factors informs
these trade-offs. An investment in dependability design and implementation can therefore repay
itself in terms of the combined development, realization and operating costs for the item.
IEC 60300-3-3 [14] describes life cycle costing and the relationships between dependability and
cost. In order to attain the minimum life cycle cost point for the item, the organization producing
it should actively manage dependability; otherwise, the item's achieved level of reliability is
usually much lower than the level required at the minimum cost point, causing project delays,
cost overruns and low levels of availability.
4.2.6 Dependabilit
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