guide to reliability, availability and maintainability
TRANSCRIPT
© State of NSW through Transport for NSW 2021
Guide to Reliability, Availability and Maintainability
T MU AM 06002 GU
Guide
Version 2.0
Issue date: 21 May 2021
T MU AM 06002 GU Guide to Reliability, Availability and Maintainability
Version 2.0 Issue date: 21 May 2021
© State of NSW through Transport for NSW 2021
Important message This document is one of a set of standards developed solely and specifically for use on
Transport Assets (as defined in the Asset Standards Authority Charter). It is not suitable for any
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Standard governance
Owner: Senior Manager, Systems Engineering, Asset Management Branch
Authoriser: Director, Asset Management Partnering and Services, Asset Management Branch
Approver: Executive Director, Asset Management Branch, on behalf of the Asset Management Branch Configuration Control Board
Document history
Version Summary of changes
1.0 First issue 27 July 2015.
2.0 Second issue changes include updates and amendments to reflect current TfNSW governance.
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Preface The Asset Management Branch, formerly known as the Asset Standards Authority (ASA), is a
key strategic branch of Transport for NSW (TfNSW). As the network design and standards
authority for NSW Transport Assets, as specified in the ASA Charter, the ASA identifies,
selects, develops, publishes, maintains and controls a suite of requirements documents on
behalf of TfNSW, the asset owner.
The ASA deploys TfNSW requirements for asset and safety assurance by creating and
managing TfNSW's governance models, documents and processes. To achieve this, the ASA
focuses on four primary tasks:
• publishing and managing TfNSW's process and requirements documents including TfNSW
plans, standards, manuals and guides
• deploying TfNSW's Authorised Engineering Organisation (AEO) framework
• continuously improving TfNSW’s Asset Management Framework
• collaborating with the Transport cluster and industry through open engagement
The AEO framework authorises engineering organisations to supply and provide asset related
products and services to TfNSW. It works to assure the safety, quality and fitness for purpose of
those products and services over the asset's whole-of-life. AEOs are expected to demonstrate
how they have applied the requirements of ASA documents, including TfNSW plans, standards
and guides, when delivering assets and related services for TfNSW.
Compliance with ASA requirements by itself is not sufficient to ensure satisfactory outcomes for
NSW Transport Assets. The ASA expects that professional judgement be used by competent
personnel when using ASA requirements to produce those outcomes.
About this document
This guide aims to provide supplier organisations with guidance in managing engineering
activities involving systems that are required to be reliable, available and maintainable. The
changes in this version include updates and amendments to reflect current TfNSW governance.
This is a second issue.
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Table of contents 1. Introduction .............................................................................................................................................. 6
2. Purpose .................................................................................................................................................... 6 2.1. Scope ..................................................................................................................................................... 6 2.2. Application ............................................................................................................................................. 7
3. Reference documents ............................................................................................................................. 7
4. Terms and definitions ............................................................................................................................. 9
5. Reliability, availability and maintainability management .................................................................. 10 5.1. Plan reliability, availability and maintainability management activities ................................................ 11 5.2. Definition of system boundaries and assumptions .............................................................................. 13 5.3. Identification of reliability, availability and maintainability requirements .............................................. 14 5.4. Allocation of reliability, availability and maintainability requirements .................................................. 14 5.5. Development of reliability, availability and maintainability acceptance criteria ................................... 15 5.6. Reliability, availability and maintainability analysis and modelling ...................................................... 15 5.7. Validation of reliability, availability and maintainability requirements .................................................. 16 5.8. Reliability, availability and maintainability deliverables ....................................................................... 17
6. Reliability, availability and maintainability tools and techniques .................................................... 17 6.1. Reliability block diagram analysis ........................................................................................................ 18 6.2. Failure mode, effects and criticality analysis ....................................................................................... 18 6.3. Fault tree analysis ................................................................................................................................ 19 6.4. Human reliability analysis .................................................................................................................... 19 6.5. Maintenance requirements analysis .................................................................................................... 22 6.6. Failure recording analysis and corrective action system ..................................................................... 23
Appendix A Additional reference documents ...................................................................................... 26
Appendix B Examples of reliability block diagrams ........................................................................... 27
Appendix C Example of FMECA table - Bogie assembly .................................................................... 29
Appendix D Examples of fault tree analysis ........................................................................................ 30
Appendix E Examples of human error analysis .................................................................................. 31
Appendix F Example of MRA - station escalator .................................................................................... 33
Appendix G Example of FRACAS incident report – CPU motherboard ............................................ 35
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1. Introduction An Authorised Engineering Organisation (AEO) engaged by Transport for NSW (TfNSW) to
undertake engineering activities is required to have reliability, availability and maintainability
(RAM) management arrangements in place that are relevant to the engineering services or
products that the AEO provides to TfNSW. These arrangements should enable the planning,
execution, and reporting of all RAM management activities for a system.
This document provides guidance on complying with the requirements of T MU MD 00009 ST
AEO Authorisation Requirements and T MU AM 06006 ST Systems Engineering Standard
which mandate RAM requirements.
This guide also elaborates on the RAM guidance described in TS 10504 AEO Guide to
Engineering Management.
AEOs should ensure that RAM management activities and outcomes are at an appropriate level
required for the scale and complexity of engineering services and systems provided, and should
incorporate RAMS requirements in the design and development of systems they are contracted
to deliver.
2. Purpose This document is intended to provide guidance to TfNSW and its supply chain AEOs applying
RAM management during engineering specification and asset life cycle stages and activities
involving systems that are required to operate dependably.
This ensures that TfNSW and its supply chain of AEOs (and non-AEOs operating under the
assurance arrangements of AEOs) are able to demonstrate sufficient control over RAM related
risks. This guidance is of particular relevance to suppliers who provide reliability-critical or
safety-critical engineering specification and design, in addition to systems engineering,
integration and maintenance services.
2.1. Scope This guide provides guidance to TfNSW and AEOs for RAM management related, in particular,
to the system specification, design and maintenance services. It also provides guidance on
RAM management principles, methods, techniques and processes used to analyse and deliver
RAM requirements from stakeholders including operational, maintenance and interfacing
targets. AEOs are assumed to have business-level policies addressing quality, performance
and safety.
For this guide, the term reliability, availability, maintainability and safety (RAMS) is used to
define an integrated management approach that includes safety. However, this guide is limited
to RAM and does not provide guidance on the safety element of RAMS management, as safety
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assurance is addressed in T MU MD 20001 ST System Safety Standard for New or Altered
Assets. Refer to this document for guidance on safety management.
The specific evidence required to demonstrate RAM management processes will depend on the
scope and nature of the work undertaken by the AEO (noting that RAM is not applicable to
certain AEO services, for example land survey). For that reason, this document does not outline
the evidence required to be an AEO, rather it provides an outline of the typical RAM methods
and processes that AEOs may need to demonstrate.
2.2. Application This document applies to Transport cluster agencies and supply chain AEOs, and applies
specifically to the management of system and element level reliability, availability and
maintainability for new or altered NSW transport assets.
The level of application of RAM management principles and methods should be scaled and
tailored according to the degree of system novelty or complexity, the use of unique or
non-standard system or product configurations, and the associated level of safety risk.
The application of RAM (in particular reliability) analysis in support of system design may be
negligible or zero for some projects where type approved products are used in standard,
repeatable, and approved system configurations. This should be reflected in contractual
requirements to avoid unnecessary and excessive effort, resources, time and cost.
The need for, and application of, RAM management has different meanings to different asset
disciplines (for example, civil and structural engineers generally use the term durability, whereas
electrical, electronic and mechanical engineers generally use RAM). The impact of RAM (or
durability) management on planning and acquisition of new or altered systems and the specific
disciplines that support the system design should be understood.
T MU AM 06006 GU Systems Engineering Guide provides guidance on the level of RAM
management activities required for engineering disciplines and identifies a range of public
transport engineering projects and the level of RAMS to be applied.
3. Reference documents The following documents are cited in the text. For dated references, only the cited edition
applies. For undated references, the latest edition of the referenced document applies.
International standards
BS EN 60706-5 Maintainability of equipment - Part 5: Testability and diagnostic testing
EN 60300-3-1 Dependability management – Part 3-1: Application guide – Analysis techniques
for dependability – Guide on methodology
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Australian standards
AS IEC 61025 Fault tree analysis (FTA)
AS IEC 61078 Reliability block diagrams
AS/NZS IEC 60812 Failure mode and effects analysis (FMEA and FMECA)
Transport for NSW standards
T MU AM 01003 F1 Blank FMECA Sheet
T MU AM 01003 ST Development of Technical Maintenance Plans
T MU AM 01010 ST Framework for Developing an Asset Spares Assessment and Strategy
T MU AM 06001 GU AEO Guide to Systems Architectural Design
T MU AM 06006 GU System Engineering Guide
T MU AM 06006 ST Systems Engineering Standard
T MU AM 06007 GU Guide to Requirements Definition and Analysis
T MU AM 06008 ST Operations Concept Definition
T MU AM 06009 ST Maintenance Concept Definition
T MU AM 06016 GU Guide to Verification and Validation
T MU HF 00001 GU Human Factors Integration – General Requirements
T MU MD 00009 ST AEO Authorisation Requirements
T MU MD 20001 ST System Safety Standard for New or Altered Assets
TS 10504 AEO Guide to Engineering Management
Other references
Department of Defense United States of America, 1991, MIL-HDBK-217F Military Handbook
Reliability Prediction of Electronic Equipment
Railtrack EE&CS Report, Infrastructure Risk Modelling Geographical Interlocking,
RT/S&S/IRM_FTA/11 Issue 1 January 1998
UNISIG, 5 August 2014, SUBSET-088-1, ETCS Application Level 1 - Safety Analysis; Part 1 -
Functional Fault Tree, Issue 3.5.4
Note: Appendix A contains a list of other documents not cited in the text that provide
additional information and guidance.
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4. Terms and definitions The following terms and definitions apply in this document:
AEO Authorised Engineering Organisation
assurance a positive declaration intended to give confidence
authorisation the conferring of authority, by means of an official instruction and supported by
assessment and audit
availability measure of the percentage of time that an item or system is available to perform its
designated function (AS 4292.4)
BRS business requirements specification
durability the capability of a structure or any component to satisfy, with planned maintenance
(if applicable), the design performance requirements over a specified period of time under the
influence of the environmental actions, or as a result of a self-ageing process (ISO 13823).
For assets either with no availability to program maintenance or for physically
inaccessible assets or parts of assets, then the durability requirement will typically
need to be increased beyond that required for a maintainable structure, to satisfy the
specified design life.
ETCS European Train Control System
failure the inability of a system or asset to perform its intended function or satisfy some
predetermined conditional attribute
fault tree logic diagram showing the faults of sub items, external events, or combinations
thereof, which cause a predefined, undesired event (IEC 60500)
FTA fault tree analysis; deductive analysis using fault trees (IEC 60500)
FMECA failure mode, effects and criticality analysis
FRACAS failure recording analysis and corrective action system
HEART human error assessment and reduction technique
HRA human reliability analysis
MRA maintenance requirements analysis
maintainability the probability that a given active maintenance action, for an item under given
conditions of use can be carried out within a stated time interval when the maintenance is
performed under stated conditions and using stated procedures and resources (IEC 60050-191)
RAMS reliability, availability, maintainability and safety
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RBD reliability block diagram; logical, graphical representation of a system showing how the
success states of its sub-items (represented by blocks) and combinations thereof, affect system
success state (AS IEC 61078)
reliability the ability of an item of equipment or a system to perform a required function under
stated conditions for a stated period of time or at a given point in time (AS 4292.4)
review a method to provide assurance by a competent person that an engineering output
complies with relevant standards and specific requirements is safe and fit for purpose
SRS system requirements specification
supplier a supplier of engineering services or products
system safety the concurrent application of a systems based approach to safety engineering
and of a risk management strategy covering the identification and analysis of hazards and the
elimination, control or management of those hazards throughout the life cycle of a system or
asset
TfNSW Transport for NSW
THERP technique for human error rate prediction
transport asset means assets used for or in connection with or to facilitate the movement of
persons and freight by road, rail, sea, air or other mode of transport, and includes transport
infrastructure
5. Reliability, availability and maintainability management T MU MD 00009 ST requires that AEOs demonstrate they have RAM management
arrangements in place, relevant to the engineering services or products provided.
The introduction of new or altered assets results in transport network complexity and RAM
implications. Implementation decisions should be made based on trade-offs between the
implementation costs and the subsequent operation and maintenance costs.
Consideration should be given to the total impact on the existing network, existing maintenance
activities such as safety working, and additional access arrangements.
The introduction of new assets that simplify the transport network configuration should generate
RAM improvements. However the introduction of new assets that do not simplify the network
may not generate RAM improvements.
Application of RAM engineering is required to ensure optimum transport network effectiveness,
safety and availability. RAM engineering is a whole-of-system life cycle philosophy and
methodology that is applied during the plan, acquire, operate/maintain, and dispose stages of
the system or asset lifecycle. RAM is most effectively applied during the plan/acquire phase for
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consideration of the future operate/maintain/dispose phases. RAM analysis in the acquisition
phase will produce a maintenance schedule that will be used in the operate/maintain phase. If
there is a change to the operations concept requiring RAM to be revisited then it is conducted in
planning phase again.
RAM engineering and management activities that include planning and producing deliverables
should be carried out by suitably qualified and experienced individuals. RAM management
deliverables should be appropriate and sufficient such as to provide assurance to relevant
stakeholders that the system can satisfy the high level performance targets as required. TfNSW
should provide the performance targets (generally based on system and service modelling,
simulation and analysis), for example, the service reliability performance target of 92% for on-
time running of trains.
The following RAM activities should be undertaken but are not necessarily limited to:
• plan the RAM management activities
• define system boundaries and assumptions, dependencies and constraints for RAM
analysis
• identify the system RAM requirements
• allocate the requirements to elements (sub-systems)
• develop the RAM system acceptance criteria
• undertake RAM analysis and modelling
• validate the system RAM requirements
System failure recording and analysis is undertaken using a range of tools and processes.
These include, but are not limited to the following:
• failure mode, effects and criticality analysis (FMECA)
• reliability block diagrams (RBDs)
• fault tree analysis (FTA)
• failure recording analysis and corrective action system (FRACAS)
5.1. Plan reliability, availability and maintainability management activities T MU AM 06006 ST requires that projects consider RAMS performance and how it relates to
operational performance for novel systems early in the system life cycle, starting with
development of the operations concept definition and maintenance concept definition in
accordance with T MU AM 06008 ST Operations Concept Definition and T MU AM 06009 ST
Maintenance Concept Definition respectively.
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T MU AM 06006 ST also requires that projects consider sustainable operation and maintenance
of the new or altered system over the full system life cycle at the beginning of the project, before
undertaking any asset life cycle stages and activities related work, AEOs should prepare a RAM
(or durability) management plan. Depending on the level of project and system complexity, the
RAM plan may be combined with other asset related plans to demonstrate how the system
RAM requirements will be achieved.
The RAM management plan should focus on managing RAM across the asset life cycle stages
and the activities, rules and principles that are required to be adopted including the following:
• reliability
o use of proven systems and equipment (assurance figures should be obtained)
o use of systems that are applicable to the conditions (systems proven in other countries
may not be suitable to NSW or Australian environmental conditions)
o human factors (human reliability and human error analysis)
o fault tolerance and graceful degradation
o the levels of redundancy designed into the system
o design life
• availability
o maintenance scheduling
o service recovery times and service availability
o network modelling to assess capacity against planned utilisation
• maintainability
o condition monitoring and diagnostics
o condition inspections
o obsolescence management
o human factors associated with the maintenance and repair task
o maintenance resources
o access arrangements for maintenance (for example, possessions, maintenance
staging areas)
o maintenance scheduling
o isolation for maintenance
o preventative maintenance
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o corrective maintenance
o human factors considerations for maintenance
The RAM management plan should also include details on the roles and responsibilities
required within the organisation to achieve the RAM objectives.
Where there are proposed changes to parts of an existing system, the RAM management plan
should consider the resulting impact to the overall system RAM from these changes. The RAM
management plan should, where practical, include an assessment of the existing system RAM
performance, and the changes to the RAM performance resulting from the new or altered
assets.
An example of an impact to the reliability is the addition of a platform display to an existing light
rail system. The light rail operating contract specifies a maximum of three isolations of the line
per year. The platform display system needs to have reliability to work within this limitation.
An example of impact to the availability is the consolidation of maintenance depots from multiple
existing locations to a new central location. The relocation of the maintenance depots results in
additional travelling distances from the central depot to faults and an increase to the
maintenance response time.
An example of impact to the maintainability is the addition of two extra running lines to an
existing double running line system. These two additional running lines alter the maintainability
(access) of the combined services route and sub-stations adjacent to the original two lines.
These assets transition from a safe location to a danger zone location and additional safety
procedures will be required to maintain these assets.
5.2. Definition of system boundaries and assumptions System and element boundaries should be defined clearly and by means of a defined system
functional and physical architecture, before starting any RAM activities.
Assumptions may be made as a result of incomplete information in instances where programs
and systems are large or complex. As system definition progresses, these assumptions should
be clarified as either statements of fact, or eliminated within the system design process.
Clarification of these assumptions should be made with the asset custodian (client
representative).
The system architecture may need to change based on iterative design decisions, based on the
inability to satisfy system RAM requirements.
Refer to T MU AM 06001 GU AEO Guide to Systems Architectural Design for more information.
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5.3. Identification of reliability, availability and maintainability requirements The asset custodian (client representative) should provide, early in the asset lifecycle, the high
level system RAM objectives that are in turn aligned to transport service availability targets.
RAM requirements captured from the stakeholders should be well-defined, demonstrable,
include explicit targets and meaningful to allow efficient RAM activities to be conducted. RAM
requirements should be considered in the context of their implementation cost. If the RAM
targets are very exacting, then the resulting implementation cost may be very high and options
analyses should be conducted to determine willingness to accept such costs.
The RAM requirements capture process should start with service reliability and availability in the
business requirements specification (BRS) and be further refined in the system requirements
specification (SRS) development process. Any requirements which fall outside these criteria
should be challenged and clarified as necessary.
Refer to T MU AM 06007 GU Guide to Requirements Definition and Analysis for more
information on requirements management in general.
5.4. Allocation of reliability, availability and maintainability requirements RAM requirements allocation assures that the high level BRS RAM targets are allocated
appropriately at system and element levels. Models based on RBDs and other modelling
techniques should be employed in the allocation process for novel, highly complex systems.
The allocations should be used as an aid to achieving the RAM objectives. These system and
element level RAM targets should then be converted into RAM requirements.
To ensure realistic allocation, system and element RAM requirements should be compared to
empirical data for identical or similar systems (that is, benchmarking) whenever possible. The
empirical data should be validated for its relevance, considering factors such as the modes of
operation, the operating environment and any fine-tuning or adjustments that have been used. If
allocated values are not achievable, design options analysis across systems and elements
should be performed to reallocate system RAM requirements. The process of allocation,
comparison with empirical data, trade-offs and iteration as required should result in system and
element RAM requirements being defined.
The allocation of a RAM target to each system and element should be specific, measurable and
attainable, taking into account the criticality and risks involved in the design, development and
installation.
Systems and elements that are critical to performance should have RAM targets set higher than
other non-critical systems, based on the system level reliability or redundancy employed. When
allocating RAM targets, the number and complexity of the system interfaces and the extent to
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which the system will be affected by external factors including the operating environment needs
to be considered.
5.5. Development of reliability, availability and maintainability acceptance criteria The acceptance criteria for RAM requirements should be agreed between the stakeholders
including the asset custodian (client representative) and system developer.
These stakeholders may include representatives from the Transport cluster.
This should include, but not be limited to, the RAM validation principles to be applied and the
tests and analysis to be carried out for the validation. Acceptance criteria should be agreed and
documented through the requirements allocation process, starting with the BRS and then the
SRS. Consideration should be given to the cost of implementing the acceptance criteria.
5.6. Reliability, availability and maintainability analysis and modelling T MU AM 06006 ST requires that projects use RAMS modelling to appropriately support option
selection and development and preliminary system design, to ensure that the new or altered
system will meet the stated operational capability and provide value for money over the
designed system lifetime.
During the plan and acquire stages of a project, reliability predictions should be used to assess
whether the allocated RAM requirements are achievable. An iterative process of comparing
RAM predictions with RAM requirement allocations combined with trade-off studies, will
eventually result in an efficient design that achieves whole of life RAM performance targets.
Reliability predictions combine lower level component or unit level reliability data through
reliability modelling, and the operating and environmental conditions, to estimate the integrated
system reliability. The validity of the reliability predictions is highly dependent upon the quality of
reliability data and assumptions made.
Whenever possible, reliability predictions should be based on data from similar components or
equipment already in use in service in similar operational environments. For electronic
equipment, parts count prediction methods based on MIL-HDBK-217F Military Handbook
Reliability Prediction of Electronic Equipment can be used to obtain reliability predictions. Where
this is not possible, reliability data may be extrapolated from tests or trials conducted by the
supplier or manufacturer. In all cases the sources of the data should be cited to maintain an
audit trail. Suppliers of original equipment and systems should provide evidence that they
satisfy all RAM requirements and that they are suitable for the intended application.
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Reliability prediction should use reliability modelling where practicable for novel, high complexity
systems, such as a RBD, fault tree or a computerised simulation model, to describe the
reliability behaviour of the system and reliability data of the constituent elements.
RAM predictions are performed predominantly for the following purposes:
• reliability
o to evaluate expected reliability performance against target risk of failure
o to identify unexpected weaknesses in a design, including single point failures
o to provide a basis for a testing program
o to predict maintenance effort and cost
• availability
o to evaluate outage times and service disruptions against economic, community and
quality criteria
o to identify critical subsystems and components
o to determine the need for redundant or stand-by equipment configurations
• maintainability
o to determine the most effective maintenance strategy
o to optimise maintenance facilities, diagnostic and training tools, spares holdings and
manning levels
o to assess need for condition monitoring
RBDs and FTA are systematic top-down reliability modelling and analysis techniques, and are
usually best applied when introducing novel, highly complex new or altered systems during
system architecture design during the acquire stage.
In addition to RAM modelling, complimentary analysis techniques should be used during design
to concentrate on areas which are critical to the system reliability, such as failure mode, effects,
and criticality analysis (FMECA).
5.7. Validation of reliability, availability and maintainability requirements Validation should include details of the validation tasks and relevant results against the RAM
acceptance criteria. Any limitations and constraints applying to the system should also be noted.
There are numerous sources of international good practice in reliability and maintainability
validation. These include MIL-HDBK 781, EN 60300-3-1 Dependability management – Part 3-1:
Application guide – Analysis techniques for dependability – Guide on methodology and
EN 60706-5 Maintainability of equipment - Part 5: Testability and diagnostic testing.
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A RAM report including results from the analysis and verification and validation activities should
be prepared and then issued to stakeholders. Refer to T MU AM 06016 GU Guide to
Verification and Validation for more information. The RAM report should clearly display all
verification and validation failures against RAM acceptance criteria. Corrective action should
then be undertaken to rectify these failures. Validation and verification activities should be
repeated following corrective actions, and the RAM report re-issued.
5.8. Reliability, availability and maintainability deliverables The following deliverables should be produced during the RAM process:
• RAM management plan including the asset life cycle stages
• BRS RAM requirements with their acceptance criteria
• SRS RAM requirements with their acceptance criteria
• element level RAM requirements with their acceptance criteria
• RAM analysis and modelling with their data
• RAM report including results from the analysis, modelling, verification and validation
activities
6. Reliability, availability and maintainability tools and techniques Careful consideration should be given to the selection of the appropriate RAM tools and design
techniques used to provide RAM results. This consideration involves a critical decision as to
whether a simple calculation or a comparison with an existing system is sufficient or whether
RAM tools and design techniques are required.
These tools and design techniques may provide different RAM results as the system definition
progresses. These progressive RAM results should be recorded in the RAM report during the
asset life cycle stages.
Different asset types may have different approaches and tools for RAM modelling and analysis.
Communications, signalling and electrical designers may use RBD analysis, failure mode,
effects, and criticality analysis (FMECA) or FTA tools, whereas bridge and structural designers
may use finite element analysis (FEA) tools.
The RAM tools and techniques are explained in Section 6.1 through to Section 6.6.
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6.1. Reliability block diagram analysis AS IEC 61078 Reliability block diagrams describes RBDs as a diagrammatic analysis method
for demonstrating the contribution of component reliability to the success or failure of a complex
system.
A RBD is drawn as a series of blocks connected in parallel or series configuration, with each
block representing a component of the system with an associated failure rate. Parallel paths are
redundant, meaning that all of the parallel paths must fail for the parallel network to fail. By
contrast, any single failure along a serial path causes the entire serial path to fail.
RBDs are used to calculate the reliability of each element and the contributory effect on the
reliability of the system. This assists in the identification of single points of failure in the system.
Examples of where a RBD would be used are for the development of a station announcement
system and a blue light emergency station provided in Appendix B.
6.2. Failure mode, effects and criticality analysis AS/NZS IEC 60812 Failure mode and effects analysis (FMEA and FMECA) describes FMECA
as a bottom up analysis method that is used to understand failure modes at the unit level and
their escalation effect, both at a local and a system wide level. This method requires the system
design to be well defined down to unit level.
Each system is decomposed into its elements, usually down to line replaceable unit (LRU) level
where each element is then analysed uniquely to identify functional failures and relevant modes
of failure, and their escalated effect on the next higher level of the system.
This process is employed to identify those elements of a system which have a significant impact
on system reliability, availability and safety. This analysis is further used to promote mitigation
measures leading to improved system reliability and availability.
FMECA is typically used for high level analysis of system reliability through the following
process:
• identification of failure modes and consequences, and facilitation of design modifications
• assessment of failure causation, performance limits and vulnerability issues
• classification of failure modes relative to the severity of their effects
An output of the FMECA should be a reliability critical items list (RCIL). This is a list of items
which have at least one failure mode classified as critical according to its criticality analysis.
Consideration should also be given to common-mode failure where an event causes multiple
systems to fail. For example a transformer explosion in a substation may cause both
transformers (main and standby) in the room to fail simultaneously.
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Appendix C provides an example of a FMECA used for the development of a train bogie
system.
Refer to T MU AM 01003 F1 Blank FMECA Sheet for further details.
6.3. Fault tree analysis FTA should be used for highly complex safety or reliability critical systems.
FTA should be done during the initial stage of the project and updated as more details become
available during subsequent stages of the project.
AS IEC 61025 Fault tree analysis (FTA) describes FTA as a top down deductive failure analysis.
An undesired state of a system (top event) is analysed using Boolean logic to combine a series
of lower-level events and associated probabilities. This analysis method is used to determine
the probability of a safety accident or a particular system level (functional) failure.
The basic symbols used in FTA are grouped as events, gates, and transfer symbols. Event
symbols are used for primary events and intermediate events. Primary events are not further
developed at a lower level on the fault tree. Intermediate events are found at the output of a
gate. Events in a fault tree are associated with statistical probabilities. Gate symbols graphically
describe the mathematical relationship between input and output events. The gate symbols are
derived from Boolean logic symbols. Transfer symbols are used to connect the inputs and
outputs of related fault trees, such as the fault tree of a subsystem to its system.
FTA incorporates the following phases:
• definition of the undesired system top event to analyse
• obtaining an understanding of the system functional breakdown
• construction of the Boolean fault tree from top event down to base events
• assignment of failure rates to the base events
• evaluation of the fault tree (software-based simulation or spreadsheet analysis)
• control of the hazards identified
An example of where FTA would be used is for analysing the risk of failure of the water deluge
system (that forms part of a broader fire suppression system) failing to operate when required.
The contributory factors that lead to this system top event are provided in Appendix D.
6.4. Human reliability analysis T MU AM 06006 ST requires that projects consider human reliability factors as part of the
overall reliability of the system.
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The purpose of conducting human reliability analysis (HRA) is to ensure that the actual
operability performance of the system is in line with its designed requirements. Humans are an
integral part of designed systems, playing important roles in operation, accident prevention and
maintenance activities.
Operators and maintainers should be trained and competent; however ‘trained and competent
people’ is not a way of preventing human error. Human error is a normal part of human
performance, and should be appropriately assessed to create resilient systems. A trained and
competent human operator can still take erroneous decisions and actions based on stress,
fatigue, distraction and other human performance degradation factors. Early, appropriate HRA
is essential to ensure the exploration of the appropriate hierarchy of controls. Delayed or
ineffective assessments tend to create dependencies on administrative risk control which can
create latent system weaknesses.
Therefore, analysing and predicting the reliability of a system without assessing human
reliability may result in an over estimation of system performance.
Although there are many ways in which a human can positively impact on system performance,
the focus within a RAM assessment is usually to identify the following:
• human errors that may impact on the RAM of the system
• mitigation measures to reduce likelihood of human errors or to reduce impact of these
errors on the system
• minimum training and capability requirements
These measures can relate to the design of the equipment or the task, or may warrant
additional redundancy or diversity to be incorporated within the overall system design.
In order to be able to identify the errors that can be made and what their likely effect on the
performance of the system would be, it is necessary to identify the following:
• the tasks that are required to be carried out by operators and maintainers
• the likely conditions under which those tasks will be performed
• the potential errors that could be made
With many other aspects of the design in the early stages, information may be at a relatively
high level and should be used to identify those areas of the system where a more detailed
assessment is of most value.
There are a number of methods available for identifying human errors ranging from utilising past
experience through to the application of structured processes based on guidewords or
checklists. Human error should be built into existing system RAM analysis techniques such as
FMECA or FTA.
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In those cases where a quantitative assessment is required, techniques for human error rate
prediction may be employed for evaluating the probability of a human error occurring and
impacting on system performance. This should then be incorporated into the system RAM and
performance models to assess the impact on the overall system performance.
Techniques to evaluate the probability of human errors fall into the following three general
categories:
• the use of screening data
• the use of historical or subjective data
• the use of human error databases
Examples of where a HRA would be used are for a ticketing system and a door release system
provided in Appendix E.
Refer to T MU HF 00001 GU Guide to Human Factors Integration – General Requirements for
more information on HRA.
6.4.1. Screening data A single screening value for human error within a system model may be used in the early
stages of an assessment. This enables an organisation to identify where the system is
particularly vulnerable to human error and to review the design in terms of the level of
redundancy or diversity that is currently built in, or to identify whether a more detailed
assessment may be required.
6.4.2. Historical or subjective data Actual performance data, if available, may be used as an estimation within reliability models.
This data is normally only available at the system level and does not specifically highlight the
human error contribution. However, it is estimated that approximately 70% - 90% of failures are
due to human error and so it is possible to factor the data in this way to obtain a more reliable
estimate.
Note: Manufacturer's data generally does not include human errors and so it will be
indicative of performance based on 100% reliability of people.
Alternatively, subjective data may be sought through consultation with users or their opinions
and may be used to modify existing data.
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6.4.3. Human error databases A number of techniques are used for quantitative human error assessments where it is possible
to look up a generic human error probability and then modify it according to the specific task.
Commonly used examples include the following:
• Human error assessment and reduction technique (HEART)
HEART method is based upon the principle that every time a task is performed there is a
possibility of failure and that the probability of failure is effected by one or more error
producing conditions to varying degrees. Error producing conditions include topics such as
training level and frequency, poor procedures, poor system feedback and so on.
Factors which have a significant effect on performance are of greatest interest. These
conditions are applied to a ‘best-case-scenario’ estimate of the failure probability under
ideal conditions to then obtain a final error probability. By forcing consideration of the error
producing conditions potentially affecting a given procedure, the application of HEART also
enables the user to identify a range of potential improvements to system performance.
An example of where HEART would be used is the assessment of a critical maintenance
task.
• Technique for human error rate prediction (THERP)
THERP models human error probabilities using an event tree approach, in a similar way to
an engineering risk assessment, but also considers performance shaping factors that may
influence these probabilities. The probabilities for the HRT event tree, which is the primary
tool for assessment, are nominally calculated from historic databases, local data including
simulated data or from accident reports. The resultant tree portrays a step by step account
of the stages involved in a task in a logical order. The technique is described as a total
human reliability assessment methodology as it simultaneously manages a number of
different activities including task analysis, error identification and human error
quantification.
6.5. Maintenance requirements analysis Maintenance requirements analysis (MRA) is the inclusion of reliability, availability and safety
integrity as a part of the maintenance requirements of the system.
MRA applies reliability theory principles within a structured process designed to identify effective
maintenance and inspection tasks that would detect or delay failures of equipment. This
ensures that maintenance requirements are incorporated into the design activities.
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The following elements are inherent in MRA:
• identify the maintenance item
o Identify the items to be maintained at system, element, assembly, unit, component
level as part of the asset breakdown structure.
• establish the function
o Identify all functions associated with the maintenance item.
• establish failure modes and effects
o Identify and analyse all possible failures to or deviations from the specified
functionality associated with the maintenance item. Analyse their escalation effects
from component level to unit level to assembly level to subsystem level to system
level.
• recognise failure
o Identify the means by which each failure is detected and communicated to the
maintainer.
• identify maintenance task options
o Identify how each maintenance item should be repaired or replaced (both preventative
and corrective maintenance tasks).
• establish maintenance task intervals
o Identify a maintenance program which includes the schedule of inspection or
replacement for all maintenance items.
An example of where MRA would be used is for the development of a station escalator provided
in Appendix F.
Refer to T MU AM 01003 ST Development of Technical Maintenance Plans and
T MU AM 01010 ST Framework for Developing an Asset Spares Assessment and Strategy for
further details.
6.6. Failure recording analysis and corrective action system A FRACAS should be applied from that point in the design cycle at which a version of the
product or service approximating the final operational version becomes available until the
product or service is decommissioned. The RAM plan should identify the existing FRACAS in
place, if any.
The FRACAS is a closed loop process incorporating data reporting, collecting, recording,
analysing, investigating and timely corrective action for all failure incidents. The objective of the
system is to aid design, identify corrective action tasks and evaluate test results in order to
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provide confidence in the results of the safety analysis activities in addition to the correct
operation of the safety features.
The effectiveness of FRACAS is dependent upon accurate input data in the form of reports
which should document all the conditions relating to the incident.
Incident reviews should be undertaken to ensure that the impact on the safety and reliability
characteristics of the product or service are quickly assessed, with any corrective actions
requiring design changes, quickly approved.
The FRACAS process is outlined as follows and is illustrated in Figure 1:
• an incident report is raised and recorded in a database
• a data search is carried out for related events to determine if there is a growing trend in a
particular failure event type
• the incident is reviewed - if the incident is a new hazard it is recorded as such in the hazard
log
• information concerning the incident is communicated to those that need to know, in order to
control risk
• corrective actions are recommended, as necessary
• if no corrective action is required, the database is updated and the process ends
• the corrective action is authorised and implemented then assessed for success
• if the corrective action is unsuccessful, the incident is re-reviewed, corrective actions are
modified as required, details are updated in the database and the action returns for further
authorisation to proceed
• if the corrective action is successful, the database is updated and the process ends
An example of where FRACAS would be used is the development of a CPU motherboard
provided in Appendix G.
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Incident raised and recorded
Search for related events
Review incident
Communicate information as necessary
Corrective action necessary?
Authorise, implement and assess plan
Corrective action successful?
Update database
No
No
Yes
Yes
Figure 1 - FRACAS process
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Appendix A Additional reference documents The following documents, not cited in the text, provide additional information and guidance.
I.S. EN 50126 (all parts) Railway Applications - The Specification and Demonstration of
Reliability, Availability, Maintainability and Safety (RAMS)
EN 50128 Railway applications – Communication, signalling and processing systems –
Software for railway control and protection systems
AS ISO 55001 Asset management - Management systems - Requirements
AS IEC 62508 Guidance on human aspects of dependability
T MU MD 00009 SP AEO Authorisation Model
T MU AM 01002 MA Maintenance Requirements Analysis Manual
Department of Defense United States of America, 1987, MIL-HDBK-781A Handbook for
Reliability Test Methods, Plans, and, Environments for Engineering, Development, Qualification
and, Production
Department of Defense United States of America, 191995, MIL-STD-2155 Handbook Failure
Reporting, Analysis and Corrective Action Taken
Williams, J.C., HEART – A proposed method for achieving high reliability in process operation
by means of human factors engineering technology in Proceedings of a Symposium on the
Achievement of Reliability in Operating Plant, Safety and Reliability Society,1985, NEC,
Birmingham
Swain, A.D. and Guttmann, H.E., Handbook of Human Reliability Analysis with Emphasis on
Nuclear Power Plant Applications. 1983, NUREG/CR-1278, USNRC
Shappell, S.A. and Wiegmann, D.A., The Human Factors Analysis and Classification System—
HFACS, February 2000, DOT/FAA/AM-00/7
Stanton, N. A., Salmon P. M. et al, Human Factors Methods A practical guide for Engineering
and Design, 2nd Edition, 2013, Ashgate, Aldershot, ISBN 978-1-4094-5754-1
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Appendix B Examples of reliability block diagrams Figure 2 and Figure 3 provide examples of RBDs for station public announcement and station blue light emergency display, respectively.
Loudspeakers
STM 64 PortP.MUX AMD II
Matrix Enhanced
MTBF=157.680 H
MTBF=183.960H
MTTR=2hours
MTTR=2hours
Network Fibre
MP50 Call Station
MTBF=163.549 H
MTTR=2hours
STM 64 Port
Matrix Enhanced
P.Mux AMD II
MTTR=2hours
MTBF=183.960H
MTBF=157.680 H
MTTR=2hours
MTBF=163.549 H
MTTR=2hours
Amplifier Module V400 Amplifier Mainframe
VIPET
P1 Ethernet Switch
PCAS Workstation
VIPA HOSTVAR 4
Network Fibre
MTBF=600000H
MTTR=24h
MTBF=600000H
MTTR=24h
MTBF=21400 H
MTBF=121354 H
MTTR=2hours
MTBF=39800H MTBF=65000H
MTBF=48681 HMTBF=96400 HMTBF=215800 HMTBF=118600H
MTTR=4hMTTR=1hMTTR=4h
MTTR=4 hoursMTTR=4 hoursMTTR=4 hoursMTTR=4 hours
Loudspeakers
MTBF=87600H
MTBF=87600H
MTTR=2hours
MTTR=2hours
Service board
MTBF=621.960 H
MTTR=2hours
Service board
MTBF=621.960 H
MTTR=2hours
Matrix Enhanced
MTBF=157.680 H
MTTR=2hours
Matrix Enhanced
MTBF=157.680 H
MTTR=2hours
Switch OS6450-24
MTTR=2hours
MTBF=894251H
Figure 2 - RBD station public announcement (sample fragment)
Note: In Figure 2 mean time between failures is expressed as MTBF and mean time to repair is expressed as MTTR.
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Emergency Push Button
with Key Reset
Relays
240V AC UPS
Blue Light Display
Comms Module Output RelayInput Relay Alarm Module
Comms Module Output RelayInput Relay Alarm Module
MTTR=1 hour
MTBF=100,000 H MTBF=50,000 H MTBF=100,000 H MTBF= 100,000 H MTBF=50,000 H
MTBF=600,000 H
MTTR=1 hour MTTR=1 hour MTTR=1 hourMTTR=4 hours
MTTR=3 hours
MTTR=1 hour
MTBF=50,000 H MTBF=100,000 H MTBF= 100,000 H MTBF=50,000 H
MTTR=1 hour MTTR=1 hour MTTR=1 hour
MTBF= 50,000 H
MTTR= 3 hoursEmergency Push Button
with Key Reset
MTBF=100,000 H
MTTR=4 hours
Figure 3 - RBD station blue light emergency display (sample fragment)
Note: In Figure 3 mean time between failures is expressed as MTBF and mean time to repair is expressed as MTTR.
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Appendix C Example of FMECA table - Bogie assembly Figure 4 provides an example of FMECA table for bogie assemble.
Figure 4 - FMECA table (sample fragment)
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Appendix D Examples of fault tree analysis Figure 5 shows an example of a FTA for failure of a fire water deluge fail.
TOP1
Fire Water Deluge Fails
GATE1
Motor Failures
GATE2
Detection Failures
PUMP
Fire Pump
I E
GATE3
Power Failures
MOTOR
Fire Pump Motor
I E
DETECT
UV fire detector
I E
PANEL
Fire Detection Panel
I E
PSU
Mains Power Supply
I E
STANDBY
Standby Generator
I E
Fire suppression system example:
• Failure Rate, λ = 1/MTBF
• MDT = Mean Down Time (hrs)
λ = 9.6
MDT 21h
λ = 59.6
MDT 144h
λ = 120 ∴ MTBF = 0.95 years
MDT 84h ∴ A=0.99
λ = 0.0096
MDT 21h
λ = 100
MDT 24h
λ = 500
MDT 168h
λ = 50
MDT 168h
λ = 5
MDT 168h
λ = 10
MDT 24h
λ = 60
MDT 24h
Figure 5 – Fault tree analysis diagram
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Appendix E Examples of human error analysis Table 1 and Table 2 provide examples of human error analysis for ticketing system and doors release system, respectively.
Table 1 – Example of human error analysis for ticketing system
Task Error Mitigation
Select ticket type at ticket vending machine Incorrect ticket type selected • Machine buttons labelled with the various ticket types • Machine visual display showing ticket type selected
Select destination at ticket vending machine
Incorrect destination selected • Machine buttons labelled with the various destinations • Machine visual display showing destinations
Enter coins into ticket vending machine Coins inserted into the notes reader • Ticket vending machine has coin slot labelled
Enter notes into ticket vending machine Notes inserted into the coins slot • Ticket vending machine has the notes reader labelled
Enter notes into ticket vending machine Notes inserted upside down or back to front • Machine labelled with a diagram showing the correct note orientations
Transport the ticket Ticket bent in transit • ‘Do not bend this ticket’ marked on the ticket • Ticket made from flexible plastic to avoid damage • Ticket size allows ticket to be placed in a wallet or purse
Transport the ticket Ticket dropped or crushed
Insert ticket into ticket reader Ticket inserted upside down • ‘Travel Card’ marked on upside of the ticket
Insert ticket into ticket reader Ticket inserted back to front • Direction arrow marked on the upside of the ticket
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Table 2 – Example of human error analysis for doors release system
Task Error Mitigation
Locate doors release button Button not located • Button labelled with doors release • Button illuminated with green lights
Press doors release button Button not pressed • Button labelled with doors release • Button illuminated with green lights
Travel on train Button pressed accidently • Button recessed to avoid accidental presses • Button must be pressed for 3 seconds to activate
Travel on train Button obscured by passengers • Door labelled requesting passengers to stand clear
Travel on train Button damaged by passengers • Button recessed to avoid accidental contact • Button made from material that can withstand high impacts
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Appendix F Example of MRA - station escalator Table 3 provides an example of MRA for station escalator.
Table 3 – Example of MRA for station escalator
Maintenance item
Functions associated
Possible failures modes
Effect Escalation effect
Failure recognition
Maintenance task options
Maintenance task intervals
Platforms Entry and exit access
Unable to provide entry or exit access
Platform blocked to passengers
Passengers unable to use the escalator
• Visual inspections for damage
• Testing
• Replace platform panels
• Daily cleaning, inspection and testing
• 6 monthly service inspection and testing
Steps Support standing or walking passengers
Unable to support passengers
Steps not safe for passengers
Passengers unable to use the escalator
• Visual inspections for damage
• Testing
• Replace steps • Daily cleaning, inspection and testing
• 6 monthly service inspection and testing
Tracks Provides running surface for the steps
Unable to provide running surface for the steps
Steps unable to move
Passengers unable to ride on escalator
• Testing • Lubricate tracks • Replace tracks
• Daily testing • 6 monthly service
inspection and testing
Tracks Provides running surface for the handrails
Unable to provide running surface for the handrails
Handrails unable to move
Passengers unable to use handrail for support
• Visual inspections for damage
• Testing
• Lubricate tracks • Replace tracks
• Daily testing • 6 monthly service
inspection and testing
Drive gears Provides coupling and speed conversion of the motor to the steps
Unable to provide coupling and speed conversion of the motor to the steps
No or slow movement of steps.
Passengers unable to ride on escalator
• Testing • Lubricate gears • Replace gears
• Daily testing • 6 monthly service
inspection and testing
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Maintenance item
Functions associated
Possible failures modes
Effect Escalation effect
Failure recognition
Maintenance task options
Maintenance task intervals
Drive gears Provides coupling and speed conversion of the motor to handrails
Unable to provide coupling and speed conversion of the motor to handrails
No or slow movement of handrails
Passengers unable to use handrail for support
• Testing • Lubricate gears • Replace gears
• Daily testing • 6 monthly service
inspection and testing
Hand rails Provides support and stability to passengers
Handrails unable to support passengers
No handrails Passengers unable to use handrail for support
• Visual inspections for damage
• Testing
• Replace handrails
• Daily cleaning, inspection and testing
• 6 monthly service inspection and testing
Motors Provides driving force for handrails and steps
Unable to drive the handrails or steps
No or slow movement of steps or handrails
Passengers unable to ride on escalator
• Testing • Replace motors • Daily testing • 6 monthly service
inspection and testing
Control system
Regulates speed of steps and handrails
Unable to drive the handrails or steps
No or slow movement of steps or handrails
Passengers unable to ride on escalator
• Testing • Replace motors • Daily testing • 6 monthly service
inspection and testing
Emergency stop system
Halts movement of steps and handrails in an emergency situation
Unable to halt steps and handrails
Steps and handrail movement
Passenger injuries
• Testing • Replace components
• Daily testing • 6 monthly service
inspection and testing
Glass screens
Protects passengers from moving components
Unable to protect passengers from moving components
Exposed moving parts
Passenger injuries
• Visual inspections for damage
• Replace components
• Daily inspection
Glass screens
Protects passengers from falling
Unable to protect passengers from falling
Passengers fall off steps
Passenger injuries
• Visual inspections for damage
• Replace components
• Daily inspection
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Appendix G Example of FRACAS incident report – CPU motherboard Figure 6 provides an example of FRACAS incident report for CPU mother board.
Figure 6 - Example of FRACAS incident report for CPU motherboard