For safety relevant or safety critical medical devices (where section 14 of IEC 60601-1 Programmable Medical Electrical Systems (PEMS) is applicable), system architects and development teams are faced with a daunting list of items to consider in their PEMS architecture specification, but with little guidance provided in IEC 60601-1.
Thankfully there are many industry sources around that can support these activities. Safety standards such as IEC 61508 give sound guidance on how to consider redundancy and diversity in the product architecture. IEC 61508-6 Annex D provides a useful checklist for design teams assessing common-cause failures.
The ISO 26262 Automotive Functional Safety standard guides design teams in understanding partitioning of functionality. Focussing on systems, hardware and software, it explores the topics of architectural decomposition and gives guidance on dependant failures analyses. If the budget is tight, sources of information do not necessarily have to come from international standards. There are also industry papers providing sources of information on types of architecture and the pros and cons of each e.g. Controller Integrity in Automotive Failsafe System Architectures from Delphi.
Minimising systematic failures is also not a simple task but following the IEC 61508 guidance on system architecture can help reduce the impact of such failures. Relatively inexpensive static analysis tools such as MISRA can be utilised to identify potential weakness in software.
As described in our previous blog titled Components with High Integrity Characteristics, the designer can find excellent guidance on topic of component failure modes and their effects in the Alessandro Birolini Reliability Engineering handbook and, using reliability standards such as IEC TR 62380 or Siemens SN 29500, can support not only in conservatively estimating the failure in time rates, but also in assessing the different ways in which specific failures may occur.
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