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MACHINE BUILDING, FRAMEWORKS & SAFETY FEATURE
MACHINERY CONTROL SYSTEM SAFETY
A new version is replacing ISO 13849-1:2015, the safety standard for machinery control systems. Stewart Robinson MIET MInstMC, principal engineer and Functional Safety Expert at TÜV SÜD, looks into the requirements
I
SO 13849 is a safety standard which applies to parts of machinery control systems that are assigned to providing safety functions.
It provides safety requirements and guidance on the principles of design, and the integration of safety-related parts of control systems (hardware or software). A new version now replaces ISO 13849-1:2015.
Once listed in the Official Journal (OJEU) as a harmonised standard there may be a transition period of up to three years before the superseded version is withdrawn from the OJ, ending its presumption of conformity. However, once it is listed (which at the time of writing we are still awaiting), this new standard will have a presumption of conformity, meaning that following it will ensure your products are in line with corresponding EU rules. Once the new version is listed in the OJEU, it will be also become listed as a designated standard in the UK.
SAFETY REQUIREMENTS ISO 13849-1 provides safety requirements and guidance on the principles for the design and integration of safety-related parts of control systems (SRP/CS). This includes the design of software, and the standard includes specific requirements for SRP/CS using programmable
electronic systems. It also specifies characteristics that include the performance level required for carrying out safety functions. The standard applies to SRP/CS for high demand and continuous mode, regardless of the type of technology and energy used (electrical, hydraulic, pneumatic, mechanical, etc.), for every type of machinery. The whole document has been reorganised to
better reflect the design and development process for control systems. It now also includes a focus on risk assessment recommendations. For example, Figure 2 of Clause 4 is an enhanced version of Figure 1 in the current version – breaking out the ‘three-step process’ of risk reduction. However, the overall process and the relationship with ISO 12100 is unchanged. The new Figure 3 is also an enhanced version of the current Figure 2, which describes the contribution to risk reduction. Alongside this, Clause 4.6 (subsystems) is new, while significant parts of Clause 4 have been moved to their own clauses for specification, design, and verification. In Clause 6 – Design Considerations, safety-
related parts of control systems (SRP/CS) has been replaced by ‘subsystem’. Again, this has been restructured and includes a new sub-clause for systematic failures (6.1.7).
OF FURTHER NOTE There is also a new Clause 7 on software safety requirements. At first glance, this is almost a repeat of Clause 4.6 in the previous version. However, changes and additions include explaining the differences between ‘limited variability’ and ‘full variability’ and guidance in deciding between them. Limited Variability Language (LVL) is typically found as application code in programmable logic controllers (PLC) systems, using graphical representation such as ladder diagrams or function block diagrams. Full Variability Language (FVL) is typically found in computers, for example C++, C#. In ISO 13849 it is mainly used for embedded software, referred to as SRESW. Both LVL and FVL can be used for safety-related application software (SRASW) in accordance with ISO 13849. Greater rigour is required for LVL due to the complexity. Clause G.5 of Annex G covers the management of functional safety, requiring a functional safety plan to be drawn up and documented for each SRP/CS design project, which should be updated as any changes to the system are made. Key features of a functional safety plan should include: • The identification of the relevant activities in the SRP/CS design process and confirmation of the order in which they should take place. • The identification of the roles and resources required for actioning and reviewing every activity. • The creation of a validation plan. There is also a new Annex L on
electromagnetic interference (EMI) immunity. Example sources of EMI are both natural, such as static & lightning, and manmade intentional transmission and unintentional interference, such as electronics, electric power, communications, machines, and ignition systems. This new part of the standard has been severely criticised by some EMC experts. It provides a scoring methodology (table L.1) that can be used where equipment that integrates electronics can be assessed by considering various measures to achieve immunity. Annex M includes additional information
for safety requirements specification, while Annex N also introduces the concept of fault- avoiding measures for the design of safety related software. It Is intended to help users of the standard apply the requirements of Clause 7 on software safety requirements. Meanwhile, Annex O now covers safety-related values of components or parts of the control systems. This is extracted from a data library format created by the German machinery manufacturers association (VDMA). It has been around for some years and is used in applications like SISTEMA and PAScal to help with the calculations for hardware reliability.
TÜV SÜD
www.tuvsud.com/uk NOVEMBER 2023 DESIGN SOLUTIONS 19
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