Machine Safety FAQs

These are the questions that are answered on the page below. Click a question to be taken to the answer, or browse down the page to read all FAQ's.

Questions (click for answer)

• Q 1. Why change?
• Q 2. What are the new standards, how are they different to BS EN 954-1?
• Q 3. Which standard should I use?
• Q 4. When will EN 954-1 be withdrawn?
• Q 5. Where can I obtain copies of the standards?
• Q 6. Who is responsible for what?
• Q 7. How can we help?

Q. Why change?

A. There are several reasons for change;

• In the past, safety-related parts of a machine’s control system were designed in accordance with EN 954-1. This was based upon the calculated risk formed into categories. The aim was to set an appropriate system behaviour against a category. With electronics and programmable electronics increasingly incorporated into safety systems, safety can no longer be measured purely in terms of categories found in BS EN 954-1. Neither can this standard provide information on the probability of failure of these systems.
• Furthermore, there has been a tendency for components specified to a high category of BS EN 954-1 to be chosen instead of components that have a lower category, but might actually have more suitable functions. This might be as a result of the misconception that the categories are hierarchical for example, category 3 is always “better” than category 2 and so on.
• New functional safety standards are intended to encourage designers to focus more on the functions that are necessary to reduce each individual risk, and what performance is required for each function, rather than simply relying on particular components.
• The new standards BS EN ISO 13849-1 and BS EN 62061 help with the weaknesses of BS EN 954-1.

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Q. What are the new standards and how are they different to BS EN 954-1?

A. Now that BS EN 954-1 will be withdrawn 29 December 2009, the available alternatives are BS EN 62061 and BS EN ISO 13849-1.

The performance of each safety function is specified as either:

• SIL (Safety Integrity Level, SIL 1 - 3) in the case of BS EN 62061
• PL (Performance Level, PLa - PLe) in the case of BS EN ISO 13849-1

BS EN ISO 13849-1: “Safety-related parts of control systems, Part 1: General principles for design”

This standard may be applied to SRP/CS (safety-related parts of control systems) and all types of machinery, regardless of the type of technology and energy used (electrical, hydraulic, pneumatic, mechanical, etc.). BS EN ISO 13849-1 also lists special requirements for SRP/CS with programmable
electronic systems.

BS EN ISO 13849-1 is based on the familiar categories from BS EN 954-1. It examines complete safety functions, including all the components involved in their design. BS EN ISO 13849-1 goes beyond the qualitative approach of EN 954-1 to include a quantitative assessment of the safety functions. A performance level (PL) is used for this, building upon the categories.

Components/devices require the following safety parameters:

• Category (structural requirement)
• PL (a – e): Performance level
• MTTFd: Mean time to dangerous failure
• B10d: Number of cycles by which 10% of a random sample of wearing components have failed dangerously
• DC: Diagnostic coverage
• CCF: Common cause failure

The standard describes how to calculate the performance level (PL) for safety-related parts of control systems, based on designated architectures. BS EN ISO 13849-1 refers any deviations to IEC 61508. Where several safety- related parts are combined into one overall system, the standard describes how to calculate the PL that can be achieved.

For additional guidelines on validation EN ISO 13849-1 refers to Part 2, which was published at the end of 2003. This part provides information on fault considerations, maintenance, technical documentation and usage guidelines.

BS EN 62061: “Functional safety of safety-related electrical, electronic and programmable electronic control systems”.

This standard defines requirements and gives recommendations for the design, integration and validation of safety-related electrical, electronic and programmable electronic control systems (SRECS) for machinery. It does not define requirements for the performance of non-electrical (e.g. hydraulic, pneumatic, electromechanical) safety-related control elements for machinery.

BS EN 62061 represents a sector-specific standard under IEC 61508. It describes the implementation of safety-related electrical and electronic control
systems on machinery and examines the overall lifecycle from the concept phase through to decommissioning. Quantitative and qualitative examinations of the safety-related control functions form the basis.
The performance level is described through the safety integrity level (SIL).

The safety functions identified from the risk analysis are divided into safety subfunctions; these safety subfunctions are then assigned to actual devices, called subsystems and subsystem elements. Both hardware and software are handled this way.

A safety-related control system is made up of several subsystems. The safety-related characteristics of these subsystems are described through parameters (SIL claim limit and PFHD).

• Safety-related parameters for subsystems:
• SILCL: SIL claim limit
• PFHD: Probability of dangerous failure per hour
• T1: Lifetime

These subsystems may in turn be made up of various interconnected subsystem elements (devices) with parameters to calculate the subsystem’s corresponding PFHD value.

Safety-related parameters for subsystem elements (devices):

• Failure rate; for wearing elements describe via the B10 value
• SFF: Safe failure fraction; for electromechanical devices the failure rate is indicated by the manufacturer as a B10 value, based on the number of cycles. The time-based failure rate and lifetime must be determined through the switching frequency for the respective application.

Internal parameters to be established during design / construction for a subsystem comprised of subsystem elements:

• T2: Diagnostic test interval
• β: Susceptibility to common cause failure
• DC: Diagnostic coverage
• PFHD: The PFHD value of the safety-related control system is calculated by adding the subsystems’ individual PFHD values.

Detailed explanation of these terms and their use are found in our downloadable Safe Machinery Guide.

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Q Which standard should I use?

A As before, for risk assessment and risk reduction use the standards BS EN 14121-1 and EN 12100. For the safety related electrical control circuits use either BS EN ISO 13849-1 or BS EN 62061. The big question is which of these two standards to use. The logical step from BS EN 954-1 is to use BS EN ISO 13849-1, however, with complex bespoke products and software, BS EN 62061 should be applied.

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Q. When will EN 954-1 be withdrawn?

A. 29 December 2009

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Q. Where can I obtain copies of the standards?

A. www.bsiglobal.com

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Q. Who is responsible for what?

A. Machine builders, buyers, owners and users all have a responsibility for safety.

Users' responsibilities - buying machines and owning existing machines

Users of machines need to ensure that newly-purchased machines are CE marked, and accompanied by a Declaration of Conformity to the Machinery Directive. Machines must be used in accordance with the manufacturer's instructions.

Existing machines taken into service before the Machinery Directive came into force do not need to comply, although they need to comply with PUWER and be safe and fit for purpose.

Modification of machines can be considered as manufacture of a new machine, even if for use in-house, and the company modifying a machine needs to be aware that it might need to issue a Declaration of Conformity and CE marking.

Existing machines – the Work Equipment Directive

This is implemented in UK law as the Provision and use of Work Equipment Regulations 1998, (PUWER 1998). It applies to the provision of all work equipment, including mobile and lifting equipment, in all workplaces and work situations where the Health and Safety at Work etc Act 1974 (HSW Act) applies. It extends outside Great Britain to some offshore activities. The regulations apply to all employers, the self-employed, and others who have control of the provision of work equipment. They require that all equipment is suitable for use and is inspected and maintained as necessary to ensure that it remains so.

Manufacturers' responsibilities & new machines

Manufacturers placing machines on the market within the European Economic Area must comply with the requirements of the Machinery Directive. Note that "placing on the market" includes an organisation supplying a machine to itself, i.e. building or modifying machines for its own use, or importing machines into the EEA.

New machines - the Machinery Directive

In the UK the Machinery Directive 98/37/EC is implemented as the Supply of Machinery (safety) regulations 1992 as amended.

From 29 December 2009 the relevant UK regulations will be the Supply of Machinery (safety) regulations 2008, which implement the European Machinery Directive 2006/42/EC.

Machines have to comply with the Essential Health and Safety Requirements (EHSRs) listed in Annex I of the Directive, thus setting a common minimum level of protection across the EEA (European Economic Area). Machine manufacturers, or their authorised representatives within the EU, must ensure that the machine is compliant, the Technical File can be made available to the enforcing authorities on request, the CE marking is affixed, and a Declaration of Conformity has been signed, before the machine may be placed on the market within the EU.

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Q. How can we help?

A. Peter Still of Schneider Electric has represented GAMBICA and in turn the UK on the IEC committee TC44 for machinery safety throughout the development of the new standard. This has given us a unique insight into the background, requirements, and implications of the new standards. Our Safe Machinery Guide can be downloaded and gives a good overview of the Machinery Directive and contains some worked examples of both BS EN ISO 13849-1 and BS EN 62061.

We have a comprehensive safety components offer which can be found in Products

We can supply the reliability data for all products used in safety applications Reliability Data in Support

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