A systematic approach to the functional safety of machinery
28 October 2014
Functional safety is one of the core elements in the design and construction of machinery. But how can the general requirements of the Machinery Directive 2006/42/EC be met in each specific case? Which codes and standards should be taken into account? And how can risk assessment be documented correctly and in compliance with the legal requirements? Dr Rolf Zöllner, risk manager at TÜV SÜD Industrie Service, explains.
Machines must operate safely and reliably at all times or, in the case of loss of function, need to be transitioned immediately to a safe state. Legal requirements in the field of machine safety prescribe risk assessments be undertaken before a machine can be sold on the European market. TÜV SÜD has assisted a Swedish supplier of mining and quarrying equipment to establish an effective and efficient management system for the functional safety of machinery.
The Machinery Directive 2006/42/EC includes only general safety requirements, offering no generally valid solution to these frequently asked questions. The applicable standards and their weighting differ, depending on the industry, type of machine and technical equipment. These standards are subject to constant monitoring, regular revision and updating to the state of the art. In view of the above, machinery manufacturers are facing the challenge of having to decide on and monitor the functional safety requirements of machinery systematically and effectively – also with respect to different requirements outside the EU, future changes in standards and regulations.
One possible solution to this challenge is the creation of an internal management system, such as that implemented by one Swedish manufacturer of mining and quarrying equipment.
The company faced the challenge of having to comply with a joint approach to functional safety provisions and requirements for the entire company when the directives relevant to their products were revised. The manufacturer made the decision to document the conformity of its mining equipment with the recognised, applicable safety standards in a new and traceable manner.
While this is one of the requirements for placing products on the EU market, non-EU markets also demand evidence of machinery safety, which is frequently in accordance with the international standards so manufacturers wishing to access international markets also need to demonstrate that their products meet the respective national requirements.
To ensure an effective, efficient and sustainable solution to this complex task, the decision was made to establish processes for a functional-safety management system. The management system was intended to bundle and refine the relevant know-how and best practice to ensure that it would be included in the design and construction of all new or modified machinery, throughout the company, from the start. To ensure that all relevant standards and directives would be taken into consideration and any possible obstacles identified and eliminated, the company called on the expertise of TÜV SÜD Industrie Service to act as a functional safety consultant.
Over a period of two years, TÜV SÜD helped the company to organise numerous workshops and training sessions for the company. At the start of the project it worked with mechanical engineers, designers, safety engineers and other employees of the corporation to determine which actions would be necessary. Once this had been done the initial concept was worked out: Which specific hazards and risks are posed by the relevant mining and quarrying equipment? Which standards and directives are relevant to the company? How would they quantify hazards and risks? Which design measures are suitable for eliminating risks or reducing them to an acceptable level?
Standards and directives
To obtain answers to these questions, the experts assessed the risks of machine prototypes according to the relevant standards - such as IEC 61508, EN ISO 62061, ISO 12100 as well as EN ISO 13849-1 and EN ISO 13849-2. At this stage of the project, the functional safety professionals focused on the inherently safe construction of the machine, the failure-safety of electrical systems and controls and the hazards posed by mechanical and hydraulic components. By examining these prototype machines, the experts identified and developed all important aspects that affected overall machinery safety.
As the project workshops progressed, risk assessment was also tailored to the next specific requirements of the company. This core element of the functional safety of machinery is firmly anchored in the Machinery Directive 2006/42/EC and end-to-end documentation in compliance with the legal requirements plays a major role in this context. Documentation, too, is company-specific and the only possibility of providing clear evidence that the design and construction of a machine is in compliance with the applicable directives and standards. If the competent market surveillance authorities doubt the machine's compliance in the fields of occupational health and safety or an accident occurs with personal injuries, only the internal records can provide exonerating arguments which help to eliminate any unclear points and/or establish responsibilities.
A core element
Risk assessment generally combines probabilistic and – to a different degree – quantitative methods. It enables the safety relevance of an event to be described on the basis of specific parameters, particularly regarding the probability of occurrence and the potential consequences. Starting from this, statistical models of the systems under assessment and the functionally linked sub-systems of the machine can be prepared by putting the failure probability of the individual components in relation with each other. The results can be used to determine the hazardous failure probability for the entire machine or sub-system. The risk rating is calculated using this probability and the possible consequences of an event. It shows whether a risk is still tolerable or whether it must be reduced by taking suitable measures (see Figure 1). The matrix maps the risk according to its consequences and probability of occurrence. According to this matrix, the risk of an event is unacceptably high if it has a high probability of occurrence and involves considerable consequences for physical well-being. These risks call for design-related technological and organisational precautions to reduce consequences and the probability of occurrence to an acceptable residual risk.
The company discussed all these aspects related to the functional safety of machinery with TÜV SÜD and summarised the results in a process manual. The manual includes the machine manufacturer‘s company-specific know-how and will, in the future, be used as part of a machinery development guide. A functional safety management representative will ensure the manual is continuously updated and accessible to all employees. Based on its in-depth analysis of the issue, the company now knows which standards and directives must be monitored for changes and revisions and documented in the manual.
Management of functional safety also involves alignment of the company’s internal processes and clarification and definition of responsibilities and competencies. Similarly to other management systems, such as quality or environmental management systems, the new organisational structures and information processes ensure that employees focus on functional safety at every step of product development – from planning and design to manufacturing. In addition, employees in the purchasing and sales departments also benefit from this know-how in areas such as the procurement of safety-relevant components or machinery exports to countries with different machinery safety requirements and provisions. The management system provides the company with an efficient tool that enables it to implement risk assessment quickly, safely and cost-efficiently, also for new products, and tailor this assessment to the respective target market.
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