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Welcome to the minefield
Festo safety consultant Klaus Gabriel outlines the challenge of complying with the new Safety standard EN ISO 13849-1 and examines risk assessment and safety function implementation.
Regardless of whether or not the new EN ISO 13849-1 safety standard replaces the existing EN 954-1 standard on December 29 - and the jury's out until the European Committee reconsiders the issue in early December - machine designers and builders would do well to honour its intentions straightaway. Throughout the EU, machine builders have to comply with the new Machinery Directive 2006/42/EC from the end of 2009. They will only benefit from the presumption of conformity to the directive if they adhere to the relevant standards.
According to Machinery Directive 2006/42/EC, machines and production equipment must be designed in such a way that people, animals, objects and the environment are protected from injury or damage. All manufacturers of machines and production equipment are thus obliged to carry out risk assessments and, based on these, to create the protective measures necessary for each individual machine. Risk assessment must be carried out using the approved method detailed in EN ISO 1050/14121. This standard defines the method of risk analysis, in which the first step is to determine the machine's performance limits:
• Initial position/standstill
• Normal operation
• Set-up and service operation
• Emergency operation
The second step is to identify each and every potential hazard and complete a risk estimation. Once this has been completed, the designer can start the three steps of risk reduction:
• Design measures e.g. inherent safety (EN 12100-2)
• Technical and supplementary safety measures (DIN EN ISO 13849-1)
• User information on the machine and in the user manual (EN 12100-2)
Safety standard DIN EN ISO 13849-1 provides a method of implementing technical safety measures for all types of machines, regardless of their technology make-up and how they are powered - ie it covers electrical, pneumatic, hydraulic and mechanical systems. The standard enables a machine performance level (PL) to be determined. Technical measures can then be put into place and safety products evaluated to determine whether or not they reach the required PL, according to their:
1. Control architecture (Category B, 1, 2, 3 or 4)
2. Mean Time To Failure (MTTF) - derived for mechanical components from a B10 life cycle value
3. Diagnostic Coverage (DC)
4. Common Cause Failure (CCF)
Festo, for example, can provide customers with B10 figures for all its mechanical components, together with MTTF data for the electrical parts of its components.
As a practical example, consider a machine comprising five modules, or stations, each of which performs a different type of operation and presents one or more potential safety hazards. Once all design measures have been implemented, it is the job of the designer to then consider technical safety measures in the form of inputs, logic components and outputs. Festo has identified ten functions where pneumatic safety can be implemented on machines, as shown in the diagram above, and has developed specific products - such as valve manifolds with built-in proximity sensors for position sensing/movement reversal and fail-safe brake units for pneumatic actuators - together with a range of services to make it easier for users to follow safety.
Suppose one module is a cutting station which accepts sheeting from the preceding station and cuts it into individual workpieces, which are then separated. This work operation represents two potential hazards: cuts and impacts. The job of the designer is thus to define suitable safety functions and implement these with the aid of EN ISO 13849-1.
Safe exhaust function
How can the required reduction of risk be achieved on the cutting station? One way would be with an isolating safety device on a guard door, with an integrated 'safe exhaust' valve manifold. If the guard door is opened, this device, switches the drives reliably to a zero-energy state. This allows safe intervention to remove a material jam. To achieve a higher category, a safety valve such as the Festo MS6-SV could be fitted. This electrically-operated pneumatic soft-start and exhaust valve provides safer start-up and quick and reliable venting of compressed air supplies in the event of an emergency. It is fully compliant with EN ISO 13849-1 and is certified to Performance Level e, categories 3 and 4.
The MS6-SV is a high reliability, self-testing mechatronic component, with built-in safety redundancy; the design incorporates parallel valves to ensure fail-safe exhaust of the system, even in the event of one of the valves developing a fault. Fully integrated sensors regularly monitor the physical position of the valve components to confirm the valve's condition and status, and built-in self-test facilities automatically check the safety functions at system power-up and every hour thereafter. Overall system safety is further enhanced by the MS6-SV's soft-start function whereby the gradual - and adjustable - pressurisation of the system ensures that pre-exhausted actuators move steadily to their start positions without shocks or impacts that could damage machinery or operators.
Complying with Machinery Directive 2006/42/EC and the EN ISO 13849-1 safety standard does not need to be an excessively onerous task. By eliciting the help of a reputable automation component supplier at the outset, designers and builders can create machines that are significantly safer than they are today, and further contribute to the production world's ideal of zero machine-related accidents.
To make the process easier, the BIS (Department for Business Innovation and Skills) and leading automation companies have produced Guidance Notes and further advice on the regulations and safety measures. Festo has produced a comprehensive 84-page booklet, entitled 'Safety Engineering Guidelines', together with an informative poster, to assist machine builders with the evaluation and assessment of safety measures; copies of these can be requested via the company's website at www.festo.co.uk, and the guidelines can also be downloaded in PDF format.
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