Innovative Online Software Speeds Machine Safety Compliance
As of the end of 2011, the machine safety standard EN954-1 has been effectively replaced by EN/ISO 13849-1. The new standard addresses the latest technologies not covered by 954-1, imposing new demands on machine manufacturers under the EU Machinery Directive.
The replacement occurred because EN 954-1 no longer represented the current state of technology. There was no consideration for direct connection between risk minimization, category, or complexity. Also time response (e.g., testing intervals, life cycles) and the failure probability of components (e.g., Common Cause Failures) were not included.
ISO 13849-1 addresses the programmable electronic safety devices that are being used increasingly in modern machines. The standard also provides a quantitative approach to risk assessment and safety validation. It makes sure that safety is not solely a matter of component reliability, but also uses common-sense safety principles such as redundancy, diversity, and fail-safe behavior.
The new standard describes assessment of complete safety functions. It also defines the requirements for the maximum permissible probability of dangerous failure for a safety function, and includes robust design as part of quantitative requirements. “The older standard was more of a product standard rather than one that addresses safety systems,” says John D’Silva of Siemens Industry, Inc. “ISO 13849-1 takes the entire application into consideration.”
Under the new standard, the risk assessment for a given safety function will yield a Performance Level (PL). This helps eliminate both over- and under-engineering, a costly and possibly risky result of 954-1’s limitations.
The requirements of 13849-1 are significantly different from 954-1. The old standard was essentially a simple risk assessment. Based on that assessment, the requisite safety level was determined. With the new standard, the requirements are more complex. Customers are required to perform a host of safety calculations based on safety data from manufacturers, the products being used, and the specific design of the application.
“It is difficult and time consuming for the customers to do these calculations manually; however, with the possibility of human errors, requirements may not be met and unexpected costs could be incurred,” says D’Silva. “So software tools have been developed that allow customers to select configurations and indicate the products to be used; then the software tool does the calculations to determine whether the required safety levels are met.”
Currently the two principal software tools available to customers are Siemens’ Safety Evaluation Tool (SET) and the IFA’s Sistema software utility, both available at no charge.
The Siemens Safety Evaluation Tool (SET)
The Safety Evaluation Tool provides valuable support with the rapid and easy assessment of safety functions in machines and systems. This TÜV-tested, online tool offers step-by-step user guidance, from specification of the safety system’s structure, to component selection, to the determination of the attained safety integrity in accordance with ISO 13849-1 and IEC 62061. SET is supported by comprehensive, integrated libraries. As an end result, the user is also provided with a standard-compliant report that can be integrated into the documentation as safety proof.
Online access to the Safety Evaluation Tool ensures that the calculations are always carried out on the basis of the currently applicable standards and the continuously updated technical data of the safety-relevant components (used within the tool).
How to Use the Tool
Using the tool is like a five-step process:
- Linking online to the Safety Evaluation Tool
- Creating new projects or uploading existing programs
- Inserting Safety Functions or sub-systems from the tool’s libraries
- Then editing the sensor logic and actuator group to meet the compliance needed
- Receiving a standard-compliant report that can be integrated in the documentation as a proof of safety
When first calling up the Safety Evaluation Tool, the user is requested to register. Following registration, the user will receive an e-mail containing access data (login name and password) to start using the SET. The subsequent first-time registration process asks some questions for information purposes only. The user interface for SET is very simple and user-friendly; it provides drop-down menus for file, project, and edit functions; project documentation; and various other logistical options.
To create a new project, the user selects the applicable standard for the project in the automatically opened dialog and then clicks “OK.” (“Project” refers to the summarization of one or several safety areas and safety functions of a system or machine.)
The information shown on the project has to be entered in the tool by clicking on “new safety area.” “Safety area” refers to a grouping of several safety functions of a project or system. At least one safety area is required. A safety area helps to structure a machine to assign the safety functions to specific system sections. Click on “new safety function” to assign these.
Prior to creating the safety function, its layout needs to be defined. The term “safety function” refers to a summarization of the individual sub-systems or SRP/CS under detection, evaluation, or reaction. The safety function’s safety integrity now needs to be selected in accordance with the selected standard.
Alternatively, the user can define the required SIL (or PL) directly with SET. SIL or PL settings will be displayed also in the standard-compliant report (Step 5).
The main areas SILs cover are severity of the possible harm, frequency, and duration of exposure; probability of occurrence of a hazardous event; and probability of avoiding or limiting the hazard to derive the SIL level. These are selectable based on the guidance chart provided in the tool.
When using the ISO 13849 standard, an integrated risk graph helps determine the PL level. PL takes into consideration severity of injury, frequency or exposure time to the hazard, and the possibility of avoiding the hazard or limiting the harm.
The values for an input group are set next (e.g., number of channels, SIL levels, type of safety input), as well as values for estimated test cycles. After setting the values, SET shows whether a SIL or PL has been reached.
“The SET is a valuable tool because of its intuitiveness and ease of use,” concludes D’Silva. “With the new standards, a great many calculations are needed, and this can be frustrating. SET and Sistema help minimize that frustration by simplifying and automating the standards verification process. It all comes at no charge, which in itself is a good step towards driving down some of the anxiety that naturally accompanies change.”
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