EN ISO 13849, divided into Parts 1 and 2, is a functional safety standard, in particular for the safety of machinery, which specifies the general principles for the design of parts of safety-related control systems (including software) and for their validation.
Basic principles of EN ISO 13849
The main purpose of ISO 13849-1, General principles for design, and ISO 13849-2, Validation, is to regulate the level of performance that the safety-related parts of control systems of a machine must have in order to be considered a suitable mitigation measure of a specific risk.
The ISO 13849-1/2 standards are type B1 (ISO 12100-1), and harmonized with the Machinery Directive 2006/42/EC.
Designers often choose to achieve a given risk reduction measure through the application of means of protection that involve one or more safety functions. Parts of control systems intended to provide safety functions are referred to as safety-related parts of control systems (SRP/CS).
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The ability of SRP/CS to perform a safety function is assigned to one of five levels, called Performance Levels, PL a, b, c, d, e, defined in terms of the probability of dangerous failure per hour.
The likelihood of a hazardous failure depends on numerous factors, such as hardware and software structure, diagnostic coverage, component reliability, common cause failure, design process, operating stresses, environmental conditions and operating procedures.
For the evaluation of the PL, the standard uses a methodology based on the subdivision into categories assigned to one of five levels, called Categories B, 1, 2, 3 and 4.
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Field of application of EN ISO 13849-1/2
The SRP/CS are the “true” field of application of the standard. Anything that cannot be defined as part of the safety-related control system is not covered by the standard.
According to ISO 13849-1, an SRP/CS is part of a control system that responds to safety-related input signals and generates safety-related output signals.
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Machinery Directive and ISO 13849
The need to provide design requirements for the construction of safe and reliable control systems arises directly from the Machinery Directive, which in requirement 1.2 focuses on control systems.
In particular, requirement 1.2.1 deals with the safety and reliability of the control systems, and specifies that:
“The control systems must be designed and constructed in such a way as to prevent hazardous situations from arising.”
Above all, they must be designed and constructed in such a way that:
- they can withstand the intended operating stresses and external influences,
- a fault in the hardware or the software of the control system does not lead to hazardous situations,
- errors in the control system logic do not lead to hazardous situations,
- reasonably foreseeable human error during operation does not lead to hazardous situations.
These points, and in particular the one on the detection and management of faults, are given special attention in ISO 13849-1, because they are decisive in obtaining the PL.
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