Each safety-related function must be implemented with a defined safety integrity level (SIL) if the control system implements safety-related functions (SFs) in addition to the standard control functions. The required SIL of the SF depends on the quantity of the risk associated with the failure of this one SF. The SIL against random failure can be expressed through the dangerous failure rate of the SF for an electronic safety-related control system (ESRCS) operating in a continuous mode of operation. The proof must be provided (among other things) that the SIL requirements for the individual SFs are met so the ESRCS can be accepted and implemented. The assessment of the impact of random failures on the SIL of the SF must be performed using the quantitative analysis method. This paper describes the procedure and derives equations for evaluating the impact of random failure on SIL of the SF using Markov chains with two absorption states. The achieved results are presented for SF implemented by ESRCS with dual architecture based on composite fail-safety technique.
One of the basic features of a safety function is the safety integrity level. This feature also affects the technical solution of the system that implements the safety function. One of the parameters that affect the safety integrity of a safetyrelevant system is its recovery to the original state after a failure has occurred. The method of recovery a safety-relevant system after the occurrence of a failure to its original state depends not only on its technical solution, but also on the method of its operation. The paper deals with the influence of various methods of recovery on the safety integrity of the safety function, which is implemented by an electronic safety-relevant system with a 2oo3 architecture.
In many applications, a light curtain is used to detect the intrusion of a human (or other protected object) into the danger zone. After detecting the interruption of the light curtain, a safety function is activated to bring the danger zone into a safe state (nonhuman). If the space taken by the light curtain is at the same time a working space, it is necessary to distinguish whether the light curtain interrupt is caused by man or material (material flows cannot cause the dangerous zone to be put into a safe state). There are several ways how to achieve this resolution, but additional (optional) sensors are commonly used. The paper deals with the parameters of additional sensors and how to connect them to the achieved Safety Integrity Level (SIL) of the safety functions realized by the help of the light curtain.
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