The determination of safety integrity levels (SIL) for E / E / PE (electrical / electronic / programmable electronic) is performed by HINTSW - T & T Systems according to the concepts, definitions and methods, both quantitative and qualitative, given in IEC 61508-5.
The safety integrity, in IEC 61508-4, is defined as the probability that a relative safety system performs satisfactorily security functions required in all of the specified conditions, and within a specified period of time. IEC 61508-5 also introduces the concept of tolerable and necessary risk reduction risk, defining that the determination of acceptable risk for a given hazardous event has the aim to establish what is considered acceptable regarding the frequency (or probability) of occurrence of a dangerous situation and about the severity of its specific consequences; the safety-related systems are therefore designed to reduce the frequency (or probability) of the dangerous event occurrence and / or its consequences.
Consequently, the safety-related systems must be designed so that:
• the frequency of failure of safety related systems is low enough to prevent the frequency of occurrence of a dangerous situation exceed the value necessary to achieve the tolerable risk level set, and / or
• the systems related to safety are of sufficient action on the consequences of failure to reach the tolerable level of risk attached.
Therefore, as has been said, fixing the "amount" of risk reduction required to charge a system E / E / PE is equivalent to fix the level of safety integrity.
IEC 61508-5 specifies that the integrity of security consists of two elements:
• integrity of the hardware security; this integrity part of the safety devices is related to the hardware random failures for dangerous failure modes. The obtaining of a predetermined integrity level can they estimated with sufficient accuracy by means of statistical methods and by means of the usual rules of combination of probability;
• systematic safety integrity; this integrity part of the safety devices is related to systematic faults for dangerous failure modes. In theory, even in these cases it would be possible to make quantitative estimates, however, in practice this is often complicated because it is difficult to predict the distribution of systematic failures that depend on design faults and / or by common cause failures, with the effect of increasing the uncertainty in the calculation. In these cases you must use special techniques to reduce the uncertainty of the calculation.
Starting from the concepts and the definitions mentioned above, for the determination of the SIL, the IEC 61508-5 standard proposes a quantitative method based on the calculation of the probability of dangerous Average fault, which should be lower than, or at most equal, to the reduction of risk minimum required, and two qualitative methods, one based on the graph of the risk, and the other on the matrix of the severity of dangerous situations.
HINTSW - T & T Systems, according to the application and the field of application, is able to effectively apply all the aforesaid methods to determine the SIL of a system E / E / PE safety-related