The calculation of the probability of dangerous failure of a generic system E / E / PE (electrical / electronic / programmable electronic) is done by HINTSW -T & T Systems in accordance with the requirements contained in IEC 61508-2 standard, according to the methods and techniques described in IEC 61508-6 standard.
In IEC 61508-2, the assertion that the goal of safety integrity has been achieved with regard to the dangerous random hardware failures of a system E / E / PE, is based on the following points:
• The tolerance requirements in failure, reported in the same Standard;
• The diagnostic coverage and frequency of periodic tests of subsystems and components, combined with the execution of a reliability analysis using suiteble data.
IT Is clear that the above steps are based on design features such as fault detection and redundancy of architectural features (for example the diversity).
IEC 61508-2 also indicates the functional steps by which you can achieve the safety integrity; in particular, as regards the two previous points, the standard indicates the following steps:
• Architecture definition of logical subsystems of the sensors and actuators.
• Development of a model for the hardware architecture of the system E / E / PE. The model must be developed separately examining each safety function and identifying the subsystems and components that must be used to take charge of these functions.
• Specification, for each subsystem, the following parameters:
or the time between the diagnostic tests for failures that are not automatically detected;
or the average duration of recovery;
or the degree of diagnostic coverage;
or the probability of failure;
or the fraction of safe failures.
• Identification of architectural hardware constraints.
• Creation of a reliability model for each safety function.
• Calculating a reliability forecast for each safety function.
HINTSW - T & T Systems has the know-how and the necessary skill to determine the parameters (range of diagnostic tests, level of diagnostic coverage, average time to restore, etc.), and to perform the calculations (probability of dangerous failure,; reliability model, prediction of reliability) specified by the standard to determine whether the hardware designed for the system E / E / PE meets the objective of integrity of the fixed security. The calculations can be performed for all hardware facilities proposed by the standard: 1oo1, 1oo2, 2oo2, 2oo3 and 1oo2D.