Thus the designation SIL, determined to a system, refers automatically on a function(s) of the E/E/EP system(Electrical/Electronics/Electronics Programmable), for then to be extended, by conseption, to the hardware(s) and/or software(s) component(s) who participate in that function(s).

More specifically, if one considers a simplified digital telephone system , the most important functions are:

• to call,
• to receive a call

Thus the level of confidence associated with this system resides in fact, on the successful implementation of its functions. Then, these functions are, by conseption, spread over different components of the system which can be software (signal processing software, the software management of the display,..), or hardware (the keyboard, the LCD screen, the electronic board, power supply,…).

Thus the safet integrity level SIL automatically refers on, a function of a system, which itself refers automatically on hardware or software, and this, of course in the case of systems E/E/EP. One will speak therefore of safety integrity of a function, safety integrityintegrity of security of an equipment and safety integrity of a software, with:

• The Safety integrity of hardware is to define the part of the safety integrity linked to random failures of the equipment which could lead to an dangerous event.
• The safety integrity of software is to define the probability, for a software, in a programmable electronic system , to execute correctly its safety related functions in all the specified conditions.