SIL (Safety Integrity Level) is a measure of safety for electrical, electronic and programmable electronic equipment. The SIL level is described by the measure of the number of actuations to the occurrence of fault defined by THR (tolerable hazard rate).
Under the directives 94/9/EC and 1999/92/EC (ATEX), gas detection equipment requires only electrical safety certification and, in some cases, parameters compliance verification. Creating an explosion protection plan for the existing individual working conditions may involve measures outside the scope of ATEX requirements for stationary gas detection equipment. The purpose of this additional safety evaluation is the reduction of risk in all applications in which incorrect behaviour results in a hazard to the safety of persons, environments and material property.
The Safety Integrity Level (SIL) is defined by four values. The higher the integrity value, the more reliable the functional circuit. Gas detection devices usually score between SIL 1 and SIL 3.
The products used in safety systems must exhibit a high level of equipment and software reliability, so that undetectable faults are only possible with a very low degree of probability. The benchmark for systems’ safety evaluation is the IEC/EN 61508 standard, also applied for risk assessment in the international process control engineering. This certification for system safety defines the requirements for availability of safety functions and probability of failure of the tested system, in accordance with the process control engineering requirements.
The EN 61508 standard, as a basic standard independent of the application, only specifies general requirements for the components and complete systems with safety functionalities. This is why it is not applicable to gas detection equipment which must also comply with other safety criteria. For this reason, also the EN 50271 and EN 50402 standards are applicable for gas detection equipment.