Level of confidence of a safety barrier:

The probability of failure on demand to perform properly a required safety function according to a given effectiveness and response time under all the stated conditions within a stated period of time. This notion is similar to the notion of SIL (Safety Integrity Level) defined in IEC 61511 for Safety Instrumented Systems but applies here to all types of safety barriers. The "design" level of confidence is assessed with the help of instruction given in Appendix 8 of the IEC standard. The barrier is assumed to be maintained so that it is as efficient as when it was installed, to have the same response time and the same level of confidence or probability of failure on demand. The "operational" level of confidence includes the influence of the safety management system. The value could be lower than the "design" one if some problems are identified during the audit of the safety management system.