Effectiveness of a safety barrier

The effectiveness is the ability for a technical safety barrier to perform a safety function for the duration of an undesired event, in a non- degraded mode, for a pre-specified set of circumstances. The effectiveness is either a percentage or a probability of that the defined safety function will perform as intended. If the effectiveness is expressed as a percentage, it may vary during the operating time of the safety barrier. For example, a valve that does not completely close on demand would not have an effectiveness of 100%.