(29b) Use and Misuse of Enabling Conditions and Conditional Modifiers in Layers of Protection Analysis

Enabling conditions and conditional modifiers are some of the most contentious factors used in Layer of Protection Analysis. Enabling conditions are situations which must occur simultaneous with a given initiating event to allow the specific cause for a scenario to propagate to a consequence of interest. Some of the most common enabling conditions are related to modes of operation such as start up and shut down of a plant or unit operation. Often an analyst will use an enabling condition when a particular scenario requires the simultaneous occurrence of two initiating event failures.

Conditional modifiers are normally defined as the three independent factors of probability of ignition, probability of occupancy, and probability of injury. Application of each of these conditional modifiers is only applicable when LOPA is used to evaluate the potential for injury from a given initiating event. If a company's risk criteria are based on release of material from primary containment and not the subsequent impact to personnel, then conditional modifiers are not used in the LOPA method.

Inclusion of enabling conditions and conditional modifiers in the LOPA method allow more accurate modeling of the risk of a given scenario from a life safety perspective. However, use of these factors is subject to potential pitfalls and misuse. Avoiding improper use of these important factors can help to prevent gross misestimates of the risk of events and assist in the safe design and operation of facilities for which LOPA is used for the risk analysis.