Abstract This paper presents experiences from a reliability analysis, performed by Norsk Hydro Research Centre. Heat detectors are a part of an Emergency Shut Down (ESD) system. Generally, there is in ESD systems, a conflict between degree of safety and production regularity, when production irregularities are caused by spurious trips (false alarms). The frequency of spurious trips can be reduced by employing voting logic for the detectors. The paper focuses on the safety assessment of large heat detector systems. It is a rather common situation to carry out voting of a large number of detectors being spread over a wide ‘physical’ fire area. This situation introduces a problem when it comes to reliability modelling: ‘Should detectors, when there is a certain physical distance between their location, be considered as redundant components or not?’ Logical fire areas are introduced in order to make the reliability modelling practicable. Logical fire areas also define the level for safety assessment. A ranking of different voting logics, with respect to safety and production regularity, is presented in the paper.