Associate Safety Professional Practice Exam

Which fire detection system experiences thermal lag?

1. Ionization detectors

2. Fixed temperature

3. Smoke detectors

4. Photoelectric detectors

The correct answer is: Fixed temperature

The fixed temperature fire detection system is characterized by thermal lag due to its reliance on the temperature reaching a specific predetermined level before activation. This type of system utilizes a heat-sensitive element, such as a bimetallic strip or a fusible link, which only responds once the ambient temperature exceeds the set threshold, indicating the presence of a fire. This inherent property means that there is a delay—thermal lag—between the initial heat generation of a fire and the system's response, as the temperature must accumulate sufficiently to trigger the alert or alarm. In contrast, ionization and photoelectric detectors are designed to respond more rapidly to changes in their environment. Ionization detectors react to the presence of smoke particles, while photoelectric detectors use a beam of light that can be obscured by smoke. Smoke detectors, as a category, operate based on the detection of smoke rather than temperature and thus do not experience thermal lag in the same manner as fixed temperature systems. Understanding the specific mechanisms of each type of detection system reveals why fixed temperature detectors are the only ones that experience thermal lag.