Which detector type is generally more responsive to smoldering fires, and why?

• A. Photoelectric detectors respond quickly to smoldering fires because they detect visible smoke sooner than ionization detectors.
• B. Ionization detectors respond more quickly to smoldering fires because they detect small particles of smoke.
• C. Photoelectric detectors detect heat rather than smoke.
• D. Ionization detectors are always superior in all fire conditions.

Answer: (A)

When a fire smolders, it produces a lot of smoke with visible particles that linger before flames appear. Detectors that sense smoke by light scattering respond quickly in this situation because the smoke particles scatter the detector’s light into the sensing chamber, triggering the alarm sooner. Photoelectric detectors use this light-scattering principle, so they’re generally more responsive to smoldering fires than ionization detectors, which depend on changes in ion flow from combustion byproducts and tend to react faster to flaming fires with small particles.

The idea that photoelectric detectors detect heat is incorrect—they detect smoke, not heat. Conversely, the notion that ionization detectors are always superior in all fire conditions isn’t accurate, since they’re more sensitive to flaming fires than to smoldering ones.

So the best choice reflects that photoelectric detectors are typically quicker to respond to smoldering fires due to their smoke-detection method via light scattering.