A colleague reminded me recently that too many designers, contractors and authorities having jurisdiction do not understand detector limitations. Many devices can be used to detect fires, but some are quicker than others. Do you know how much smoke it takes for a spot-type smoke detector to operate? Some would quickly answer 2%–4% obscuration. But what does that percentage really mean?

Throughout NFPA 72, National Fire Alarm and Signaling Code, smoke detector sensitivity is described in terms of the percent obscuration required to alarm or produce a signal. As Annex A states, “Smoke detectors are tested using various smoke sources that have different characteristics (e.g., color, particle size, number of particles, particle shape).” 

The confusion is that smoke detectors are normally described in manufacturers’ literature as detecting a certain percent obscuration produced using a specific type of gray smoke. Actual detector response in a room will vary depending on the smoke’s characteristics. Typically, actual smoke conditions are different than what is used in testing detector sensitivity.

Again, from Annex A: “The percent per foot sensitivity marked on the smoke detector is derived from testing in a smoke chamber, usually referred to as the ANSI/UL 268 Smoke Box. These measurements are only valid in the context of the apparatus and cannot be used outside the context of the smoke box. The operating point of a spot-type smoke detector is measured with certain prevailing conditions such as no air movement in the space, a measured distance from the fire source and specific materials used for the fire source.”

None of that information is provided in the specifications for the room where it will be installed. As ceiling height increases, the detection of the smoke emanating from a fire source will take longer. Depending on the air movement, detection may be delayed to a point where it is too late for early warning. It is imperative to understand what early warning means.

The goal of fire protection is to provide enough time after detection for people to safely evacuate the building. I suspect that not many people can assess what that time is or should be because every scenario is different. 

You may assume that using the most sensitive detection device will always provide safe evacuation time. This is wrong. 

Ionization versus photoelectric

To provide the correct detection device for the right detection time, you need to know—at minimum—the size of the space, ceiling height, type of detector used (ionization or photoelectric), combustible loading in the space (what’s going to burn), airflow and distance the smoke must travel to get to a detector.

Although ionization and photoelectric smoke detectors must meet the requirements of the sixth edition of the ANSI/UL 268, Standard for Smoke Detectors for Fire Alarm Systems, they respond differently to the smoke’s particle size when it reaches the detector. Particles of combustion are different depending on what is burning.

Tests have shown that ionization detectors are most responsive to smaller, invisible submicron-sized particles found in flaming fires. Photoelectric smoke detectors are most responsive to larger, visible particles in low-energy smoldering fires.

Airflow in a room can cause smoke stratification that can hinder air containing smoke particles or gaseous combustion products from reaching ceiling-mounted smoke detectors or carbon monoxide detectors.

Stratification also happens when the smoke must travel a long distance to the detector, cooling on the way. The smoke will also become less dense as it rises and will reach a level where there is no temperature difference compared to the air.

Getting smoke from the fire to the detector happens quicker in fast-burning fires. Smoldering or low-energy fires will be more difficult to detect and be affected by airflow.

Early detection

Early detection requires more spot-type smoke detectors or a different type such as a linear beam or active sampling smoke detector.

Some designers have attempted to use duct-mounted (passive air sampling) smoke or carbon monoxide detectors to provide open area smoke detection. The assumption is that the air, and therefore smoke, will be drawn into the HVAC system, allowing for detection in the duct. This would save the owner the installation cost of the spot type smoke detectors. The code states in Section 17.7.4.3 that “detectors placed in environmental air ducts or plenums shall not be used as a substitute for open area detectors.”

We use duct smoke detectors to shut down HVAC systems or initiate smoke management. As smoke is drawing into the duct system it is mixed with the air in the duct, thus diluting the smoke as it travels through the duct to the duct smoke detector. Smoke from the room or space served by the HVAC system may not be drawn into the duct when the ventilating system is shut down.

Know the limitations of the smoke detector that you choose. Don’t be driven into a poor design choice based on saving money for the owner.

stock.adobe.com / Oleksandr