FPEeXTRA Issue 75

UL 268: Separating Fact from Fiction

By: Maria Marks, CFPS, SET

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What is UL 268, and why are we concerned about changes to this standard? Simply stated, UL 268 Smoke Detectors for Fire Alarm Systems is the Underwriters Laboratories safety standard for smoke detectors connected to fire alarm systems. There is a separate document UL 217 Standard for Smoke Alarms, for stand-alone devices that detect and sound an integral horn and are typically found in residential occupancies. 

So what has changed? First, the materials we have in our homes and offices today. The UL 268 6th and UL 217 7th edition testing standards were based on a legacy room that was filled with natural materials such as wood, linen, cotton, and silk. Today’s modern rooms contain more synthetic materials such as polyurethane foam, polyester, nylon, and engineered wood. In a legacy room, a fire can take up to 25 - 30 minutes for flashover to occur. In a modern room, that time is shortened drastically, with flashover occurring in 3 - 4 minutes. With the change in common furnishings and building materials, egress or escape time has gone from 17 minutes in 1970 to 3 minutes today. Due to this decrease in time available for egress, it is necessary to ensure we have early warning while minimizing nuisance events. The new 7th edition 268 standard will narrow the detector sensitivity range to provide the early warning for notification and occupants.

The second major change to the UL 268 standard was to address nuisance alarms. NFPA 72 2022 edition defines a nuisance alarm as “An unwanted activation of a signaling system or an alarm initiating device in response to a stimulus or condition that is not the result of a potentially hazardous condition.” These changes were made to the existing UL 268 and UL 217 standards based on the results of several studies including the “Smoke Characterization Project” done by UL for the NFPA Fire Protection Research Foundation. This project had the following objectives: Develop smoke test protocols on selected materials in residential settings, develop smoke particle size distribution data and profiles for non-flaming and flaming modes of combustion, and provide data / analysis for potential initiatives. These initiatives included recommendations for The Standard for Smoke Alarms, UL 217, as well as providing data on materials and additives that could impact the advancement of technology for smoke sensing devices and smoke suppression.  

The most significant of the changes to UL 268 are the additions of three fire tests: one based on cooking nuisance, and the other two based on polyurethane foam fire tests. Detection technologies for years had been based on either photoelectric or ionization principles. Ionization devices monitor the change in current created when smoke particles impact the ionized air between electrically charged plates. Photoelectric detectors detect the scattering or obscuration of light due to the presence of smoke particles in the detection chamber. It is critical that the engineer of record select the appropriate detector based on the space to be protected. However, these changes to materials in the spaces led to challenges with detection. Newer devices are based on a multi-sensor or multi-criteria to meet the early warning requirements while providing system stability. The new tests that were added utilize polyurethane foam to verify that detectors respond sooner to flaming and smoldering synthetic materials. They are also focused on nuisance alarm resistance to provide detectors that are less sensitive to nuisance events such as burned toast or hamburgers. These tests compress the sensitivity window by 70% from 3.5%/FT to 1.3%/FT. 

This shift in sensitivity will challenge single sensor technology devices. Multi-sensor detectors will become the dominant solution as they are able to provide the early warning and minimize nuisance alarms.

In summary, the change from UL 268 6th edition to 7th is the biggest change to UL test standards for smoke detectors in the past 25 years. It was necessary due to the changes in materials in our homes and businesses. The new tests are focused on nuisance avoidance and providing a quicker response to smoldering synthetic materials. In today’s built environment, the furnishings and building materials used on a regular basis burn much faster. Occupant egress time has decreased from 17 minutes in 1970 to just 3 minutes. The updated standards will provide notification to occupants in a timely manner while minimizing nuisance events.

Maria Marks, CFPS, SET is with Siemens