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Fundamentals of thermal protective performance
It is important to keep the measurement techniques as they are now
It has often been said that if there is one property that firefighters feel they know about, it is thermal protective performance, or what most of us refer as TPP. TPP is what defines the thermal insulation of structural firefighter protective clothing.
It is more likely to be specified by a fire department than any one single requirement, yet do we really understand what thermal protective performance means and how it should be applied?
The TPP numbers that the fire service has become accustomed to may change, and not for any particular reason other than to switch the test method referenced for its measurement. Therefore, it is worth revisiting this subject and discussing why it is important to keep the measurement techniques where they are today.
On the origin of TPP
There are multiple roots for how thermal protective performance testing came about. Originally, a test method was devised to measure insulation materials in clothing and a variety of materials.
The first method involved a material sample positioned horizontally over a single Bunsen burner with a special sensor called a calorimeter to measure the rate of heat transfer through the material. The measured energy that passed through the material predicted whether a second-degree burn would occur. This test was first developed for protection of industrial workers or military personnel against flash fire.
Fire service interest in TPP testing arose during Project FIRES in the early 1980s, which was the Firefighter Integrated Response Ensemble System work led by the International Association of Fire Fighters. This project culminated in defining the modern day firefighter protective ensemble of coat and pants.
Part of this development included the evolution of a TPP test for firefighter clothing. The single-flame burner was replaced with two burners angled upward toward the center face of the sample clothing composite — a panel of super heated quartz tubes was added between the two burners to provide a combined convective and radiant heat exposure.
While the overall heat level remained the same as a fire ball (flash fire for industrial workers), the heat composition in the modified test was changed to better approximate the emergency conditions for firefighters.
TPP rating defined
IAFF worked to identify appropriate levels of heat insulation in clothing by investigating fireground conditions and determining needed escape time. This research led to the current TPP rating of 35 that has been the minimum standard for firefighter clothing since its introduction in 1986. Incidentally, prior to that time, firefighter clothing insulation was defined only on the basis of the overall thickness of the composite (combination of material layers) used in the clothing.
The test result is based on the predicted elapsed time that will occur before an average individual will sustain a second-degree burn. The actual TPP rating is this time multiplied by the energy level of the test exposure.
In this case, the exposure level is at 2.0 calories per square centimeter per second. This represents the amount of heat energy that a firefighter would face while being in the same room during a flashover or backdraft.
Thus, the minimum TPP rating of 35 is 2.0 cal/cm2-sec times 17.5 seconds. The 17.5 seconds is the time that a firefighter would have to escape from the environment under that exposure condition before sustaining second-degree burns.
What makes TPP important
For many years, the TPP rating was the sole measurement of firefighter protective clothing. It has been the benchmark for how the various material layers — outer shell, moisture barrier and thermal barrier — are combined to form the primary three-layer composite used in turnout gear construction.
The idea has been that clothing with a TPP rating of 35 or more will provide adequate protection for firefighters who find themselves in dire situations and permit escape with survival. This level of clothing performance does not mean that the firefighter will not be burned or that his or her clothing will remain undamaged.
On the contrary, this minimum rating is in essence the basis for protecting a firefighter during the infrequent emergency conditions that some firefighters experience. It comes with the expectation that in providing that protection, the clothing will be charred and show other forms of high heat damage.
Very few departments gauge the thermal insulation of their gear on the basis of the minimum TPP rating of 35. Instead, most organizations opt for a higher rating to provide a better level of protection.
The TPP rating is generally balanced with the stress-related features of structural firefighting protective clothing. Without this thinking, one natural response would be simply to specify an increasingly higher TPP rating for improved protection.
However, this comes with the penalty that the gear becomes more bulky and heavier that in turn creates physiological stress on the wearer. Since 2000, TPP ratings have been compared with a property called total heat loss that measures these stress related effects for the same three-layer composite.
Firefighters have become accustomed to their clothing constructed at a specific TPP level. Through training and actual use during structural fires, they learn how long they can be in a certain situation and what their respective TPP value is for the clothing they wear.
Limitations of TPP ratings
TPP is not the end all in thermal insulation. As the test is designed to mimic the high end of the fireground exposure conditions, it is only a snapshot of the total range of conditions that a firefighter can face.
Fireground conditions are dynamic, but the test is static using the exact same condition over the total time of the test. TPP testing does not account for the various factors that can affect insulation — clothing condition, levels of moisture, fit of the clothing on the individual, and any of the myriad ways that the heat exposure can occur.
To use the basis of the TPP rating of 17.5 seconds as a safe time for having protection is not only ill-fated assumption, but one that would result in a firefighters getting burned or worse.
The group responsible for writing the NFPA 1971 standard which includes the TPP test has labored for years to identify other tests that will provide improved protection to firefighters. Concepts such as compression of clothing in reinforced areas (knees and shoulder) and heat storage during prolonged exposures to moderate levels of radiant heat have been added to NFPA 1971 to supplement how thermal protection is provided.
In its attempt to modernize and improve some of the test methods applied throughout NFPA 1971, the committee has decided to replace the TPP method based with a newer method from ASTM International. While this change appears innocuous on the surface, it was recently learned that the new method will result in an average decrease in the TPP ratings of 2 to 3 for exact same materials.
This means that the current 35 becomes a 32 with a similar offset in any TPP rating. The committee never intended such a change and there is no plan to move the TPP requirement downward. This discovery comes as a surprise to those involved in the standard development process.
The argument supporting the change is that the ASTM test method is better defined and more precise. Yet, it is different because it causes already compliant material composites that meet the 35 TPP rating requirement to drop 2-3 point due only to a difference in how the measurement is made.
In fact, even the individuals at the test laboratories routinely involved in this testing are still scratching their heads trying to figure out why this is so. The bigger problem is that the fire service has a long history in the current test and what these TPP values means in terms of their expectations for thermal performance.
This history and fire service understanding of TPP is about to become undermined.
Potential improvements should be investigated. But changes in a standard should never be implemented until the changes are fully validated through extensive inter-laboratory testing and the changes are determined to have the intended effect or benefit to the fire service. Modifications to a standard, which have not been vetted should always be postponed until the fundamentals of the test or change are fully understood.
In this case, the TPP test and the associated rating work as intended.