Understanding the Thermal Protective Performance of Your PPE

If you’re aware of any test method behind the design and performance of your protective clothing, it is probably the thermal protective performance (TPP) test. This article will review the test, its limitations and possible improvements.

Understanding TPP
The TPP test was first included in the 1986 revision of NFPA 1971: Standard on Protective Ensembles for Structural Fire Fighting and Proximity Fire Fighting. Before 1986, thermal protection was measured by the thickness of the turnout gear’s three layers–outer shell, thermal barrier and moisture barrier. In 1986, however, new technology allowed us to measure the rate at which radiant and convective heat transfer through the three layers to a level that will predict a second-degree burn. The mechanism for doing this: the TPP test. As a result, thickness of turnouts was no longer considered an effective measure of the garment’s level of protection.

Simply stated, the TPP test apparatus is a bench-top configuration consisting of two burners that provide a convective/radiant heat source and a bank of quartz tubes that provide additional radiant heat. The three-layer composite sample is a 6″ square. Heat is applied at a rate of 84 kW/m², ± 2 kW/m², (2.0 cal/cm², ± 0.05 cal/cm²). The heat transfer is measured by a copper calorimeter.

The collected data is plotted on a time and temperature chart and applied over another pre-plotted line called the “Stoll Curve.” The name comes from burn-injury research conducted by Alice Stoll and Maria Chianta in the late 1950s and early 1960s at the Aerospace Medical Research Department, Naval Air Development Center. Some of the testing was performed on sailors (the story goes that sailors volunteered to get second-degree burns in exchange for weekend passes). Stoll and Chianta’s research led to the development of the Stoll Curve in 1960; it has been recognized internationally for measuring burn injuries ever since.

The point where the two plotted lines (time/temperature and the Stoll Curve) intersect is the predicted time to second-degree burns. To generate the TPP number, the predicted time to second-degree burn is multiplied by the heat exposure, which, in this case, is typically 2 cal/cm2 (roughly approximates flashover conditions).

Here’s an example: The minimum TPP rating for NFPA 1971-compliant coats and trousers is 35, which provides 17.5 seconds of protection until a second-degree burn is caused, in dry gear. Note: This is a very simplified description of the test. The test method is fully described in chapter 8.10 of NFPA 1971. As an indication of the specificity of the test method, the word “shall” appears 60 times in prescribing the test.  

TPP Test Limitations
Firefighters must understand that higher TPP numbers do not necessarily equate to better overall protection. First, the TPP test will not be accurate for any measurement above 60. Second, a composite rating of 60 would be an extremely heavy, cumbersome ensemble that would restrict mobility and add considerably to heat stress. For these reasons, most composite materials selected are in the upper 30 to lower 40 TPP range.

The TPP test is one of the most useful and widely accepted thermal protection tests in existence, and it has been a staple of PPE testing for over a quarter of a century. However, it has limitations:

  • The test is run on a flat surface, in a static environment, with a constant heat source–hardly representative of conditions in the field. Are firefighters flat? Do firefighters remain perfectly still while fighting fire? Is the temperature constant in a fire environment?
  • The test does not factor in all the other features of a set of turnout gear that exceed the basic three-layer composite, including but not limited to pockets, reflective trim, the stormflap, reinforcements, overlaps and the clothing worn beneath the turnout gear.
  • The test does not measure other factors, such as heat transfer in compressed areas, such as the areas beneath the SCBA shoulder straps.
  • The test does not measure heat transfer in wet gear.  
  • And, perhaps most importantly, it is strictly a component test (as are almost all of the PPE tests) versus an ensemble test.

The Perfect Test?
So if the current TPP test has limitations, what would a better test look like? The perfect test would test the entire ensemble (helmets, hoods, turnout coats and pants, gloves, boots and SCBA) as a PPE “envelope” on an instrumented manikin capable of movement, especially crawling, and would test the fire environment with temperature fluctuation and forced air movement.

Why is this test not readily available? Money–it takes a lot, and then some. Instrumented manikins are very expensive, and very few are in use. In the United States, only three instrumented manikins exist: DuPont’s Thermo-Man, North Carolina State University’s Pyro-Man and one at Natick Research Labs. And, each test destroys a full PPE ensemble.

Testing with instrumented manikins continues to provide valuable data for the fire service that is more representative than the TPP test; it’s none too soon to consider instrumented manikin testing as a logical next step. Keep in mind that current manikins are limited because (1) they are static, (2) the burn chambers do not provide heat fluctuation, and (3) there is no injection of air movement.

Despite the high cost, there is reason for optimism. N.C. State University’s College of Textiles Textile Protection and Comfort Center (TPACC) has developed Pyro-Hand, an instrumented hand form with finger movement. Perhaps, with interest from the military and cross-over into consumer market applications, the cost of instrumented manikin development and subsequent implementation of new test methods might become feasible.

Other PPE Tests
As stated earlier, the TPP test is one of the most useful and accepted thermal protection tests in existence. It is not a bad test for what it was meant to be–and for something designed over a quarter of a century ago. But firefighters should be aware that there are several other PPE-related tests required in NFPA 1971. Just for structural coats and trousers, the following “component” tests are performed:

  • Flame Resistance
  • Thread Melting
  • Tear Resistance
  • Seam Strength
  • Cleaning Shrinkage Resistance
  • Water Absorption Resistance
  • Water Penetration Resistance
  • Liquid Penetration Resistance
  • Viral Penetration Resistance
  • Corrosion Resistance
  • Label Durability
  • Trim Retroreflectivity
  • Trim Fluorescence
  • CCHR (Conductive and Compressive Heat Resistance)
  • Light Degradation Resistance
  • DRD (Drag Rescue Device) Function
  • DRD Material Strength
  • THL (Total Heat Loss)

Note: Only the TPP and THL tests use the composite layers. The Overall Liquid Penetration Test is the only test that involves both the coat and the pants as assembled/donned. (This test does not include a helmet, hood, gloves, boots or SCBA.)

Beyond TPP
But it’s more than just limitations in testing that should make us wary of putting too much emphasis on TPP. Manufacturers have consistently improved TPP and other protections with every new release of PPE. Such improvements are welcome–after all, burn injuries can be the most painful, disabling, disfiguring and costly of all injuries; any measurable progress in reducing the number and seriousness of these injuries is a worthy pursuit. But PPE improvements must also be balanced against the changing fire environment.

Recent research indicates that today’s firefighting environment has changed more than most firefighters realize. This is because building construction and fire loads have changed. The National Institute of Standards and Technology (NIST) and UL have recently conducted burns with new sensor technology instruments that provide scientific data about fire behavior that previously was not possible to gather. This research is legitimately calling into question current methods of fire extinguishment. Also, live instrumented burns by the “Kill the Flashover Project” (www.JoeStarnes.com) challenge conventional wisdom on fire behavior and suppression.

More and more science on fire behavior, and subsequently fire suppression, will soon emerge. For example, NIST will soon open a new facility at its National Fire Research Facility in Gaithersburg, Md., which will be large enough to measure fire conditions in a two-story building and will be able to put stress loads on the structure.

What does this have to do with PPE? In the Report of the Second National Fire Service Research Agenda Symposium, a cross-section of the fire service identified the number one research need for fire service tools and equipment as an “assessment of current personal protective equipment (entire ensemble) performance, functionality and related safety features for today’s fire environment” (emphasis added).

Put simply: The fire service needs to collect and analyze refined data on the burn injuries firefighters currently experience. It should keep an open mind to the emerging science on fire behavior. The outcome should help determine the best PPE technology, materials, design and performance for firefighter safety in realistic fire environments.

At the same time, PPE performance can’t be evaluated without also considering firefighting tactics. As we gain a deeper understanding of fire behavior (especially flashovers), there’s a school of thought emerging: Maybe the minimum TPP of 35 is too high. PPE with a high TPP may contribute to heat stress, and more fireground fatalities are related to heat stress than to burns. Flashovers can easily be prevented with the proper strategy and tactics. A lower TPP would be less stressful for firefighters.

However, the “what if” question remains. That is, “what if” things go wrong on the fire scene and firefighters get caught in a flashover?

PPE Symposium
Questions such as these and many other issues will be addressed at the F.I.E.R.O. (Fire Industry Equipment Research Organization) Fire PPE Symposium, March 4—6, 2013, at the Sheraton Raleigh (N.C.) Hotel. Attendees will tour the TPACC at the College of Textiles at N.C. State University and experience first-hand NFPA (and several other) test runs in the leading research facility for firefighter protective clothing in the world. Detailed information and online registration are available at www.fireppesymposium.com.

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