Thermal Protection Performance (TPP)In the 1986 revision of NFPA 1971, Protective Clothing for Structural Fire Fighting, a new test method for measuring thermal protection was introduced and a minimum Thermal Protective Performance (TPP) rating was established. This sophisticated test method replaced the requirement for a minimum composite thickness. Its purpose is to measure the time elapsed for convective and radiant heat to penetrate through the composite system – Outer Shell, Thermal Liner and Moisture Barrier – to damage the human skin.
The illustration below is of a standard TPP tester. The test fabrics – all three layers in composite form – are placed beneath the sensor, which records skin temperature. The layers are placed onto the machine in the same order in which they are found in the protective system, with the Thermal Liner next to the sensor, Moisture Barrier in the center and the Outer Shell next to the energy source. The movable shutter enables the technician to control the time and the amount of exposure; gas burners provide the actual flame. At the same time, the heated tubes provide the radiant heat and a flashover situation can be simulated. The point at which heat transfer through all three layers is enough to cause second degree burn is determined graphically by using a recorder chart of the sensor readings. During testing, the recording continuously traces the average temperature rise on paper, depicted as a curved line representing higher and higher temperatures as more heat penetrates through the sample materials to the sensor.
After the test is completed, its tracing is compared with a second curve, called the Stoll's curve, which shows the blister point of human skin as a function of heat and time. The point of intersection between these two curves is the actual TPP rating. For the purpose of measuring actual time to burn, the TPP rating is divided in half to determine the number of seconds until the human tissue reaches second degree burn. Thus, the NFPA minimum requirement of a TPP rating of 35 equates to 17 ½ seconds until 2nd degree burn occurs in a flashover situation.
A popular misconception is that if 35 is good, a rating of 40, 50, or even 60 must be better. It is important to remember, however, that the only way to increase your TPP rating is to add more insulation, usually by specifying heavier material components. Generally speaking, added insulation will mean increased weight of the total system, resulting in greater heat stress for the firefighter. The question one must ask is whether the added seconds of protection in the event of a flashover environment is a good trade-off against today's lighter weight systems. It is also important to note that by selecting the three layers to be used in your system, you have automatically specified the TPP rating, since it is a function of the materials chosen and not an independent requirement.
Consider the following charts when evaluating your system's TPP.
Turnout Composite Thermal Performance
- Garment design and features dominate the thermal and heat stress performance of the clothing system.
- More than 50% of a typical turnout is reinforced by additional fabrics, trim, labels or overlap.
- Base Composite: TPP = 50
- KEVLAR® / NOMEX® Outer Shell PTFE / E-89® Moisture Barrier aramid batt Thermal Liner.