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Understanding Key Turnout Gear Tests
The turnout gear worn by firefighters is intended to provide protection under myriad fireground conditions. This category of clothing includes some of the most extensively tested items in the protective clothing industry. In fact, there are hundreds of tests administered to the many materials, components, and overall clothing itself in order to certify that they meet the National Fire Protection Association (NFPA) 1971 standard on protective clothing for structural fire fighting.
Since there is a broad range of industry offerings for different composites and components for turnout clothing, it is important to have a good understanding of the key tests that are used to ensure their quality and performance. In this article, I’ll summarize the key tests for comparing garment product performance.
Flame and heat resistance are two of the fundamental properties used in NFPA 1971. Flame resistance testing determines how readily clothing, material, and components resist ignition when contacted by a flame and their burning behavior once the flame is removed. Heat resistance testing determines how clothing, materials, or components respond to high heat. As such, it is a test of the thermal stability of a material.
Flame resistance testing is performed by contacting a clothing item, material, or component with a flame for a specific time period and then noting or measuring the result. Clothing materials for structural firefighting are evaluated using a method in which the cut edge of the material is vertically suspended in a Bunsen burner flame for 12 seconds.
After the specimen is removed from the flame, the time the material continues to burn is reported as the “afterflame” time, while the distance that the material is damaged is determined as “char length.” The tested materials are also observed for melting and dripping. NFPA 1971 dictates that outer shell, moisture barrier, and thermal barrier materials cannot have an afterflame time of more than 2 seconds, char lengths greater than 4 inches, or exhibit melting or dripping. This test is also applied to most other materials, including reinforcements and padding.
Heat resistance testing is conducted by placing the test item in a special oven for a period of time and then examining the condition of the item after it is removed. The test involves a 5-minute exposure in an oven set at 500oF (260oC). The oven must meet certain specifications in order to provide a controlled temperature exposure to tested material specimens. Following exposure to heat, specimen materials cannot ignite, melt, drip, or separate, and must not exhibit shrinkage of more than 10% in any direction.
The oven exposure is not intended to simulate use, but rather to provide a uniform manner of testing consistent with the performance of clothing in the field. When first selected, the set of exposure conditions was chosen to help simulate the damage to clothing that was observed in the field.
Thermal Protective Performance (TPP)
Thermal protective performance testing measures the amount of heat transfer through the clothing composite (the combination of all layers) when exposed to a combination of convective heat and thermal radiation. TPP testing was first introduced in the 1986 edition of NFPA 1971 and has since become one of the primary thermal insulation tests used for determining adequacy of firefighter protective clothing material composites.
Composites are laid flat in the TPP test apparatus and are exposed from the bottom to the heat from two burners and a radiant panel. The exposure level is intended to simulate the heat energy associated with a flashover. The test uses a special sensor called a calorimeter that measures the heat transfer through material composite. This information is used to predict a time-to-burn. The reported TPP rating is this time-to-burn multiplied by the exposure energy (2.0 calories per square centimeter per second).
It is important to recognize that TPP measurements do not imply a certain protection time, because the testing only simulates one among an unlimited set of clothing exposure conditions. The required TPP rating for garment and glove composites used in structural fire fighting is at least 35. Higher TPP ratings mean higher levels of insulation, but this comes as a tradeoff in terms of garment bulkiness, weight, and flexibility.
Conductive and Compressive Heat Resistance (CCHR)
The conductive and compressive heat resistance test was added for evaluating the garment shoulder and knee areas of firefighter protective clothing. The test is a variant of a conductive heat resistance test that is commonly applied to gloves and footwear, but evaluates shoulder and knee areas under pressure (2 pounds per square inch for shoulder areas and 8 pounds per square inch for knee areas to simulate the weight of the firefighter).
In this test, the composite samples are placed on a laboratory hot plate set at a temperature of 536oF. A calorimeter is used to measure the time required for a 43oF temperature rise to occur (called the CCHR Rating). Higher CCHR times indicate better conductive and compressive heat resistance. This measure does not apply any specific level of protection but rather is intended to provide a way of comparing the insulation provided by garment reinforcements when in contact with hot surfaces.
Testing is performed with the reinforcements both dry and wet. The CCHR requirement in the NFPA 1971 standard has recently increased from greater than 13.5 seconds to greater than 25 seconds. Since ordinary composites cannot pass under the new requirement, all garments must now have additional insulation at both the shoulders and knees.
Total Heat Loss (THL)
The total heat loss test is used to measure how well garments allow body heat to escape. The test assesses the loss of heat both by the evaporation of sweat and the conduction of heat through the garment layers. As clothing is made more insulative to high heat exposure (such as by increasing its TPP rating), there is a tradeoff with how well the heat build-up in the firefighter's body (that can lead to heat stress) is alleviated.
Garments that include non-breathable moisture barriers or very heavy thermal barriers prevent or limit the transmission of sweat moisture, which carries much of the heat away from the body. If this heat is kept inside the ensemble, the firefighter's core temperature can rise to dangerous levels if other measures are not employed (e.g., limiting time on scene, rotating firefighters, providing rehabilitation at the scene).
The total heat loss test has been included in NFPA 1971 to provide a balance between thermal insulation for protection and evaporative cooling insulation for stress reduction. In this test, heat flow through material composites is measured under both dry and wet conditions using a hot plate that simulates skin temperature control. This information is then used to calculate a total heat loss value. Higher THL values are considered to be an indication of increased material “breathability,” even though the overall value reflects both conductive and evaporative heat transfer from the body.
The original total heat loss requirement in NFPA 1971 mandated that the total heat loss for the three layer composite must be 135 Watts per square (W/m2) meter or higher. In the new 2007 edition, the THL must now be at least 205 W/m2.
NFPA 1971 includes a number of other tests, but the ones described in this article are some of the most commonly used in comparing the performance of garments and are often used as part of purchase specifications. Ideally, it is best to provide the highest levels of protection in selected garments as based on performance tests. Nevertheless, some tests such as TPP and THL are opposed to each other, so departments must determine the right balance between protection and imposed stress and restrictions in firefighter function. Having an understanding of these tests and comparing clothing on the basis of these properties is one way to determine the right balance.