By Jeffrey O. and Grace G. Stull
Most firefighters that examine a copy of NFPA 1971, the standard that establishes the design, performance, and label requirements for firefighter protective clothing, are generally overwhelmed by the level of detail. The current edition (2007) spans 126 pages. While the standard contains separate sections on its scope, definitions, certification, label, design, performance, and testing, it is the section on testing methods that dominates this content.
The standard lists some 72 different test methods that apply to both structural and proximity firefighting protective garments, helmets, gloves, footwear, hoods, and overall ensembles. Naturally, it is the combination of these tests taken collectively that forms the boundaries for what is and isn’t acceptable.
Several stringent requirements exist and it is difficult to comprehend exactly how all these tests, and the other criteria in the standard, come together to define the clothing that firefighters routinely wear for their health and safety.
Firefighters realize that the clothing is flame resistant, heat resistant, provides insulation from heat during fire, and is also breathable. These are just some of the properties that firefighter protective clothing must offer, but it helps to understand just how these qualifications are demonstrated in each article of clothing. There are four fundamental properties that are applied to different elements of the firefighting protective ensemble: flame resistance, heat resistance, thermal protective performance, and total heat loss.
Flame Resistance
Flame resistance testing is performed by contacting a clothing item, material, or component with a flame for a specific duration and then noting or measuring what happens. Clothing materials for structural and proximity firefighting are evaluated using a technique in which the cut edge of the material is vertically suspended ¾ of inch into a 1½ inch high 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 measuring the tear that occurs along the char line by placing a weight on the corner of the fabric; this measurement is known as “char length.” The burning behavior of tested materials is also observed to determine if melting and dripping occurs. Materials that liquefy (such as polyester) are known to worsen burn injuries.
NFPA standards generally apply the following acceptance criteria:
- No afterflame times greater than 2 seconds
- No char lengths greater than 4 inches
- No melting or dripping
Materials that meet each of these criteria are considered flame resistant. Variations of this test are carried out on other types of protective elements. Whole helmets are tested by directly exposing different parts of them to the same flame described above using a 15 second exposure, but char length is not measured and afterflame times can be as long as 5 seconds. In contrast, whole gloves and footwear items are also tested but with a higher energy flame and larger burner.
Footwear is impinged by the flame on several parts of the boots, representing different materials of construction. Afterflame times must be less than 2 seconds and the flame cannot burn through the footwear material. Gloves are tested as a composite, but the flame is applied to a 90 degree bend.
In addition to the measurements above, the loss of specimen weight is determined and used to judge performance. These differences in test approaches illustrate how different elements of the ensemble are tested and how some parts of the ensemble may not be evaluated for flame resistance. For example, internal helmet suspension straps, interior glove layers and footwear linings are not tested for flame resistance
Heat Resistance
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 NFPA standard specifies a 5-minute exposure in an oven set at 500°F (260°C). Contrary to some statements in the industry, this exposure is not intended to simulate use but rather to provide a reproducible and controlled manner of testing that is consistent with how clothing and equipment perform in the field.
Following their exposure, test items are examined for evidence of ignition, melting, dripping, and separation. For some items, materials, and components, additional assessments are made:
- Outer shell materials are examined for char (the formation of a brittle residue from heat exposure).
- Helmets are inspected for deformation and drooping of the shell.
- Gloves are evaluated for their continued functionality.
- Heat-exposed footwear must show continued functionality of components and demonstrate that water-tightness is unaffected.
- Hardware must remain operable.
In testing products, garment materials are tested individually, while helmets, gloves, footwear, and hoods are tested as whole items. Both helmets and hoods are placed on a non-conductive
headform during testing while whole gloves and footwear are filled with small glass beads. Glove lining materials are tested separately.
High heat exposures can also cause some materials and components to shrink. Shrinkage following oven exposures is measured for garment, helmet ear cover, hood materials, and whole gloves. In this test, the distances between marks placed on the material or item are checked after the heat exposure to determine the percent loss in distance. Garment, helmet ear cover, and hood materials are permitted to shrink up to 10 percent, while glove shrinkage cannot exceed 8 percent.
Thermal Protective Performance
Thermal protective performance or TPP is one of the key tests that assess the ability of clothing and equipment to limit heat transfer through the clothing. For all applications where exposure to heat can occur, the protective clothing and equipment must limit the passage of heat to the person’s body. Burns occur as the consequence of heat transfer to the skin at a rate greater than the body’s ability to dissipate that heat.
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, replacing thickness as the principal measurement of composite insulation.
Composites laid flat in the TPP test apparatus 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 calorimeter (a heat sensing device that mimics the skin’s response to heat) to measure the time-to-burn. The time-to-burn is based on comparing the calorimeter response to study data that show what exposure energies are needed to cause the onset of a second degree 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 condition among an unlimited set of clothing exposure conditions.
The required TPP rating for garment and glove composites must be at least 35. Garment and glove wristlets, helmet ear covers, and hoods require TPP ratings of 20 or greater. These lower levels of insulation performance are justified because these areas are considered interfaces where more than one item or set of clothing layers overlap.
TPP testing is not applied to helmets or footwear. The insulation provided by helmets is considered to be in excess of the soft clothing layer elsewhere in the ensemble, while footwear uses other tests to assess insulation from contact with hot surfaces and high radiant loads.
Total Heat Loss (THL)
Total heat loss is used to measure how well garments allow body heat to escape (the exact opposite of the TPP test). 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 exposures, there is a trade-off 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 efforts are not undertaken (i.e., limiting time on scene, rotating firefighters, providing rehabilitation at the scene). Thus the total heat loss test has been the NFPA standard to provide a balance between thermal insulation for protection and evaporative cooling insulation for stress reduction.
For structural firefighting protective clothing, a new requirement of 205 watts per square meter (W/m2) is set replacing an earlier requirement of 130 W/m2. The new level is consistent with a recommendation from a major study of clothing effects on firefighter heat stress conducted by the IAFF.
The study showed that using material composites with higher values of total heat loss creates less stress on firefighters and that the total heat loss test was able to predict the stress-related effects of clothing on the firefighter. At the current time, total heat loss testing is only used for garments. Materials used in gloves, footwear, hoods, and helmet covers do not have to be breathable.
Summary
In the case of these four fundamental tests, it can be seen that while each of these items functions as part of an overall ensemble, the ways that the items are tested can different. Sometimes, these differences are based on the geometry of the item, but different criteria are applied and some test exceptions exist. Thus, the understanding of key test methods in NFPA 1971 can provide an appreciation for just what limitations different clothing items can have for the firefighter.
