Turnout clothing is generally judged on how well the clothing meets relevant specifications, primarily NFPA 1971 Standard on Protective Ensembles for Structural Fire Fighting and Proximity Fire Fighting.
A principal part of this standard is the series of performance requirements, which define the minimum criteria for the clothing, materials, and components. In turn, performance criteria are based on measurements using standardized test methods.
These test methods are critical in shaping the current products in the marketplace today. While firefighters would expect that the test methods relate to the environments they face during emergency responses, the establishment of suitable test methods is not always simple and many methods often do not simulate how firefighters are exposed.
The fire service may be becoming familiar with tests such as thermal protective performance (TPP), total heat loss (THL), and conductive and compressive heat resistance (CCHR), but seeing the manner in which these methods are conducted might leave some firefighters puzzled or at least asking questions.
Understanding the way that test methods are selected or developed helps the fire service recognize the limitations for performance of their gear.
For example, it goes without saying that firefighter clothing gets tested for both flame and heat resistance. But it might be a surprise how this testing is actually conducted.
Individual materials
First, it’s important to understand that the clothing itself is not tested but rather the individual materials and pieces that go into its construction. In testing materials for flame resistance, a material is judged to be flame resistant when the cut edge of a narrow piece of fabric, suspended in a laboratory controlled Bunsen burner flame for 12 seconds, does not continue to burn more than 2 seconds after the flame is removed.
The material specimen also cannot demonstrate a char length of more than 4 inches, nor exhibit melting and dripping. The test does not mimic flame exposure the way it occurs in the field; however, this test approach is the mainstay methodology for evaluating fabric throughout the fire service clothing textile industry.
Similarly, a material’s heat resistance is evaluated by hanging a square piece of fabric in a special oven that is set at 500 F for a period of 5 minutes. Materials qualifying in this test do not ignite, melt, drip, or separate.
In addition, the dimensions of specimens for principal clothing layers are measured to determine the extent of shrinkage caused by heat exposure. Many in the fire service see this test as being overly severe because they believe that no firefighter can survive under those conditions.
This disconnection between reality and the test itself has led to a number of debates over the years that the test has been in use.
In one more example, the thermal protective performance or TPP test also gives rise to questions as to what the test means for its application to turnout clothing.
In this test, it is easier to see how the test conditions can represent a flashover environment. Still, as with all tests, the test represents just one set of conditions among an unlimited range of variables.
Moreover, the exposure of the clothing materials is at a fixed energy level unlike the dynamic nature of fire environments. Yet, the TPP test has sometimes been mistakenly used to infer protection times.
Time that lapses
The principal measurement of this test is the amount of time that lapses from the initial exposure of a turnout clothing composite specimen to the time a second degree burn would be predicted by the sensor in contact with the interior side of the sample.
This burn protection time is generally hidden in the result because the burn time is multiplied by the exposure energy, which is 2 calories per square centimeter per second. So the minimum TPP requirement of a value of 35 actually means that the time to second degree burn is 17 1/2 seconds. Interpreting the test results in this way is not only incorrect, but also dangerous since the TPP test cannot possibly account for all the variables on the fireground.
Each of these examples shows how tests do not completely relate to the fireground. But these tests also show the principles involved in selecting a test that is being used to establishing minimum criteria. The development of a new test method requires a very conscientious and systematic approach, which must involve several specific elements. A good test method is one that meets the following criteria:
First, the test method must measure the target attribute of the product’s performance. This principle should seem obvious, but is not always readily attainable. In many cases, it is relatively difficult to simulate a specific type of hazard exposure in a manner that accommodates a wide range of the factors faced by the firefighter.
Many end users prefer that tests take on the appearance of what they do because it is easy for them to look at the test and agree that it represents a type of exposure they may encounter.
However, such approaches are not always practical. There are many, many examples of tests that are performed in NFPA 1971 which do not directly simulate the actual exposure, such as the heat resistance test that was described earlier.
Yet the test remains viable and accomplishes the intended purpose which is to keep materials that easily degrade in high heat exposures, like polyester and Nylon, from being used in firefighter clothing,
Acceptable ranges
Second, the test method must be repeatable. New test methods should have an acceptable range of variation and where there is variation in the method, there must be an adequate explanation for the imprecision of the test.
For example, TPP testing is done at a constant heat energy exposure level because it’s easier to maintain that level from test to test. Test precision must be demonstrated through both intra-laboratory (within an individual laboratory) and in interlaboratory studies (between several laboratories).
This is a critical area for evaluating a new test method because performance criteria based on new test methods must account for the level of variability for the test.
Precision studies must be carefully designed to yield information that can not only be used for adjustment of any proposed performance criteria, but can help to identify sources of variation in the test method itself that can be remedied.
Test results must be reproducible to establish confidence in the test, and that the same materials will always yield the same results no matter who does the testing.
The last and most important criterion for the development of a test method is that the test method ranks product performance consistent with field observations. There is no doubt that a well designed test method will yield data that shows differences between products.
Yet, those differences in samples are meaningless unless they show the same trends in product performance as observed by firefighters in the field. The fact that a test simply provides high levels of discrimination is not an acceptable basis in and of itself for indicating the test is successful in achieving its intended use.
Why gear works
A good test must confirm why good products work and why they protect the firefighter. This type of information can be very difficult to come by, particularly if the hazards involved are relatively dangerous (i.e., the many types of thermal exposures that firefighters face).
Nevertheless, a responsible part of test method development must involve some sort of field study in order to validate the efficacy of any proposed method and performance criteria based on that method.
These types of studies are complicated and difficult to execute, but they can be carried out successfully and to the benefit of setting meaningful performance criteria.
Perhaps one of the best examples of a detailed study that can not only show the relevance of a test but also how to establish the appropriate performance level is a study performed by the International Association of Fire Fighters back in 1998, which justified the implementation of the total heat loss test.
That study showed that the THL test best corresponded with the physiological impact of turnout clothing with different composites. The study also was able to suggest a defensible performance requirement that was eventually adopted in NFPA 1971.
The fire service needs to be wary of new tests proposed by different sources until or unless they have been completely validated to show that they are repeatable and that they will provide the basis of meaningful performance criteria that improves the safety of firefighters.
Just because a new test looks “neat” and may even relate through its appearance to what a firefighter sees on the fireground, doesn’t always make it a good test. Therefore, it is important to ask questions and understand tests before you rely on them for making judgments of acceptable clothing performance.
