Why a Fabric with an ATPV 8.0 cal/cm² rating is so Much, Much Better than a Fabric with ATPV 7.9 cal/cm²?
(A non-orthodox and slightly humorous look at some iconic numbers)
First of all – there has been one single and simple gradual change in a last fifteen - twenty years that has made a huge difference in electric arc protection. Ignitable untreated cotton and melting synthetic materials, which in arcflash events very often resulted in clothing ignition and fatality from massive skin burns, were replaced by materials highly resistant to flame. Thus, a very solid foundation has been created to turn the arcflash fatality trend downward.
Advances in development of inherently fire retardant (FR) and treated materials like Nomex, Modacrylic, FR-threated cottons and others have revolutionized the workwear market not only for firefighters but for electrical personnel as well.
Second of all – there is an apparent disconnection between the foundation and the “structure” that has been built on that same foundation. The “structure” is test and electrical safety standards guiding end user through the process of selecting PPE for protection against thermal effects of electric arc. There are signs that the “structure” might need some renovation and further development.
Here come iconic numbers and one of them is ATPV=8 cal/cm²
Try to answer one simple question: why is ATPV 8.0 cal/cm² so much, much better than ATPV 7.9 cal/cm²?
Or another question: why is fabric with one of these two ratings “excellent and superb” but nobody wants even think using a fabric with the other rating?
Or yet another one: what is the fundamental difference between these two fabrics or is there any?
The “correct” answer on first two questions is “just because”. Life is good, stop here, no further reading, let’s avoid “confusion”.
But, if you are still curious, let’s see what can be done with question number three. An ATPV rating comes, of course from testing. And there are number of things in a test process that can affect a numerical ATPV outcome that can vary for exactly the same fabric tested:
Number of panel sensors involved in the interpretation of results (sneak peek into ASTM F1959 test method is desirable here and it is not an easy read). An average reading of two panel sensors is used according to the test method. Panel sensors are covered with test fabric and measure heat energy penetrating though the test fabric. The average is always less than the highest reading. Obviously, an ATPV based on the highest reading is different from one based on the averaged reading.
Statistical variation in a fabric, thread and weaving/knitting process and statistical variations of the temperature sensors and other test equipment. Standard deviation as it is stated in the “Precision and Bios” section of the current test method is 5%. Recent test results showing standard deviations up to 10%.
Fabric preparation process. An ASTM test method requires three washes followed by one drying cycle, while an IEC test method requires five washing-drying cycles. More washings cycles and drying after each washing of an IEC test method result in higher shrinkage and higher weight of a fabric going into testing. The higher the weight – the higher the ATPV.
Relative position of the Stoll curve (used as a skin burn criteria) and heat energy curve acquired by panel sensors. Both curves are plotted as heat energy vs time. A graph is available as a part of a test’s data package. Fabric evaluation and ATPV calculations are based on a cross/no-cross interaction between these two curves. Stoll curve staring time is synchronized with the start of the test current. Then there is time delay and time shift for heat energy curve resulted from several sources. Few to mention are an inherent calorimeter (copper slug and thermocouple) heating inertia, inherent delay of the signal processing instrumentation, inherent delays in power equipment operating times, and software delays. Considering the shapes of the two curves, increasing the time shift increases the probability of no-crossing and potentially increases ATPV (generally and within the range of 6-8 cal/cm² inclusively).
Instrumentation and equipment delays are irrelevant to the nature of the test method but could affect an outcome of testing. Time shift is not clearly addressed in the current standard and is a subject tof a particular lab’s interpretation.
Magnitude of arc test current. A test current of 8,000 A is used in the current ASTM standard. It is known fact from ASTM 2004 testing that ATPV is higher for higher test current (16,000 A) and lower for lower test current (4,000 A). The resulting difference in ATPV can reach 50%. Results of recent (September 2014) testing of 7 oz/cm² FR Cotton fabric are shown in the chart below.
Test panel design. There are two designs for test panels, specifically two ways of mounting the monitoring censors. Sensors are used to measure incident heat energy of arc exposure. Monitor sensors can be mounted on clamps that hold the test fabric in place (see drawing below) or independently of any panel fixture.
For an accurate measurement the sensor must be perpendicular to the heat flux direction (vector). Another words it should be normal to a radius drawn from arc electrode center line towards the sensor. It is true for the censor calibrated position without test fabric. When a fabric sample is in place for testing, the monitor sensor is moved from the calibration position introducing measurement error due to: a) sensor surface is no longer normal to the radius (vector error), b) exposed surface area of the sensor is reduced from a circle to an ellipse, and c) distance from arc electrode is increased. Each error is small and can be neglected. But accumulation of all three may be too significant to ignore.
This error is rather small for single layer fabrics and increases with the number of layers or with fabric thickness.
After all the bullets are shot, ATPV of an 8 cal/cm² fabric is statistically somewhere between 7 cal/cm² and 9 cal/cm² for 8,000A arc current value. Evidently, ATPV of 7.9 cal/cm² and ATPV of 8.0 cal/cm² are both statistically the same thing and 7.9 cal/cm² fabric is as good as 8.0 cal/cm².
Fabrics with ATPVs 7.9 and 8.0 have the same electric arc thermal protective properties. But the system makes one of them bad and the other one good.
The existing test method is an excellent way of comparing thermal properties between different fabrics. However, it is wrong to iconize the measured ATPV and assign this constant to a fabric as a universal measure of its thermal protective properties, no matter what.
There is a way, however to eliminate 7.9 vs 8.0 controversy. Fabric protective property cannot be described in a complete way with only one single number (ATPV or EBT). The fact of variability of the thermal protective properties of any given fabric with variability of arc current calls for description of these properties with Time-Current type of curve (TCC) (see chart below).
When a fault current value in a particular spot of electrical system is known, the fabric/garment protection time (time before second degree skin burn is acquired) can be calculated from TCC.