Monday, April 6, 2015

Issue #27 - Ignition and Melting Characteristics of Protective Clothing

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In many cases, if exposed to a sufficiently large and sustained exposure fire, the fire-retardant (FR) fabrics will burn vigorously. Fabric's time to ignition or melting versus heat flux data can be used for determining the textile TDU value. For example, Figure 1 shows that it takes approximately 5cal/cm2/sec heat flux for igniting denim textile listed in Table 1 within 3 seconds time interval. The TDU value for the denim fabric can be calculated by substituting the values for heat flux and time to ignition into Equation 4, and it is roughly equal to 3800(kW/m2)4/3 * sec. Behavior of the fabrics other than those listed in Table 1 during convective and radiant heating can be predicted using a simple test that can be quite easily reproduced in the field.
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"The 2015 edition of nfpa 70e no longer has a PPE Category 0, however my software still calculates Level 0. What am I missing?"

Indeed, the AFA V5.0 software does not show anywhere the term "PPE Category" or "Hazard Risk Category" (HRC). The program displays term "Level". The new NFPA 70E 2015 does not prohibit using the term "Level". As long as you don't call it "PPE Category" there should be nothing preventing you from doing so. Remember that not that long ago people were told not to mix HRC with incident energy. So they began to call it 1, 2, 3 and 4 PPE categories to get away from the term HRC. This time around the NFPA 70E  claimed ownership for the term "PPE category" that people moved to, and say you can't use that term with incident energy. Also, the NFPA Handbook shows a sample label with fields to be filled in for "Available Incident Energy" and "Level of PPE" while just half a page earlier states that "available incident energy" cannot be included with the "PPE category" in table 130.7(C)15(A)(b). It makes sense since the "Level of PPE" and "PPE Category" are technically two different terms.

Clicking on Settings menu, selecting Incident Energy at AFB and switching to Evaluated option will also prevent the Level 0 displayed on arc flash labels created using the AFA V5.0 program. Also, you can edit default site-specific PPE by clicking on Settings menu, selecting Site Specific PPE and making required changes.


IEEE 1584 Guide for Performing Arc Flash Calculations has established practice when modelling HRC fuses for arc flash analysis. The IEEE 1584 4.6 Step 5 reads "For fuses, the manufacturer's time-current curves may include both melting and clearing time. If so, use the clearing time. If they show only the average melt time, add to that time 15%, up to 0.03 seconds, and 10% above 0.03 seconds to determine total clearing time. If the arcing fault current is above the total clearing time at the bottom of the curve (0.01 seconds), use 0.01 seconds for the time". The opening time tolerances differ between fuse designs and the percentage of the overload relative to the amp rating. Incrementing the time according to IEEE 1584 guidelines should effectively compensate for the margin of error in published time-current curves in most cases.
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