Behavior of Apparel Fabrics during Convective and Radiant Heating.

Personal protective equipment (PPE) recommended for arc flash is not always designed for arc flash exposure. The purpose of this paper is to warn of the dangers posed by using the improper materials in arc flash exposures until standards have caught up on this issue.

The table below shows a representative range of everyday textiles along with some of the measurements of importance in establishing their response towards convective and radiant heating[1]:



Times to ignition or melting of the 20 fabrics in Table above were reported by Wulff[2], [3] for different incident heat fluxes. The Wulff's data have been used to develop a methodology by which ignition and melting times may be forecast. A semi-empirical relationship between ignition/melting time and radiative heat flux has been derived[1]:



where [NF0] is the non-dimensional destruction time of the fabric (that is, time to ignition or melting) and is given by:



where:

(k/l) - average thermal conductance, W / (m^2 * C);
t - ignition/melting time, sec;
pl - mass/unit area, kg / m^2;
c - average specific heat, W * sec / (kg * C).

NBi is the Biot number which is defined as the ratio of the average convective heat transfer coefficient of the fabric to the average thermal conductance of the fabric. It is obtained experimentally for each fabric.

The non-dimensional radiative heat flux [qxrad] is given by:



where:

ads - adsorptance of the charred fabric;
W0 - incident heat flux, W / m^2;
Tim - mean ignition/melting temperature, C;
T0 - ambient temperature.

The constants a and b are specific to a particular fabric group and have experimentally derived values.

By substituting the relevant values for a specific fabric and by applying the particular fabric type constants, a and b, an estimate of the ignition or melting time for any incident heat flux can be obtained. Substituting the piloted ignition temperature instead of the self-ignition temperature allows an estimate to be made of the time to piloted ignition. The calculator below computes time to non-piloted ignition or melting of the 20 fabrics as a function of heat flux. It also predicts the amount of incident energy required for self-ignition or melting of the fabrics:

Time to Non-Piloted Ignition or Melting vs. Thermal Radiation Level Calculator

The figure below displays time to damage as a function of radiant heat flux for two different fabrics from the Table 1.



The level of intensity and duration of exposure result in a received thermal dose unit TDU that can be calculated and used to estimate these progressive effects from the following equation[4], [1]:



where I is radiated heat flux in W/m^2 and t is exposure time in seconds.

Threshold incident energy for ignition or melting of apparel fabrics can be evaluated from Equation 4 above as:



The TDU for melting of nylon and polyester fabrics is ranging from about 1500 to 3000[1]. As an example, assuming nylons' TDU is equal to 1500, substituting different time intervals into Equation 5 and converting the calculated onset incident energy for melting of nylon to J/cm^2 and cal/cm^2:



While TDU value depends on type of textile, the textile chemical and physical properties, it is seen that the threshold energy for ignition or melting apparel textiles is variable function of heat flux. Any amount of heat delivered within a long enough time interval will have no impact on the fabrics' integrity while a limited amount of heat delivered within short enough time interval may ignite or melt the fabric.

It is also worth noting that the optical response varies widely depending on the fabrics' type, weight and color. For light-colored textiles listed in Table 1, 40 to 60% of IR radiation is reflected, 10 to 40% is transmitted and 14 to 50% is absorbed. The amount of energy exceeding threshold incident energy for a 2nd degree burn for bare skin exposure[5] may pass through protective clothing and destroy the skin before any visible damage is done to the clothing.

NFPA 70E allows flammable clothing with a fabric weight at least 4.5 oz/yd^2 ( > 0.16kg/m^2) to be worn when working on tasks stated low risk. Stating a task as low risk and allowing the use of apparel fabrics may lead to severe injuries and fatalities from clothing ignitions in arc flash incidents. The NFPA 70E based arc flash analysis fails to consider the variable nature of onset to second degree burn energy on bare skin[5]. Also, there seem to be issues in NFPA 70E when the standard calculation procedure for determining arc flash boundaries allowing flammable clothing to be worn in situations that could result in ignition of the clothing.

References:

1. Ian Hymes, Warren Boydell, 1996, Thermal Radiation: Physiological and Pathological Effects (Belinda Prescott).
2. Wulff, W. et al, 1973, Ignition of fabrics under radiative heating, Comb Sci Tech, 6, 321-334.
3. Wulff, W., 1973, Fabric Ignition, Tex Res J, October, 577-588.
4. HSE, 2010, Methods of Approximation and Determination of Human Vulnerability for Offshore Major Accident Hazard Assessment.
5. M. Furtak, L. Silecky, 2012, Evaluation of Onset to Second Degree Burn Energy in Arc Flash, IAEI, July, 65-69

Two things don't make sense to me here.

First, all the raw data refers to ignition, not melting. Based on my reading of the same reports the clothing acts as an insulator up until the point that it melts at which point it becomes conductive. So we're not really interested on whether or not it lights on fire but whether or not it melts. Technically the key point would be how much additional energy is accumulated by the victim (passing through the Stoll curve) but just as with many of the researchers looking at flash fire survivability it might be simpler to simply assume that once melting occurs, injury exceeds Stoll curve. And why this matters is that all the data is talking about ignition, not melting.

Second, and more self-contained, the data shows that at 0.01 seconds of exposure, the limit is 1.3 cal/cm^3 and then jumps to 4 cal/cm^2 at 1 second? This seems backwards.

You said, "Personal protective equipment (PPE) recommended for arc flash is not always designed for arc flash exposure. The purpose of this paper is to warn of the dangers posed by using the improper materials in arc flash exposures until standards have caught up on this issue."

PPE for arc flash is AR (arc-rated) so it is tested FOR arc exposure. The 2015 NFPA 70E has eliminated HRC 0 which was "implied" PPE but not recommended by ASTM or IEC standards.
ASTM F1506 is for flame resistant, arc rated fabrics and they are all "designed for arc flash exposure". Your post cited non-arc flash research which those of us who write the standards are aware of (it drove my research back in 1990's) but this is on mainly non-FR materials and none of the things you cite are related to arc flash. ONLY AR materials are supposed to be used in arc flash exposures. Don't understand what you mean by "until standards have caught up". The standards are light years past this info. Definitely don't wear melting fabrics or non-AR fabrics. Don't see any value other than that in this compilation of papers on convective and radiant heat. Additionally studies like this are basically useless (other than as a starting point) because the heat flux in an arc flash is much more than what is used in these papers.

Hugh Hoagland
ArcWear (we do 90% of the arc ratings but do not sell AR PPE).

"NFPA 70E allows flammable clothing with a fabric weight at least 4.5 oz/yd^2 ( > 0.16kg/m^2) to be worn when working on tasks stated low risk. Stating a task as low risk and allowing the use of apparel fabrics may lead to severe injuries and fatalities from clothing ignitions in arc flash incidents. The NFPA 70E based arc flash analysis fails to consider the variable nature of onset to second degree burn energy on bare skin[5]. Also, there seem to be issues in NFPA 70E when the standard calculation procedure for determining arc flash boundaries allowing flammable clothing to be worn in situations that could result in ignition of the clothing."

This probably over simplifies the issue and is a little inaccurate reading of the new 2015 NFPA 70E risk approach. The risk approach is for "acceptable risk". I don't know any plant which has EVERYTHING properly installed, maintained and no historical or visible sign of impending failure so the use of ANY fabric is indefensible. I read this as the committee's attempt to take away the silly PPE requirements some interpreted from the last version and some very poorly done arc flash studies requiring AR PPE for light switches and basic controls. In doing this, the committee also allowed NO PPE for any operation which met the "acceptable risk" model (I disagree with this approach and prefer to limit it to Arc Resistant equipment and single phase equipment less than 480V). I applaud your attempt to keep people out of melting clothing but NFPA 70E doesn't call this PPE.

Hugh Hoagland

I'd like to know where the author came up with the statement "NFPA 70E allows flammable clothing with a fabric weight at least 4.5 oz/yd^2 ( > 0.16kg/m^2) to be worn when working on tasks stated low risk. "

Hugh, I'm on board with the "acceptable risk" approach vis a vis looking at likelihood of the hazard occurring in the first place. Certainly if you don't wear PPE rated equal to or greater than the hazard, then injury WILL occur. The approach here is simply stating that if its a rare event, that we don't need to do silly things to try to prevent something that is very rare in the first place.

BUT the new version essentially takes the stance that once the incident energy rating drops below a certain threshold that PPE is no longer required...being outside the arc flash boundary. This has sort of been the theme all along BUT the old "H/RC 0" (nonmeltable clothing) category is still out there and still being used to address potential injury outside the arc flash boundary. Shouldn't there be a "less hazardous" zone, akin to the hazardous locations (class X, division 2)? The case that has been made over and over again is that wearing meltable fiber clothing either as an underlayer, over, or in lieu of arc rated PPE will exacerbate the injury. 70E-2015 simply eliminates this altogether and only bans meltable fiber clothing as underlayers/overlayers with respect to when arc rated PPE is required. Previously there has always been "low risk" areas where an arc flash hazard might still exist but was outside the arc flash boundary so nonmeltable clothing (H/RC 0) was recommended. Now that has totally disappeared.

Is the research showing that at 1.2 cal/cm^2 or lower exposure that the injury will be no greater than bare skin (just as safe or safer than being naked), even if wearing a Clinton pants suit, or is there a second "arc flash boundary" where nonmelting clothing should be required, and then a further zone where for lack of a better word, anything goes? Because to sum it up, Arcad's point is trying to answer this question and as of today, 70E-2015 is not helping when the hazard definitely exists but where the incident energy is under 1.2 cal/cm^2.

I have combined both ignition and melting temperatures under one column. When the clothing ignites, a significant amount of energy is released. The amount of energy can be evaluated using the heat of combustion measure. I can provide you with heat of combustion values for the fabrics listed in Table 1 if needed.



This is right although it may seem "backwards". For exposures lasting less than one (1) second the heat flux required for ignition or melting the fabrics would significantly increase as the duration of exposure decreased. However, the amount of incident energy required to cause the damage would decrease.

See NFPA 70E table 130.7(C)(11) Protective Clothing Characteristics