ARC RATING: The value attributed to materials that describes their performance to exposure to an electrical arc discharge. Source: ASTM F1959 (American Society for Testing and Materials).
Discussion: The arc rating expressed in cal/cm2 describes the protection provided the wearer against the thermal energy from an electric arc flash. The arc rating may be an ATPV or EBT based on the fabric’s response to thermal energy.
ARC THERMAL PERFORMANCE VALUE (ATPV): The incident energy, expressed in cal/cm2, on a material or a multilayer system of materials that results in a 50% probability that sufficient heat transfer through the tested specimen is predicted to cause the onset of a 2nd degree burn injury based on the Stoll curve. Source: ASTM F1959.
BREAKOPEN THRESHOLD ENERGY (EBT): The incident energy on a material or material system that results in a 50% probability of breakopen. Source: ASTM F1959.
Discussion: While ATPV and EBT describe different responses to electric arc flash, both may be considered the arc rating of a fabric. Both values can be determined for a fabric or layered system, but the lower of the two values is reported as the arc rating. The EBT is sometimes thought to be less desirable than ATPV, but both are a fair measure of the protection provided by a fabric. Woven fabrics will typically have ATPV ratings while some knits, especially fleeces will report an EBT. Fabrics with more strength than insulation will usually have an ATPV while fabrics with more insulation than strength will have and EBT.
CALORIE: The amount of energy needed to raise one gram (or cubic centimeter or one milliliter) of water 1 degree Celsius. Caloric values of food are measured in kilocalories, which is equivalent to 1,000 calories.
Discussion: In arc testing, energy per unit area is reported as calories per square centimeter (cal/cm2). This is also the format for reporting the arc rating of a fabric. Heatflux may be reported in cal/cm2/sec.
CELLULOSE: A carbohydrate that makes up the cell walls of all plants. Cellulosic fibers and fibers made from cellulose include cotton, rayon and lyocell.
COTTON: A natural staple fiber that is almost pure cellulose. Cotton is the most important and most widely used textile fiber in the world. Cotton is valued for textiles for its versatility, softness, strength, absorbency and dyeability.
Discussion: MVM is the second-largest purchaser of cotton fiber in the U.S. Only American-grown United States Dept. of Agriculture (USDA)-classified cotton is used in our products. Cotton is evaluated by the USDA for fiber length, uniformity, strength, micronaire (fineness), color and trash content. MVM uses a computer program to select cotton in groups of 80 bales from our warehouse to control yarn and fabric properties. This selection allows for control of yarn and fabric characteristics over time to produce a more consistent product.
ELECTRIC ARC FLASH: Is not an electrical shock, it is an electrical discharge through air, from a high voltage source to another conductor or ground that typically lasts less than 1 second.
Discussion: An electric arc flash can generate temperatures high enough to melt nearby plastics and metals and can damage the electric circuits involved. Extremely high radiant energy from the arc can cause 2nd and 3rd degree burns on exposed skin and ignite flammable clothing. Burns can even occur through clothing.
FLAME RESISTANCE: The property of a material whereby combustion is prevented, terminated, or inhibited following the application of a flaming or non-flaming source of ignition, with or without subsequent removal of the ignition source. Source: NFPA 2112 (National Fire Protection Association).
FLAME RETARDANT: A chemical substance used to impart flame resistance. A flame retardant may be added to a fabric to make it flame resistant. Source: Bulwark Industry Update.
FLASH FIRE: A type of short-duration fire that spreads by means of a flame front rapidly through a diffuse fuel, such as dust, gas, or the vapors of an ignitable liquid, without the production of damaging pressure. Source: NFPA 2112 standard.
FR and AR : Clothing that resists ignition and can self-extinguish may be described as flame resistant or FR. Clothing made from materials that have also been arc tested to establish an arc rating may also be described as arc rated or AR.
Discussion: Only materials that have been tested for flame resistance by ASTM D6413 and have char lengths of less than 6” (150 mm) and afterflame of less than 2 seconds may be arc tested by ASTM F1959. This means that all AR materials are FR, but not all FR materials may be AR (some FR materials may not have been arc tested). Protective garments made from Mount Vernon Mills fabrics will be both FR and AR.
HEATFLUX: The thermal intensity indicated by the amount of energy transmitted divided by area and time kW/m2 [cal ⁄cm2s]. Source: ASTM F1959
Discussion: In thermal testing the heatflux is a measurement of the rate of energy transfer. In some thermal tests in NFPA 2112 and heatflux of 2 cal/cm2/sec (84kW/m2) is used for testing.
INHERENTLY FLAME-RESISTANT FABRICS AND TREATED FABRICS: Flame resistant fibers and fabrics can generally be divided into two groups: those that are inherently flame resistant, and those that achieve flame resistance through special treatments. Inherently flame resistant fibers/fabrics are those where flame resistance is an essential characteristic of the fiber from which textiles are made. Treated fibers/fabrics have an applied chemical treatment to change the original level of a specific property or properties, in this instance flame resistance. Source: Bulwark Industry Update.
Discussion: Treated fabrics may also incorporate some inherent fibers to improve or impart additional properties.
LYOCELL: A cellulosic fiber made from purpose-grown trees by an energy efficient, environmentally friendly process employing a non-toxic solvent that is recovered and reused. Tencel® is the branded lyocell fiber produced by Lensing.
Discussion: Lyocell fiber properties are similar to but typically exceed cotton for strength, absorbency and elasticity. This makes lyocell an excellent blending partner for cotton.
META-ARAMID: A synthetic polymer known for high strength and its inherent resistance to heat and flame. These fibers are dyed by use of a solvent at elevated temperature. Nomex® is a branded meta-aramid fiber produced by DuPont.
MODACRYLIC: A synthetic polymer comprised of less that 85% but at least 35% by weight of acrylonitrile groups. Modacrylic fibers are inherently flame resistant but sensitive to heat and commonly blended with other fibers like cotton. Protex® is a family of branded modacrylic fibers produced by Kaneka.
Discussion: Fibers containing 85% or more acrylonitrile groups are called acrylic. The name modacrylic comes from the term “modified acrylic” as the acrylic fiber is modified to make it flame resistant.
NYLON: A synthetic polymer characterized by its strength, flexibility, toughness, elasticity, abrasion resistance and dyeability. Nylon is incorporated in FR fabrics to increase durability, especially resistance to abrasion. Nylon is known as a polyamide (PA) in Europe.
PARA-ARAMID: A synthetic inherently flame resistant polymer with high strength and heat resistance properties. Although not dyeable by conventional methods, small amounts are added to FR fabrics to increase thermal performance or abrasion resistance. Kevlar® is a branded para-aramid fiber produced by DuPont.
POLYMER: The name given to high molecular weight long chain molecule comprised of many repeats of a basic structure known as a monomer.
PPE (Personal Protective Equipment): Equipment worn to minimize exposure to hazards that cause serious workplace injuries and illnesses. These injuries and illnesses may result from contact with chemical, radiological, physical, electrical, mechanical, or other workplace hazards. Source: OSHA.
Discussion: The 2018 edition of NFPA 70E designated arc rated clothing as PPE.
SPANDEX: A synthetic elastomeric polymer consisting of at least 85% by weight of polyurethane. This fiber can be used to impart stretch properties to specially designed flame resistant fabrics to increase comfort and range of motion. Lycra® is a branded spandex produced by Invista.
Discussion: Although the fiber is flammable, small amounts of spandex can be used in FR garments. Higher amounts of spandex in a fabric do not guarantee higher levels of stretch. Fabric construction and spandex content determine the stretch properties of a fabric.
STOLL CURVE: An empirical predicted second-degree skin burn injury model, also commonly referred to as the Stoll Response. Source: ASTM F1959.
Discussion: The Stoll curve established a relationship between human skin response to thermal energy and the response of a copper calorimeter to the same energy levels. This relationship allows for study of thermal properties of materials without burn injury to humans or animals.