Arc Thermal Performance Value (ATPV): This value is presented in calories per square centimeter and represents the maximum capability for arc-flash protection of a particular garment.  This rating also applies to fabrics.  A garment made from more than one layer of arc-flash rated fabric will have a calories per square centimeter rating greater than the sum of the ATPV ratings of the original fabrics.  The calories per square centimeter rating of most arc-flash protective suits, coveralls, and coats is commonly sewn into the fabric in large letters on the outside of the garment.

Flame Resistant (FR): Flame resistant can describe a fabric naturally resistant to burning but also can represent a material with special treatment applied to the fabric.  Occasionally, the letters FR are used to represent flame retardant. This can lead to some confusion because a flame-retardant treated fabric is flame resistant, but a flame-resistant fabric is not necessarily flame retardant.

Flame Retardant: This term could be used to describe a fabric made up of a flammable fabric treated in such a way that it will provide arc-flash protection.

Fabric Weight: This is usually represented in one of two ways: ounces per square yard or grams per square meter. Both of these values essentially refer to the thickness of the fabric. The more ounces per square yard, the more material exists in the same square yard of fabric.

Heat Attenuation Factor (HAF): This is the amount of heat blocked by the fabric. Even though a fabric may be 100 per cent flame resistant, that does not mean it will block all of the heat to which it is exposed. An HAF of 85 percent means that it will block 85 percent of the heat the fabric encounters. This applies to a short burst of heat – typically less than one second. In the event of prolonged heat exposure, the HAF would be much lower.

Calories per Centimeter Squared: This is a number identifying the amount of energy that can be delivered to a point at a particular distance from an explosion. Once this value is known, the ATPV rating of the required for work at that distance from the potential flash hazard is also known.

Energy Break-Open Threshold (EBT): Primarily, this addresses the physical strength of the fabric with respect to thermal energy and at what value the fabric will fail.

Hazard Risk Category (HRC): This is a 2004 NFPA 70E rating of exposure levels for particular types of equipment. The values range from zero to four, with a zero HRC with a zero HRC not requiring any ATPV-rated. The minimum ATPV rating for Categories One through Four are as follows:

• Category One: five calories per square centimeter
• Category Two: eight calories per square centimeter
• Category Three: 25 calories per square centimeter
• Category Four: 40 calories per square centimeter

Arc flash clothing has been around for years now.  Early personal protective equipment was typically made of one or two fabric layers.  Heavier clothing was used when the need for greater protection existed.  Today’s arc flash clothing is made with a multi-layered construction, making the clothes lighter and easier to wear.  Wearing layers of clothing increases the ability of the PPE to dissipate the heat from arc flash.   It is important to view PPE as a system, because of the need to use individual articles of clothing and equipment together to improve performance against arc flash hazards.

A general guideline is that for every layer of clothing worn under arc-rated clothing, heat will drop by fifty percent per layer.  Keep in mind this is for bodily areas that are covered.  The air between the layers of clothing will increase the ability of the arc-clothing to dissipate heat.  Now, if a person wears an underlayer made of meltable fabrics such as spandex, the chance greatly increases of residual burns from arc flash.  Spandax, being a synthetic fabric can melt even at 180 degrees F. temperatures.    Enough heat can transfer through arc-rated clothing to melt underlayers.  For this reason, cotton underlayers are recommended in most cases although fire resistant underlayers are ideal.

Understanding the importance of a layered system of protective clothing and additional PPE is part of being a qualified electrical worker as well as providing protection in the case of electrical incident.  Layering clothing provides extended performance of arc-rated clothing and offers a method to minimize burns to heat transfer during an arc flash.