As the standards for PPE improve and become more specialized, it becomes increasingly complicated to understand the codes and names given to various types of protective clothing. There are two acronyms that seem to be causing a lot of confusion – AR (arc rated) vs FR (flame resistant) clothing.
I’m going to be discussing both types of fire protective clothing in detail and provide in depth, detailed information on how the fabrics are tested, the type of protection they offer, and what you should look for when choosing the best fire PPE for your job.
Before tackling the intricate details, I’ll start with the basics, understanding the difference between AR and FR clothing and how the fabrics work.
The first thing that you need to know is that AR clothing is also FR but the same doesn’t apply in reverse. FR clothing is not AR. Why make things so complicated, you may be asking?
What is the Difference Between AR and FR Fabric?
To understand why different standards apply to the fabrics used to make AR and FR clothing we need to have a clear definition of the risks involved.
There is one basic rule that applies to all types of FR clothing. This is that it must be made from a fabric that resists ignition. In other words, it won’t catch fire when exposed to extremely high temperatures or flames. FR clothing must also insulate the wearer from heat and self-extinguish flames rapidly.
There are very specific testing methods to determine the safety ratings for FR clothing, but more about that later.
We need to have a clear understanding of how heat works to clearly distinguish between FR and AR clothing. It’s all about heat energy, how fires are started and how they burn.
How Heat Works
Energy (calories) will vary for different types of fire hazards. A large explosion or flash fire will have more energy than a regular fire caused by the ignition of flammable materials. Naturally, the fire with more energy will be more destructive and the risk of injury will, therefore, be higher.
If you’re exposed to an arc flash, molten metal, or a flash fire, your risk of second and third-degree burns increases greatly. This is because the fire lasts much longer.
Your clothing will continue to burn long after the initial exposure to the heat source. As a result, working where these conditions exist will require FR clothing that will provide the extra protection that is required.
As the name suggests, arc resistant clothing is designed to minimize the risk when exposed to an arc flash. It will also provide greater protection against flash fires and molten metal.
What Fabrics are Used for FR Clothing?
When analyzing flame resistant clothing of any type, the fabric used to make the garment is everything. Knowledge of these fabrics and how they work is essential when you head out to buy fire protective PPE.
There are two categories for these fabrics.
Inherent Vs Treated FR Fabric
The quality and level protection provided by an FR garment is determined mostly by the type of fabric that is used. It is also important to note that better, generally more expensive FR fabrics will usually last longer.
Inherent FR fabric is the best and will invariably be more expensive. These fabrics are made from scientifically formulated fibers that will not burn when ignited and include:
Treated FR fabric is made from conventional fibers. Cotton is one of the most common as it is naturally fire retardant, burns slowly, and has a tendency to self-extinguish. Though other fibers, like nylon, are also used.
The fibers are coated with fire retardant chemicals that prevent them from igniting. Treated FR fabrics are usually cheaper but will become less effective over time. Detergents used to wash the garment deteriorates the FR treatment. As a result, treated FR clothing should be washed using natural detergents.
Flame Resistant Clothing
Flame resistant fabric is certified by means of a vertical flame test (ASTM D-6413). This is the minimum standard for fabrics that resist ignition and further testing is conducted on more advanced protection against arc flash or flash fire.
The pass/fail nature of this testing method means that most FR fabric is only suitable for conventional fires and does not provide lasting protection against second and third-degree burns.
Arc Resistant Clothing
In order for PPE to be AR rated, more rigorous testing is required than other types of FR fabric. The National Fire Protection Agency (NFPA) introduced the NFPA 70E standard which stipulates the requirements for PPE required when the risk of an electrical arc is imminent.
AR clothing is tested in accordance with ASTM 1959 to determine its Arc Thermal Performance Value (ATPV). This is the amount of calories the fabric can withstand during an arc flash.
To ensure accurate testing, 21 samples of the fabric are exposed to an arc flash. Heat sensors then measure the heat transfer through the fabric. Stoll curve is the scientific method used to calculate the possibility of second-degree burns occurring.
Essentially, the primary difference between AR clothing and most other types of FR clothing is the level of insulation it provides. To achieve greater protection, AR clothing usually has an outer layer made from inherently FR fabric, like Nomex or Kevlar. One or more layers of insulation are used to provide protection against heat transfer. Generally, another layer of softer treated FR fabric, like FR cotton will be used for the inner lining of the garment which provides a final layer of protection close to the skin and makes the garment more comfortable.
ATPV values are expressed as single number representing the calories per CM² that the fabric can withstand. This is correlated against the Heat Risk Category (HRC).
When choosing the correct AR clothing you should look for the appropriate HRC rating for the work you do and then use AR clothing with the corresponding arc rating. For example, HRC1, requires a minimum arc rating of 4. Whereas the minimum arc rating for HRC4 is 40.