Cal/cm² & ATPV How They Work to Keep Us Safe
How often have you heard someone say, “I need an 8 cal shirt.” Or, “My clothing needs a minimum of an 8 cal rating.” Or even, “My blast suit is 40 calories.” These common statements are the reason the NFPA 70E committee changed their terminology from hazard risk category (HRC) levels to personal protection equipment (PPE) levels in the 2015 revision of this standard. People unintentionally used the calorie rating of an HRC as a protection level of PPE when instead it is a way of determining the potential level of exposure during an arc flash event which then allows us to select the proper PPE which will then protect the worker in a worst case scenario involving an arc flash.
When an arc flash occurs, among other things, there is a release of incident energy. Calories/cm² (Cal/cm²) is a unit we use to measure this incident energy. You may also hear this energy measured in Joules/CM² but Cal/cm² is more common. An arc flash duration is measured in milliseconds meaning arc flashes typically last less than a second. The exposure to flame is very short but the absorption of incident energy by the body during this event can have drastic consequences leading to sever burns.
1 Cal/cm² of incident energy will raise 1 gram of water 1 degree Celsius. This is important to us because the human adult body contains roughly 57-60% water. When we are exposed to incident energy the water in our tissues heats up and we can incur a burn even if our clothing is not ignited. The onset of second degree burn can occur at 1.2 Cal/cm². Many people use the analogy of 1.2 Cal/cm² being equal to holding a finger over a flame for one second.
Incident energy affects the human body differently than normal thermal events. As we know we measure burn degree by depth of burn through the skin. The deeper the burn is the higher the degree the burn is given. With incident energy exposure the water in our tissues become superheated and burn depth occurs very quickly. To avoid this we need to know what the potential incident energy level is during a particular arc flash event and label that measurement in Cal/cm². Once this assessment is complete the information is shown on a label which is then attached to the equipment in a visible location. When the level of the hazard is known we can proceed with selecting a garment that will help protect the employee.
When looking at an arc rated flame resistant garment the rating you should see included on a label, as required by the American Society for Testing and Materials (ASTM) standard F1506, is the arc thermal performance value (ATPV) of the fabric. As an alternative some fabric manufacturer’s report an EBT or energy breakthrough value for the garment. EBT is the point at which the fabric breaks open while ATPV is an insulating value. The ATPV number tells us the point at which 1.2 Cal/cm² of incident energy is transferred through the fabric and, as we discussed earlier, is the point at which a 2nd degree burn can be expected. Either number can be reported but the lower of the two numbers must be used. For instance if breakthrough occurs before a thermal value can be given then the EBT number must be used.
Understand that the garment being selected is not arc rated. The fabric the garment is made from is arc rated.. Flame resistant fabric intended to be used for arc flash hazard protection must be tested using the ASTM F1959 test standard. The test requires swatches of fabric to be exposed to different levels of incident energy. Remember this test rates FR fabric and it is not a pass or fail test. After testing is complete the manufacture then reports out either an ATPV or EBT number for that fabric.
The FR arc rated fabric can now be used to manufacture different types of garments such as coveralls, shirts, pants etc. If you have ever wonder how a mesh vest gets an ATPV or EBT rating it’s because the fabric used to make the vest actually has the rating. Common sense should tell us that a mesh vest is not going to protect us from burns in an arc flash incident. However, when worn over an arc rated shirt or coat, the vest rating tells us that the material the vest was made from will not melt, drip, or add to the flammability of the arc rated shirt or coat in an arc flash incident. This same common sense should be used when looking at short sleeve shirts. These products are great as an added safety feature but should never be worn with the expectation of protecting the entire upper body.
So now we understand what Cal/cm² means and what ATPV/EBT means. Cal/cm² is a rating for what level of incident energy we might expect should an arc flash occur and is directly related to equipment. ATPV and EBT are values given to FR fabrics that tell us, when exposed to incident energy, how that fabric will perform and what level of protection we can expect in an arc flash incident. Once again Cal/cm² is for equipment and incident energy. ATPV and EBT are values given to fabric. The Cal/cm² number is displayed on the equipment and the ATPV or EBT value is displayed on the garment label. The garment must have an ATPV or EBT value that is at least equal to or greater than the Cal/cm² number.
To ensure a successful arc rated garment program the employer should take the time to train employees on these two key values. Because many work environments allow for employees to work on their own, they need to understand what level of protection they need to perform their tasks safely. Also just because several pieces of equipment are on the same electrical feed does not mean they have the same level of hazard potential. Factors such as distance and resistance from the substation will affect the level of hazard even if they are the same type of equipment. Employees need to know what the label on the equipment means and what the Cal/cm² value means. They must then know to compare that number to the ATPV or EBT value and be sure that this value is greater or equal the incident energy potential to ensure the mitigation of serious injury.
If the ATPV or EBT value is not equal to or greater than the Cal/cm² then the employee must be able to recognize this and take the proper step to increase their protection level. This could include adding another layer of arc rated garments such as a coverall but know that in order to ensure a protection level through layering, all the fabrics used for a layering solution must have been tested together using the ASTM F1959 tests method to achieve a true ATPV or EBT value. You cannot use an addition method for determining layer protection. In other words 8 ATPV over 8 ATPV does mean 16 ATPV. Different fabrics react differently together resulting in various ATPV values so testing must be done to ensure a safe system.
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