Hazard Risk Category (HRC) is a rating range directly related to ATPV. There are 5 HRCs ranging from 0 to 4, with a hazard risk of 0 presenting the least risk and a hazard risk of 4 being the greatest risk.

FR shirt and FR pants or FR coveralls (1 layer)

4

Cotton underwear plus FR shirt and FR pants (1 or 2 layers)

8

Cotton underwear plus FR shirt and FR pants plus FR coveralls, or Cotton underwear plus two FR coveralls (2 or 3 layers)

25

Cotton underwear plus FR shirt and FR pants plus multilayer flash suit (3 or more layers)

40

Arc Thermal Performance Value (ATPV) is a rating assigned to FRC indicating the level of protection provided. Higher fabric weights typically have higher ATPV's and provide increased protection as does the layering of flame-resistant clothing. ATPV is measured in calories per centimeter squared (cal/cm2).

The National Fire Protection Agency's (NFPA) 2112 is the Standard on Flame-Resistant Garments for Protection of Industrial Personnel Against Flash Fire. The standard provides requirements for design, performance, certification requirements and test methods for flame-resistant garments for use in areas at risk from flash fires.

The National Electric Safety Code (NESC) is the standard used by electric utilities to implement safety procedures for utility workers. NESC is also the standard OSHA uses when enforcing electrical utility safety. The latest revision, NESC 2007, includes flame-resistant clothing as a requirement. Similar to NFPA70E, the NESC standard requires utilities to perform a risk assessment and then to require workers to wear flame-resistant clothing with an effective rating equal to the risk. NESC is effective starting January, 2009.

The National Fire Protection Agency's (NFPA) 70E is the Standard for Electrical Safety Requirements for Employee Workplaces. NFPA70E requires employees to wear flame resistant protective clothing wherever there is a possible exposure to electric arc flash. Although it is a voluntary standard, NFPA70E is considered a "generally accepted industry standard" and thus OSHA will fine companies under the general duty clause, which requires employers to take the appropriate steps to protect workers. NFPA70E is widely accepted throughout general manufacturing as well as the electrical industries.

NFPA 70E requires employers to perform a flash hazard analysis to determine the flash protection boundary distance. The standard is designed to protect employees working inside these flash protection boundaries by requiring protective clothing for the corresponding Hazard/Risk Category (HRC) that has an arc thermal performance value (ATPV) of a least the value listed in the "Protective Clothing Characteristics" section of the standard.