NFPA® 70E – Basic Terms & Definitions

ARC Flash – An ARC flash is a fault current flowing through air between energized phase conductors, from either a phase to ground or a phase to phase fault. This results in a rapid, explosive release of radiant (90%) and convection (10%) heat energy resulting from the breakdown of air insulation into a highly conductive plasma reaching temperatures 4 times hotter than the sun (35,000 ºF).

An ARC flash can be caused by a worker moving near or coming into contact with energized conductors, which causes a spark that breaks down air insulation between conductors. Also, failure of equipment may create a spark that causes an ARC flash between energized conductors. ARC flash heat energy is measured in Joules/cm2 (J/cm2) or Calories/cm2(Cal/cm2).

ARC Rating – This is the value of energy needed to be able to pass through any particular fabric, with a 50% probability of causing a second or third degree burn. The ARC rating value is measured in calories/cm². The ARC Rating for an article of clothing is determined by a Hazard/Risk Assessment and the relevant Hazard Risk Category (HRC), and is usually measured in terms of ATPV or EBT.

Basically the ARC rating determines the protective characteristics of the fabric. The higher the ARC rating, the more protection it provides.

ATPV – The Arc Thermal Protective Value is the maximum incident heat energy that a fabric is able to absorb. This will help to lessen the severity of an injury to a 2nd degree burn. For example, if you are exposed to a potential incident heat energy level of less than 4.0 cal/cm2, the proper ATPV clothing system is 4 cal/cm2.

Calorie – The calorie is the energy required to raise one gram of water by one degree Celsius at one unit of atmospheric pressure. Second-degree burns can occur at 1.2 calories per centimeter squared per second (cal/cm²).

Flash Hazard – This is a very dangerous condition caused by the release of energy from an electric ARC.

Flash Hazard Analysis – This analysis is a study investigating the potential exposure to ARC-flash energy that may occur or be present during a specific job task. The data collected from an analysis is used for to determine safe work practices, and the appropriate FR clothing and PPE needed to prevent injuries.

Flash Protection Boundary –This is the distance from an exposed live electrical line within which a person could receive a second-degree burn if an electrical ARC were to occur.

FR (Flame Resistant) – Flame resistance refers to a material’s ability to self-extinguish upon the removal of an ignition source.

NFPA® 70E – The work standard published by the National Fire Protection Association (NFPA®) that covers all aspects of electrical safety in the workplace. This includes the necessary recommendations for adequate protection and FR clothing required for those working with, or around energized equipment.