The Arc Rating and Hazard/Risk Category (HRC) of an FR garment are two terms that are used a lot and for some they can be misunderstood. Every FR garment being used to protect against an electric arc flash should have an arc rating assigned to it. Both NFPA 70E and the National Electrical Safety Code (NESC) require the use of arc rated materials in the FR clothing used by the workers covered under these safety standards.

What is an Arc Rating?
The arc rating of a material is defined by ASTM as the “value attributed to materials that describe their performance to exposure to an electrical arc discharge”. The arc rating is expressed in calories per square centimeter (cal/cm²) and is derived from the determined arc thermal performance value (ATPV) or energy of breakopen threshold (EBT). Both are considered the arc rating for the material. An ATPV is the amount of thermal energy, in cal/cm², applied to a material or layers of materials that result in a 50% probability sufficient heat transfer is predicted to cause the onset of a second-degree skin burn injury based on the Stoll skin burn injury model. An EBT on the other hand is the incident energy on a material or layers of materials that result in a 50% probability the material will break open due to the exposure. Both of these arc ratings are determined by performing the ASTM 1959 test method titled “Standard Test Method for Determining the Arc Rating of Materials for Clothing”. The test exposes flat fabric samples to varying intensities of electric arcs. The samples are placed over heat sensors which measure the heat rise during the arc exposures. The results are recorded and analyzed to determine when there is a 50% probability of a second degree burn which is its ATPV. If the material breaks open before the heat rise gets to the ATPV an EBT is assigned in lieu of an ATPV. Either an ATPV or EBT are acceptable arc rating to meet the standards. Typical arc ratings are numbers like 5.1 cal/cm², 8.7cal/cm², and can be as high as 35.8 cal/cm² or higher. An EBT can be a similar number and is more commonly seen as a rating for knits, fleeces or higher rated insulated materials.

What is a Hazard/Risk Category (HRC)?
HRC is a term used specifically in the NFPA 70E Standard for Electrical Safety in the Workplace. The NFPA 70E Standard suggests two methods of determining the appropriate protective clothing and other personal protective equipment (PPE) for protection against an arc flash. One is based on performing an actual incident energy (cal/cm²) analysis for each specific task, while the other uses the Hazard/Risk Category table found in the standard. If the incident energy analysis is used, the protective clothing and other PPE must be equal to or greater than the incident energy (cal/cm²) established from the analysis. If the table is used, each specific task listed establishes one of five HRC category requirements for the protective clothing and PPE to be worn while performing that task. Each category outlines the minimum arc rating (cal/cm²) for the FR clothing and other PPE required. Here are the basic requirements for the 5 HRC categories.

HRC 0 – FR clothing is not required but the clothing must be non-melting or untreated natural fibers. PPE includes safety glasses or goggles, hearing protection and leather gloves.

HRC 1 – FR clothing with a minimum arc rating of 4 cal/cm² (long sleeve shirt & pants or coverall, and arc rated face shield or hood). PPE includes hard hat, safety glasses or goggles, hearing protection, leather gloves and leather work shoes optional.

HRC 2 – FR clothing with a minimum arc rating of 8 cal/cm² (long sleeve shirt & pants or coverall, and arc rated face shield or hood). PPE includes hard hat, safety glasses or goggles, hearing protection, leather gloves and leather work shoes optional.

HRC 3 – FR clothing with a minimum arc rating of 25 cal/cm² (either single layer shirt and pant, coverall or a multilayer flash suit system and arc rated flash suit hood). PPE includes hard hat, FR hard hat liner, safety glasses or goggles, hearing protection, arc rated gloves and leather work shoes.

HRC 4 – FR clothing with a minimum arc rating of 40 cal/cm² (either single layer shirt and pant, coverall or multilayer flash suit system and arc rated flash suit hood). PPE includes hard hat, safety glasses or goggles, hearing protection, leather gloves and leather work shoes.

What is the NESC ARC Rating System?
The NESC has a slightly different arc rating system. It requires a hazard risk assessment to establish the exposure potential of an arc flash for the various work practices. If the hazard has an exposure potential of 2 cal/cm² or greater then arc rated clothing must be worn. Like NFPA 70E there is an option to wear protective clothing with an arc rating equal to or greater than the exposure potential established, or follow one of the simplified tables. In the NESC tables assign a 4, 8 or 12 cal/cm² clothing system to the various work functions based on voltage, fault current and clearing times. However, it does have a couple of exceptions. The first is to allow for lower rated garments if the exposure is below 1000 volts. In this case you simply wear a 4 cal/cm² clothing system. The second is if the designated clothing system can create a greater risk in which case you can move to the next lower rating.

Summary
Whether you look for an arc rating or an HRC the key is to provide protective clothing and other PPE that is rated to protect at or above the potential arc flash exposure of the workers. Performing a proper hazard assessment, researching the standard that applies to your situation, and purchasing the appropriate protective clothing and PPE will help keep workers protected.

Mark Saner – Technical Manager