A CAT rating (short for PPE Category rating) identifies the level of protection that a FR/AR garment offers in the event of an arc flash.
The categories themselves correlate to a more specific measurement of protection known as the Arc rating. The Arc rating is measured in calories/cm² and determines how many calories of energy the garment can withstand from an arc flash. Arc rating is defined as the amount of energy a given fabric can withstand before a 50 percent likelihood of the onset of second-degree burn through the fabric. In other words, what is really measured is insulation.
Before revisions to the NFPA 70E standard in 2015 that altered the terminology, CAT ratings were known as HRC, or Hazard Risk Category. HRC levels were also on a scale of 1-4 and were correlated with identical minimum Arc ratings.