An Arc number is a method used to identify the rating and the Arc Flash protection provided by a garment worn for a given work task. Some tasks will demand higher protection levels and this information should be provided to team members by you as a result of your risk assessment or Arc Flash study.
The National Fire Protection Association (NFPA) has identified four Arc risk category levels, which are numbered by severity from 1 to 4 (1 being the lowest and 4 being the highest risk). The Arc Category (Arc) is the level of Arc Flash protection clothing you must wear to protect against a minimum level of incident energy measured in calories/cm2.
Faulty electrical equipment, depending upon the energy delivering capability, can cause an explosion or Arc Flash incident of a certain magnitude, again measured in calories/cm2. The resulting explosion is able to deliver an amount of heat to a certain distance.
Each level of Arc Flash protection is rated to deliver a certain amount of flame resistance.