Arc flash labels produced up to the NFPA 70E 2012 arc flash standards had defined four boundaries: three are shock protection boundaries and an outer flash protection boundary based on calculations of the voltage of the electrical equipment affected. You may encounter these in the field so they will be covered briefly.
NFPA 70E 2015 arc flash boundaries has only two shock protection boundaries as it has eliminated the "Prohibited Approach" Boundary.
For 2012 labels (superseded by NFPA 70E 2015)
Flash Protection Boundary - This is the outermost boundary, furthest away from the energy source of the potential arc flash hazard an unqualified person can approach unaccompanied. An unqualified person may only cross this boundary accompanied by a qualified worker. Should an arc flash occur, the person exposed to the blast would be subjected to the heat generated by the flash resulting in second degree burns (1.2 calories/cm2) which is typically curable. At this distance PPE must be worn to prevent 2nd degree (or greater) burns if exposed to the arc flash.
Limited Approach Boundary - Only qualified persons within this boundary. This is the distance an unqualified person may enter when continuously accompanied by a qualified worker, wear the appropriate level of PPE and trained for the task.
Restricted Approach Boundary - Shock Protection and Flash Protection. This boundary is restricted to unqualified personnel. Only a qualified worker, wearing the appropriate level of PPE and properly trained for the task to be performed may enter this area. It also requires an approved work permit with a written plan of the task to be performed. The written plan should include well defined procedures to prevent exposure to shock hazards in the Prohibitive Approach Boundary defined below.
Prohibited Approach Boundary (the center most region removed in NFPA 70E 2015) - Treat as if in contact with live parts. Energized, exposed conductors or parts. This area exposes unprotected, conductive body parts and tools to direct electrical contact or arc. Only qualified workers can cross this boundary wearing the appropriate level of PPE rated for direct contact with live electrical equipment. A risk assessment must have been performed prior to entering.
Figure 1: Arc Flash Boundaries 2012