The NFPA 70E defines three boundaries for electrical workers to observe. Two boundaries deal specifically with shock hazards and are intended to prevent shock and electrocution. The third boundary is the arc flash boundary. The arc flash boundary has a primary purpose of preventing burn injury due to arc flash incidents.
The Limited, and Restricted approach boundaries deal strictly with shock prevention and are based upon nominal system voltage. These boundaries are not calculated and apply only when exposed energized conductors or circuit parts are present.
Arc Flash Boundary
The Arc Flash Boundary deals specifically with arc flash hazards and determines when arc flash PPE shall be required. It is defined as the distance at which a worker is exposed to 1.2 calories/cm2 of incident energy, which represents the onset of a second degree burn. This boundary is calculated based upon specific system information, and can apply when exposed energized conductors or circuit parts are present, or with doors and equipment closed if interacting in a manner that could cause an arc. Detailed formulas for determining the arc flash boundary can be found in the NFPA 70E and IEEE 1584.
Both Qualified and Non-qualified can cross the Arc Flash Boundary as long as long as Qualified personnel supervision is present. No matter who crosses the Arc Flash Boundary, proper PPE is required that meets the requirements of the NFPA 70E.
Limited Approach Boundary
Distance from an exposed live part within which a shock hazard exists. To cross the limited approach boundary a worker must be qualified or otherwise escorted by a qualified person. In many cases the limited approach boundary is the farthest electrical boundary that exists. However, when the arc flash boundary is larger, be sure proper arc flash PPE is worn within the limited approach boundary.
Restricted Approach Boundary
The Restricted Approach Boundary may only be crossed by a qualified worker. Non-qualified workers are prohibited from crossing the restricted approach boundary and entering the restricted space. Qualified workers must don applicable arc flash PPE as well as shock PPE appropriate for the voltage a worker is exposed to. An approved work permit where applicable is needed to cross this boundary, with some form of documentation in place at a minimum.
Determining Protection Boundaries
The boundaries that focus on protecting workers from electrical shock and electrocution; Limited Approach, and Restricted Approach, are determined by referencing the NFPA 70E Tables 130.4(C)(a) for AC systems and 130.4(C)(b) for DC systems. The boundaries are directly related to their voltage levels and do not change based upon various system attributes. Know your voltage and have access to the table is all that is needed to determine the shock boundaries.
Determining the Arc Flash Boundary is not nearly as easy as determining the three shock boundaries. The Arc Flash Boundary is only determined through a detailed power system analysis referred to as an Arc Flash Analysis. This takes into account various system variable beyond knowing the system voltage in order to accurately determine the proper distance for the Arc Flash Boundary.