The National Fire Protection Association (NFPA) recommends identifying two boundaries to define the safe working limits for personnel working in an area with shock hazards. Each area is associated with a level of training and PPE. NFPA 70E Tables 130.4(D)(a) (ac voltages) and 130.4(D)(b) are used based on the nominal voltage of the equipment.
The arc flash boundary (AFB) is the minimum ‘safe’ distance from exposed energized conductors or circuit parts that has a potential for an arc fault. It is defined as the distance at which, in the event of an arc flash, a worker would be exposed to a thermal event with incident energy of 1.2 cal/cm² for 1.0 second. With this exposure, a worker may receive the onset of a second degree burn to exposed skin, typically the neck, face and hands. If it is necessary for workers to cross the arc flash boundary, and potentially be exposed to higher incident energies from any arc flash, they must be wearing appropriate personal protective equipment (PPE).
The required arc-rated clothing and PPE increases rapidly as a worker approaches the potential source of an arc flash. Incident energy, the heat generated by an arcing fault, increases by the inverse square of the distance (the Inverse Square Law) as it is approached. More simply stated, the incident energy increases rapidly as the distance decreases. When troubleshooting or any task where there is a potential for an arc flash, workers should always be aware of their body position and working distance.
The limited approach boundary
This is the minimum permitted distance that unqualified and unprotected personnel may approach an energized component. Before crossing the limited approach boundary and entering the limited space, a suitably qualified person must use the appropriate PPE and be trained to perform the required work. An unqualified person may enter the limited approach area if they are continuously escorted by a qualified person. If the qualified person leaves the limited space for any reason, the unqualified person must also exit. Under no circumstances can an unqualified person cross the restricted approach boundary.
The restricted boundary
To cross the restricted boundary and access the restricted space, personnel need to have been trained in shock protection techniques, be wearing the correct PPE and have a written and approved plan for any work in the zone. The plan must make it clear that the worker must not enter the prohibited space or cross the prohibited boundary either personally or by using any equipment or tool.
When performing a risk assessment, it must be determined where to place safety barriers to limit access for unqualified persons. Either the limited approach boundary or the arc flash boundary is used. So, for example, if the arc flash boundary is greater than the limited approach boundary then no unqualified person can be permitted to cross the arc flash boundary and even qualified workers must wear appropriate arc-resistant PPE.
If the limited approach boundary is greater than the arc flash boundary, the barrier is set to the limited approach boundary. Refer to your company’s electrical safety program for their requirements for setting up barriers and signage.