The US National Fire Protection Association (NFPA) recommends identifying three 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 data (contained in Table 130.7(C)(2)) allows you to calculate the boundaries using a formula based on the voltage of the equipment.
The arc flash protection boundary (FPB) is the minimum ‘safe’ distance from energized equipment 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 0.1 second. With this exposure, a worker may receive a 2nd degree burn to exposed skin. If it is necessary for workers to cross the flash protection boundary, and potentially be exposed to higher incident energies from any arc flash, they must be wearing appropriate PPE.
The limited approach boundary
This is the minimum permitted distance that unqualified and unprotected personnel may approach a live 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 under the supervision of a qualified person.
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.
In terms of safety, any worker crossing this boundary must be equipped and protected as they would be for making direct contact with the exposed live equipment. Establishing these boundaries is an important step in protecting staff from the dangers of electrocution. No worker should cross the prohibited boundary and enter the prohibited area unless:
The flash protection boundary and the rules governing access within it take precedence over the shock hazard boundaries. So, for example, if the flash protection boundary is greater than the limited approach boundary then no unqualified person can be permitted in the limited approach area and even qualified workers must wear appropriate arc-resistant PPE.