Limited Approach Boundary
NFPA 70E and CSA Z462 have developed requirements to reduce the risk of injury due to shock and arc flash hazards.
There are three shock approach boundaries:
The limited approach boundary, restricted approach boundary and prohibited approach boundary classifications are based on the voltage of the energized equipment.
But before a worker can approach exposed electric conductors or circuit parts that have not been placed in a safe work condition, a flash hazard assessment must be performed. Until equipment is placed in a safe work condition, it is considered live. (It is important to note that conductors and equipment are considered live when checking for voltage while putting equipment in a safe work condition as well.)
The flash hazard analysis should determine the flash protection boundary (FPB) and level of personal protective equipment (PPE) that the worker must wear. The flash protection boundary is based on voltage, the available fault current and the time it takes for the upstream protective device to operate and clear the fault.
Limited Approach Boundary
NFPA 70 defines the limited approach boundary as a shock protection boundary to be crossed by only qualified persons (at a distance from a live part), which is not to be crossed by unqualified persons unless escorted by a qualified person.
The limited approach boundary is the minimum distance from the energized item where unqualified personnel may safely stand.
No untrained personnel may approach any closer to the energized item than this boundary. A qualified person must use the appropriate PPE and be trained to perform the required work to cross the limited approach boundary and enter the limited space.
Restricted Approach Boundary
This consists of the shock protection boundary, and may only be crossed by qualified persons (at a distance from a live part).
Due to its proximity to a shock hazard, the use of shock protection techniques and equipment are required. To cross the Restricted Approach Boundary into the Restricted Space, the qualified person (who has completed required training) must wear appropriate PPE. Also, he/she must have a written approved plan for the work to be performed and plan the work to keep all parts of the body out of the Prohibited Space.
Prohibited Approach Boundary
This boundary may be crossed by only qualified persons (at a distance from a live part) which, when crossed by a body part or object, requires the same protection as if direct contact is made with a live part. Only qualified personnel wearing appropriate PPE, having specified training to work on energized conductors or components, and a documented plan justifying the need to perform the work may cross the boundary entering the Prohibited Space. Therefore, personnel must obtain a risk assessment before the prohibited boundary is crossed.
All of these limited approach boundaries are determined in tables outlined in NFPA 70E and CSA Z462, and all are based upon the voltage of the equipment.
Flash Protection Boundary (FPB)
The FPB is a safe approach distance from energized equipment or parts. NFPA 70E establishes the default flash protection boundary at 4 feet for low voltage ( < 600V ) systems where the total fault exposure is less than 5000 amperes-seconds (fault current in amperes multiplied by the upstream device clearing time in seconds).
The FPB can also be calculated. In some instances, calculations may decrease the boundary distance. Persons crossing into the flash protection boundary are required to wear the appropriate PPE. In addition, a qualified person must accompany unqualified persons. The boundary is defined as the distance at which the worker is exposed to 1.2 cal/cm2 for 0.1 second.
For details on the procedure and the required equations for arc flash calculations, consult IEEE Std 1584 - 2002. The equations are used to calculate the incident energy and flash boundary. The IEEE procedure is valid for voltages ranging from 208V volts to 15kV with gap ranges between 3 mm. and 153 mm.
For more information, see: