What are the shock approach and arc flash boundaries?
NFPA 70E has developed requirements to reduce the risk of injury to workers due to shock and arc flash hazards. There are three shock approach boundaries (limited, restricted and prohibited) required to be observed in NFPA 70E 2000. The limited, restricted and prohibited approach boundaries are based on the voltage of the energized equipment. Also, NFPA 70E 2000 requires that before a worker approaches 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 (NFPA 70E 2000 Part II 2-1.1.3), 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. The arc flash hazard analysis should determine the arc flash boundary (AFB) and level of personal protective equipment (PPE) that the worker must wear. The arc flash 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. The boundaries are summarized below:
NFPA 70 defines 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. The boundary is determined by NFPA 70E Table 2-1.3.4 and is based on the voltage of the equipment (2000 edition). 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.
A shock protection boundary to be crossed by only qualified persons (at a distance from a live part) which, due to its proximity to a shock hazard, requires the use of shock protection techniques and equipment when crossed. To cross the Restricted Approach Boundary into the Restricted Space, the qualified person, who has completed required training, must wear appropriate personal protective equipment (PPE). Also, he must have a written approved plan for the work that they will perform and plan the work to keep all parts of the body out of the Prohibited Space. This boundary is determined from NFPA Table 2-1.3.4 (2000 Edition) and is based on the voltage of the equipment.
A shock protection boundary to 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 personal protective equipment (PPE), having specified training to work on energized conductors or components, and a documented plan justifying the need to perform this work may cross the boundary and enter the Prohibited Space. Therefore, personnel must obtain a risk assessment before the prohibited boundary is crossed. This boundary is determined by
NFPA 70E Table 2-1.3.4 (2000 Addition) and is based upon the voltage of the equipment.
The AFB is a safe approach distance from energized equipment or parts. NFPA 70E establishes the default arc flash 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). NFPA 70E also allows the AFB to be calculated. In some instances, calculations may decrease the boundary distance. Persons crossing into the arc flash boundary are required to wear the appropriate Personal Protective Equipment (PPE) as determined by calculating methods contained in NFPA 70E. In addition, a qualified person must accompany unqualified persons. The boundary is defined by NFPA 70E as the distance at which the worker is exposed to 1.2 cal/cm2. IEEE Standard 1584 details the procedure and needed equations for arc flash calculations. The equations are used to calculate the incident energy and arc 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 cases where voltage is over 15 kV, or gap is outside the range of the model, the theoretically derived Lee method can be applied. ARCAD Arc Flash Analytic software automatically selects either IEEE empirical model or Lee method for arc flash calculations based on input system parameters.
