NFPA 70E defines three shock boundaries based on the voltage of electrical equipment, as well as a flash protection boundary. The various shock boundaries are determined by NFPA 70E Table 2-1.3.4. Both NFPA 70E and IEEE 1584 provide formulas for calculating the flash protection boundary.
Flash Protection Boundary
The Flash Protection Boundary (FPB) is defined as the distance at which a worker is exposed to 1.2 calories/cm2 of incident energy. The represents the distance at which PPE must be worn to prevent 2nd-degree or greater burns in the event of an arc flash.
Unqualified persons may cross the FPB only when accompanied by a qualified worker. Any persons crossing the FPB must wear appropriate PPE.
Real-world advice and insight into preventing arc flash accidents.
Shock Protection Boundaries
Limited approach boundary
The Limited Approach Boundary is the distance from a live part to which unqualified persons may approach unaccompanied. To cross the Limited Approach Boundary, unqualified persons must be accompanied by a qualified worker wearing the appropriate PPE and trained on the task to be performed.
Restricted approach boundary
The Restricted Approach Boundary may only be crossed by qualified workers with appropriate PPE and training on the task to be performed. In addition, the worker must have an approved work permit and written plan for the task. The plan should include shock-prevention procedures designed to keep all portions of the worker's body from crossing the Prohibited Approach Boundary (see below) at any time.
Prohibited approach boundary
The Prohibited Approach Boundary is defined as the distance from a live part which is equivalent to direct contact. Crossing this boundary with unprotected (conductive) body parts or tools risks an electrical arc. Any body part which crosses this boundary must be protected with PPE rated appropriately for direct contact with the part. A risk assessment must be performed and a
Calculating Protection Boundaries
The three shock protection boundaries (limited approach, restricted approach, and prohibited approach) are based solely on the voltage of the equipment. Distances for these boundaries can be found in NFPA 70E Table 2-1.3.4.
Calculating the Flash Protection Boundary is more complex, and is a major goal of conducting an arc flash hazard analysis. There are three major methods for determining the FPB:
- NFPA 70E provides a formula for calculating incident energy based on the theoretical work of Dr. Ralph Lee. This formula is detailed below.
- IEEE 1584 provides an alternative empirical formula based on extensive laboratory tests. This formula is widely held to provide more accurate results than the NFPA 70E method.
- For systems rated at <600V, with fault clearing times of less than 5000 amp-seconds, NFPA allows the assumption of a default FPB of 4 feet. (Since exact incident energy is not calculated under this method, NFPA 70E provides tables listing appropriate PPE by equipment voltage and type of work performed).
Commercially-available software can calculate FPB according to either NFPA 70E or IEEE 1584 methods, as well as performing many of the other tasks associated with an arc flash analysis. Duke Power's Heat Flux Calculator is free software for calculating incident energy for single-phase faults. Links to firms offering more advanced software can be found on our Other Resources page.
NFPA 70E flash protection boundary formula
Dc = (2(MVAbf)(t))1/2
where Dc is the distance (in feet) at which incident energy is 1.2 cal/cm2; MVAbf is the available bolted fault capacity) in megavolt-amperes); t is time duration of the fault (in seconds)