This is the fourth article in a series that provides a step-by-step approach for performing arc-flash hazard calculations. The first three parts appeared in the January, March and May 2016 issues of Electrical Contractor and can be found at www.ecmag.com.
Arc flash boundary
The 2015 edition of NFPA 70E defines the arc flash boundary (AFB) as, “When an arc flash hazard exists, an approach limit at a distance from a prospective arc source within which a person could receive a second-degree burn if an electrical arc flash were to occur.”
An energy exposure of 1.2 calories per centimeter squared (cal/cm2) is the widely accepted incident energy value considered to be where the onset of a second-degree burn could occur. The AFB must be determined as part of an arc flash risk assessment and a requirement for arc flash labels. It can be calculated using an equation defined by IEEE 1584—IEEE Guide for Performing Arc Flash Hazard Calculations. The calculation is based on using the results of the normalized incident-energy calculations from part 3 of this series, and the rate at which the incident energy decreases with distance is defined by the distance exponent X found in the table above. The distance exponent is a function of both voltage and the type of equipment.
The IEEE 1584 equation for the AFB is:
DB = [(4.184 × Cf × Ein × (t/0.2) × (610x/EB)]1/x.
The terms used in this equation include:
DB = Distance in millimeters of the AFB
Cf = Calculation factor of 1.5 for voltages < 1 kV as discussed in part 3
Ein = Normalized incident energy determined in part 3
EB = Incident energy in Joules per cm2 (J/cm2) at the AFB. This is normally 5 J/cm2, which is equal to 1.2 cal/cm2.
4.184 = Conversion from cal/cm2 to J/cm2
t = Arcing time in seconds normally determined by the clearing time of the upstream protective device as discussed in part 3
X = Distance exponent from Table 1
610 = Normalized distance of 24 inches converted to millimeters
0.2 = Normalized 0.2 second arcing time
Example
This example illustrates how to calculate the AFB for the 480-volt (V) panel that has been used for this series. I calculated the normalized incident energy in part 3 as 4.82059 cal/cm2. I also previously provided the arcing time as 0.05 seconds (3 cycles).
Similar to the previous parts in this series, I developed a worksheet below to simplify the calculation process.
Step 1: Enter the distance exponent X and incident energy EB at the AFB in terms of J/cm2. EB is normally 5 J/cm2.
Step 2: Enter the previously provided arcing time of 0.05 seconds. Since Ein is normalized to 0.2 seconds, this step scales the calculation by the ratio of the arcing time that is used divided by the normalized arcing time.
Step 3: Multiply the normalized incident energy by 4.184 to convert from cal/cm2 to J/cm2. Also, multiply by the 1.5 calculation factor Cf for voltages <1 kV.
Step 4: Multiply answers from 1–3.
Step 5: Select 1.641 as the distance exponent X from the table above for the 480V panel, and determine its reciprocal. For example, since X is 1.641, the reciprocal is 1/1.641, which is 0.609. After determining this value, raise the answer from step 4 to the power of this number.
Step 6: The IEEE 1584 equations are based on metric units, so the arc flash boundary will be in millimeters.
For this example, the AFB has been determined to be 781.7 mm or 30.8 inches Working within this boundary when an arc flash hazard exists requires appropriate arc rated clothing and personal protective equipment. However, keep in mind the AFB only addresses the thermal hazard. There may also be other hazards such as shrapnel, ultraviolet light and sound that should be considered.
The next part in this series will tie all of the calculations together.
About The Author
PHILLIPS, P.E., is founder of brainfiller.com and provides training globally. He is Vice-Chair of IEEE 1584 Arc Flash Working Group, International Chair of IEC TC78 Live Working Standards and Technical Committee Member of NFPA 70E. He can be reached at [email protected].
