After having consultants crawl all over the place, asking questions and gathering mounds of data, the arc flash study for your facility is finally done. Now the big question: with endless calculations based on IEEE 1584, IEEE Guide for Performing Arc-Flash Hazard Calculations, what do the numbers mean? More important, how do you incorporate the results into your electrical safety program? The good news is most arc flash studies include a table that summarizes the results in an easy-to-read format such as the one shown below. Understanding it is easier than you might think.
The bus name provides a unique identifier for the equipment. It typically will be representative of the equipment.
NFPA 70E requires the voltage exposure to be identified as part of the shock hazard analysis and that it is included on the energized electrical work permit and arc flash warning labels. This item documents the voltage for the specific equipment.
Depending on the equipment type, such as panel or switchgear, the IEEE 1584 arc flash calculations may use different arc gaps, working distance and rate of decay of the incident energy as a function of distance. Each of these values affects the results and makes defining the correct equipment type very important.
An arc flash occurs when short-circuit current jumps across an air gap. The gap’s size affects the arcing short-circuit current calculations and possibly also the duration. These values are used to calculate the prospective incident energy.
Bolted fault current
“Bolted” refers to a short circuit where the faulted conductors act as though they are solidly connected or bolted together. This current is used to evaluate equipment’s interrupting or withstand rating and to calculate the arcing fault current.
Estimated arcing fault current
The arcing fault current is the current that flows across the arc gap. It is used for determining the arcing time and incident energy and is influenced by variables, such as system grounding, the arc gap, and whether the arc flash occurs in equipment or in open air.
The duration of an arc flash is normally defined by the time it takes an upstream protective device to clear the fault. Time-current curves from a coordination study and the estimated arcing short circuit are used to determine this value. (Editor’s note: Stay tuned for Jim Phillips’ article about coordination in the May issue.)
Arc flash boundary
The distance from an arc that it takes for the incident energy to fall to 1.2 cal/cm2 is known as the arc flash boundary and is part of the calculation process. Defining this boundary is an NFPA 70E requirement for the arc flash hazard analysis.
The working distance defines the distance from the arc flash where the incident energy is calculated. It is normally considered where a worker’s head and torso would be located.
The incident energy at a specific working distance defines the severity of the arc flash and is used to determine the minimum protection for the worker.
Although there sometimes is debate about using personal protective equipment levels, it is still a common practice. Many in the industry will assign a level using numbers that typically correspond to a minimum arc-rating requirement for protective clothing and equipment of 4, 8, 25 and 40 cal/cm2 respectively
Although an arc flash study can contain many calculations and could seem confusing, the summary table helps the user more easily incorporate the results into an electrical safety program.