The first step in conducting an arc flash study is to obtain the data necessary to accurately represent the electrical system. Equations defined by IEEE 1584–IEEE Guide for Performing Arc Flash Hazard Calculations are at the heart of most studies and require a lot of data.
Depending on the size, age and complexity of the system, as well as what data is readily available from existing drawings and previous studies, the data collection could take quite an effort.
How much effort? I asked this survey question at www.arcflashforum.com: “What percentage of the study effort is data collection?” Respondents had a choice of answers, including the following: Less than 20 percent, 20 to 40 percent, 40 to 60 percent, and more than 60 percent. The vast majority of people agreed that data collection can often be 40 to 60 percent of the study effort.
Data collection begins by obtaining and reviewing any existing available documents, such as electrical design drawings, manufacturer’s equipment drawings, and previous short-circuit and coordination studies. Once the existing information has been reviewed, a field survey is usually necessary to validate it and to keep track of any changes or additions that have occurred as well as obtain any missing data.
Missing data can be handwritten directly on a single-line diagram and/or data collection forms, as it is obtained at the project site. A single-line diagram—whether you create or update it—is the most effective document to use for recording and organizing electrical data. An updated single-line can also be used as the record document for listing conductor, source, motor, transformer and protective device data as well as other pertinent information.
Short-circuit current data
Since incident energy is dependent on the magnitude of short-circuit current that could produce the arc flash, the available short-circuit current must be obtained from the electric utility company. In addition, if on-site generation is available, it should also be considered part of the study.
When requesting short-circuit data from the utility, emphasize that it will be used for an arc flash study. In addition to obtaining the data for normal conditions, it should also be obtained based on minimum short-circuit current conditions, if available. The minimum condition could be based on a utility transformer, out-of-service transmission line, or a similar scenario. The lower minimum current can be used to see if it would cause a protective device to operate more slowly and increase the incident energy.
During a short circuit, all directly connected running motors can act like generators and contribute short-circuit current to the location of the fault. Motor contribution must be considered because it can affect the arc flash study. The motor contribution addition could cause the incident energy to increase or decrease depending on how it affects the protective device’s operating time. As a minimum, IEEE 1584 2002 suggests including all motors 50 horsepower and greater.
Since the available short-circuit current at a given location is dependent on impedance, the impedance of individual circuit components—such as transformers, conductors and bus duct—must be considered. The higher the impedance is, the lower the short-circuit current that will flow. The lower the impedance, the more short-circuit current will flow. Many commercially available computer programs contain large databases of conductor, transformer and bus impedances, which help simplify the calculation process. In many cases, these applications only require a description of the component to automatically retrieve the correct impedance data.
Duration, equipment type and more
The duration of an arc flash is normally defined by how long it takes an upstream protective device to interrupt and clear the fault. Clearing times can be determined by evaluating a protective device’s time-current curve, which defines the specific tripping characteristics. Time- current curves are unique to each overcurrent device and can be obtained from the protective device manufacturer or from the extensive libraries of curves in many software applications.
Incident energy is not only dependent on the short-circuit current and device-clearing time, but also the equipment type. Whether the equipment is a panel, motor control center, switchboard or switchgear will affect several other specific variables used in the arc flash calculations. There also can be a difference in the calculation results, depending on whether the arc flash occurs in open air instead of inside equipment and whether the system is effectively grounded.
A well-organized and executed data-collection effort can make quite a difference for what typically accounts for 40 to 60 percent of the arc flash study.