The final part of this series examines incident energy and arc flash boundary calculations for the 480V panelboard example used in this series. IEEE 1584—IEEE Guide for Performing Arc Flash Hazard Calculations defines incident energy as, “The amount of thermal energy impressed on a surface, a certain distance from the source, generated during an electric arc event.” It can be used to determine the arc rating of protective clothing and equipment as part of an arc flash risk assessment.
Two important variables are the arcing fault current and the arc duration. The arcing fault current was addressed in part 2. The arc duration defines the total exposure by evaluating the time current characteristic of the protective device that is expected to operate and clear the arc flash. Part 4 concluded that the device for this example will operate in 1 electrical cycle, or 0.167 seconds.
IEEE 1584 defines the working distance as the distance between the potential arc source and the face and chest of the worker. Incident energy varies exponentially with the inverse of the distance, the greater the distance, the less incident energy and vice versa. IEEE 1584 lists “typical” working distances of 18 inches for low-voltage equipment such as panels and motor control centers, 24 inches for low-voltage switchgear and 36 inches for medium-voltage equipment. However, the specific task must also be considered when defining this distance. For the 480V panelboard example, 18 inches is used.
Another important variable is the electrode configuration. The first edition of IEEE 1584 was based on two electrode configurations: referred to as box configuration (enclosure) and an open configuration (in air). In each case the electrodes were oriented vertically, which is today’s equivalent of vertical conductors/electrodes inside a metal box/enclosure (VCB) and vertical conductors/electrodes in open air (VOA). Based on subsequent research, it was determined that the incident energy can be greatly affected by other electrode orientations and influence the trajectory of the arc. For an enclosure, this led to the introduction of vertical conductors/electrodes terminated in an insulating barrier inside a metal box/enclosure (VCBB) and horizontal conductors/electrodes inside a metal box/enclosure (HCB). HOA is for open air. For more about electrode configurations see “Questions and Answers” from the May 2019 issue of ELECTRICAL CONTRACTOR.
Table 1 provides a comparison of the incident energy and arc flash boundary based on the VCB, VCBB and HCB configurations. The incident energy increases with VCBB, and increases more with HCB. For panelboards such as this example, VCB is typically used, as well as VCBB if the feeder terminates into a circuit breaker, possibly behaving like VCBB. The temptation to default to the worst case HCB should be avoided unless it is a realistic configuration. It would likely not be used unless a conductor or bus was directed toward the worker.
When an arc flash occurs, the enclosure has a focusing effect on the incident energy. The IEEE 1584 equations for the incident energy and arc flash boundary calculations are based on a normalized 20-inch-by-20-inch enclosure opening. If the enclosure opening is larger, the energy will less focused, resulting in less energy per unit area. Table 2 compares the calculated incident energy using the VCB configuration and 18-inch working distance with different enclosure sizes ranging from 20-inches-by-20-inches to 36-inches-by-36 inches.
Arc flash boundary
IEEE 1584 defines the arc flash boundary as “A distance from a prospective arc source at which the incident energy is calculated to 1.2 cal/cm2.” This is the value of incident energy where arc rated clothing and associated protective equipment is required. Tables 1, 2 and 3 also include the arc flash boundary to illustrate the effect of the electrode configuration, enclosure size and arc duration on the boundary.
The calculation results in part 5 indicates the incident energy is low, and in some cases less than 1.2 cal/cm2. This is due to the very fast 1-cycle clearing time. The duration is often considered the most critical variable, and if the duration increases, the incident energy increases proportionally. Table 3 illustrates the effect that longer arc durations may have.
Arc flash studies can be complex, requiring important decisions regarding the variables used. Each must be carefully considered because they can all significantly impact the results.
Note: Parts 1 through 4 of this series can be found at ecmag.com and on ecmagdigital.com.