In the earlier years of NFPA 70E and the emergence of arc flash protection requirements, many people would use the NFPA 70E Hazard/Risk Tables to determine what arc rated PPE to wear. This approach continues to shift towards the use of arc flash studies involving incident energy and arc flash boundary calculations based on IEEE 1584.
It seems like more companies are performing Arc Flash Hazard Calculation Studies (AFHCS) than ever before. One of the biggest attractions is that the study results provide just about everything you need to comply with many of the NFPA 70E requirements. Incident energy and arc flash boundary calculations, protective clothing and equipment ratings, arc flash warning labels, recommendations to reduce the hazard and even an updated single line diagram can all be provided as part of the study. With a list like this, you might be thinking “What are we waiting for – How do I begin?” The book “Complete Guide to Arc Flash Calculation Studies” that I wrote, provides a step by step approach which is summarized in this article.
Data Collection and Modeling
The largest effort is gathering all of the necessary data. Information such as the equipment type, conductors, grounding, working distance, protective devices, utility company data and more are all part of the process. Data collection can often account for more than half of the entire study effort.
With data in hand, the next step is to create a model, or “road map” of the power system. Many computer programs are available that can simplify this process by organizing the data and performing a series of complex calculations based on IEEE 1584 – IEEE Guide for Arc Flash Hazard Calculations.
In addition to performing the study based on normal operating conditions, it may also be necessary to evaluate alternate scenarios such as a tie being open or closed. These additional scenarios can sometimes produce results that are worse than the normal case.
Short circuit calculations
Short circuit calculations are the next step. The traditional “bolted” short circuit current is determined for each location and then used to calculate the arcing short circuit current. The “bolted” condition assumes there is a solid connection at the point of the fault with no additional impedance. However during an arc flash, the arc creates additional impedance that will cause the “arcing” short circuit current to be less than the bolted current.
Arc flash duration and time current curves
Not only is the severity of an arc flash related to the short circuit current, it also depends on how long it lasts. Time current curves are normally used to evaluate how long an upstream protective device takes to operate and clear the arc flash. These curves are graphical representations of how the device responds for a given amount of current.
If a time current curve indicates the device may take a long time to operate, usually due to a very low short circuit current, IEEE 1584 suggests capping the maximum time at two seconds. This assumes a person will react and jump out of the way as long as there are adequate conditions.
Incident energy calculations
The most important component of the study are the incident energy calculations which are based on the arcing short circuit current and arc flash duration. These calculations are used to predict the prospective incident energy that could reach a worker at a specific distance from the arc, known as the working distance. Caution must be used because if any part of the person is closer than the working distance, the incident energy could be significantly greater than calculated.
Arc flash Boundary
Another major component of the study is the arc flash boundary. Incident energy decreases exponentially with distance and this boundary is defined as the distance from the arc flash where the incident energy falls to 1.2 cal/cm^2. This value is considered the energy that can produce the onset of a second degree burn and working closer than this distance requires the use of proper protection.
PPE Selection and Warning Labels
Two final components of the study are the selection of appropriate protective clothing and equipment and creating arc flash warning labels.
Protective clothing and equipment must be selected with an arc rating that is sufficient for the calculated incident energy. Quite often two arc ratings are specified. The first is for protection that is used on a more regular basis and is suitable for the majority of locations. The second rating is higher and is used at locations where the incident energy is much greater.
The Arc Flash Study makes creating warning labels much easier because all of the necessary information is part of the study results. Many computer programs programs will automatically generate the labels by incorporating study results such as the incident energy, working distance, arc flash boundary and other important information directly into the labels.
It’s All There!
Although the results of the Arc Flash Study can make complying with NFPA 70E much easier, it is important to recognize that the calculations are approximate at best and are based on theory and research. Workers can still be injured or worse during an arc flash. The safest approach will always be to place equipment in an electrically safe working condition.