Arc flash studies should be simple. Jim Phillips shows how to use the incident energy to validate an arc rating selection of PPE. His approach is to perform the arc flash study “backwards” as he describes it. Select the PPE arc rating that is considered reasonable and then use the study results to verify the locations where the arc rating is sufficient.
Jim uses the analogy of short circuit studies where the short circuit calculations are used to verify an interrupting rating. This same approach can be used for arc flash studies. This video is based on Jim’s class “How to Perform an Arc Flash Study” and is also discussed in his book: “Complete Guide to Arc Flash Hazard Calculation Studies”
When you’re performing an arc flash study, especially if this is one of the first arc flash studies that you’re undertaking, it’s really easy to get lost in all the data and in all the details. There’s just so much going on. But one of the things that you need to keep in mind– actually, to keep the focus on– is the primary objective of an arc flash study is simply to determine what level of PPE to wear. Whether you want to wear 8 calorie PPE, 8 calorie per square centimeter, or 12, or whatever your rating is.
And a couple years ago, I wrote an article. Basically, the premise of the article was to perform an arc flash study, perform it backwards. You might think, what do you mean by backwards? Well, what I mean is determine what you want your answer to be first. And again, you might think that is kind of backwards. Your answer first? That doesn’t sound quite right.
Well, actually, it makes sense. Because what happens is most people, most companies and organizations, have an idea of what type of PPE, what rating, what arc rating of PPE they think is reasonable. And right now for most companies and most people, that would be 8 calorie or maybe 12 calorie per square centimeter for an arc rating. And so that’s actually your answer. And the arc flash study is really to support that answer.
So as an example, let’s take a look at the results of this arc flash study. That, in this case, I’m showing the results. And we have the main switch gear, panel 1A, motor control center 1, and panel number 2. And really, what you’re doing in this case is you want to focus on the highest incident energies. And you actually want to look at all of the incident energies to make sure that they all are below the arc rating that you want to use. So in this case, if this group we’re going to be using, let’s say, 8 calorie per square centimeter as an arc rating for PPE, what this study did is it just confirmed that 8 calorie would be sufficient.
I use the analogy of a short circuit study. Some of you have performed short circuit studies before. And it’s kind of similar. Because in a short circuit steady, yes, you’re performing detailed calculations of the prospective short circuit current. But what you’re really doing is you’re comparing those currents against an interrupting rating that’s already in place or an interrupting rating that you want to use. And so this is kind of the same thing. That an arc flash study, what you’re really doing is performing the incident energy calculations to verify that the arc rating that you want to use is going to be sufficient.