Without getting to far into copyright violations I'll show you a quick excerpt. The first chart attached shows the idea of what the conference article explains in more detail. The dotted line is the conventional "TCC curve" (time-current curve). It's an extremely inverse curve with an "instantaneous" trip setting. But superimposed on that it shows the incident energy calculation for every current as the solid line and the "2 second cutoff" as a dashed line.
So if we focus specifically on the incident energy which is the solid line and starting from the left, we see that incident energy is rising as current rises. This should come as no surprise. It keeps rising until we get to the point where the 2 second horizontal line intersects the extremely inverse time curve.
At two second cutoff point the breaker is now tripping faster than 2 seconds. This is where we need to really pay attention though because this curve is typical of what actually happens more often than not. Although current is rising, the corresponding speed of the breaker is increasing faster than the current increase so the NET result is that incident energy actually decreases even though the fault current is rising. Look closely because this result is totally counterintuitive but is exactly what happens with most breakers and fuses.
Finally once we reach the point where the instantaneous trip point is reached, incident energy takes a dramatic dive down (due to a 50 millisecond trip time) and then slowly rises again as we would expect it to (current increasing but trip time remains constant).
There are several implications here to be away of...
1. Lowered fault current in the inverse time region results in INCREASED incident energy rather than decreased incident energy like we would expect to happen.
2. No matter what the fault current is, incident energy will NEVER exceed about 8 cal/cm2 with this set of breaker settings because for any given fault current, that is the maximum incident energy. Out another way, we can actually predict maximum incident energy WITHOUT ever doing any kind of power system analysis at all, only an analysis of the TCC curve. Granted actual incident energy might indeed be lower.
3. Another implication of this technique is that for instance we can quickly see what would happen if the arcing fault current estimate isn't good. If for instance the arcing fault current was overestimated especially near the intantaneous trip point we might estimate incident energy at well under 1.2 cal/cm2. But if the actual fault current is a little lower, suddenly it's in the range of 6-8 cal/cm2. This should be obvious to anyone that has looked at arc flash mitigation...often small changes to the instantaneous setting of a breaker that have zero impact on "nuisance trips" dramatically lower the incident energy.
4. Another implication is that inexperienced end users frequently try to push doing arc flash analysis onto the vendors. Vendors push back because that's a site issue since they have data only on their equipment and not on anyone else's equipment. But this type of analysis makes it possible to look at incident energy if there is a main breaker/fuse everywhere except at the main breaker/fuse cabinet itself.
4. Do not assume that the curve is always shaped this way. That's what I typically see but there are some cases where it doesn't work. The "second curve" file shows one of those cases. This is definitely a bit of a contrived example of what can happen but in this case the "2 second cutoff" has been increased to 10 seconds and the a moderately inverse curve. In this case the lower and upper segments of the curve are the same as before but instead of a decreased incident energy we get a parabola where only the transition points are peaks. In this case if the arcing fault current and 85% of the current are predicted on either side of one of the transition points, the value in between could result in an increased incident energy. There is a lot more detail than this in the full article, showing cases where incident energy increase all the way except at instantaneous tripping like one would expect to happen, a couple cases with a "concave down" parabola, and several more. Really neat stuff.
At this point I have to say that honestly almost all plants I've run into use either the standard inverse or extremely inverse curves. I just don't see any other curves used in most cases, so a lot of the really strange curves like parabolas never show up.
The only other concern that I have is that this type of analysis does not consider for instance fault current flowing from inductors to the fault after a trip. So it might be possible to have somewhat higher incident energy values if the transient time exceeds the breaker trip time by a significant amount. If this is not the case then this method can indeed predict maximum incident energy without doing a full power system softwae study.
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second curve to look at.png [ 51.62 KiB | Viewed 3554 times ] |
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graphical incident energy.png [ 68.01 KiB | Viewed 3554 times ] |