stevenal wrote:

PaulEngr,

Let me see if I understand the process. Assuming you use the two second cutoff, find where 2 seconds intersects your TCC. Use this current, two seconds, and arc length to calculate the incident energy downstream. Perform another IE calculation using the the time and current at the point on the curve where it transitions to instantaneous. Then use the higher of the two values for selecting the most conservative PPE. Did I get this right?

Almost.

First, you might get different results if you have multiple inverse time curves. If you have just an LI breaker then what you've described is probably going to work but if it's LSG then you should check the intermediate (long to short term transition) point. I have no idea whether or not as a general rule this will give a higher or lower result compared to the 2 second time point.

For the two second result, there is one minor issue to be aware of. We're looking for an arcing current at that value so this is not the same as the available fault current. IEEE 1584 calculates a lower arcing current and if you are doing this in a software setting it's going to take your number and calculate the arcing current when in fact that value you want to use at 2 seconds is the arcing current. It would be easy to do this calculatiion by modifying the IEEE 1584 Excel worksheet (or doing the calculation by hand) but not so easy with commercial software.

The instantaneous region though follows what you expect to happen...higher current results in higher incident energy, without an upper limit other than the point where we exceed the interrupting capacity of the breaker so it will no longer open. So we use the "instantaneous" (opening time) of the breaker but we use the available fault current that we are given to calculate incident energy using the conventional procedure. The infinite bus assumption for this purpose might be preposterous but we've got to use our best estimate for this part. The arcing current calculation of IEEE 1584 of course would apply here so the unmodified calculations can be used. Using the instantaneous current trip point is a lower limit at which this occurs, and there is really no upper bound except maybe the breaker current withstand (if it is not undersized).