Beo wrote:

I have a problem with calculations with multiple branches. Hope someone can help.

The system is a switchboard with four generators divided by a bus tie. Two generators on each main bus.

The bus tie is set to trip at 0.3 seconds and the generator breakers at 0.5 seconds.

With a short circuit on the main bus bars, the 3-phase current is calculated to 49kA. Incident energy is about 39 cal/cm2 and arc boundary 658 cm. This is at 0.3 seconds arc duration.

The remaining fault after bus tie tripping is 24,5 kA for 0.2 seconds. Incident energy about 13 cal/cm2 and boundary 300 cm.

How do I create a warning label for the main switchboard? Do I add up the incident energies and calculate new boundary zone based on that incident energy (39 + 13 cal/cm2)? Or is it as simple as saying that the worst case is 658 cm with 39 cal/cm2 and that the incident energy after bus tie tripping is 13 with a 300 cm boundary?

This is the most straight forward solution method. Look at it in steps is (for now) about the only way to go. We had some language in the next draft of IEEE 1584 about this stepped approach. As you show in your post, the sum of the two steps is what the total calculated incident energy could be. There is a bit of a wrinkle, generator short circuit current tends to decrease rather quickly. This requires a very complex solution and most chose to ignore it. It would necessitate using generator decrement curves based on sub transient reactance and other data.

Hope it helps!