Good evening,

I have a situation where I will occasionally need to parallel feed a bus. The primary feed is a utility feed, and the other feed is an emergency generator. The emergency generator is typically only used during blackout scenarios where the utility feed is removed. However, our client has requested us to provide arc flash labels during scenarios of parallel feeding the bus.

When I run the scenario of parallel feeding (utility and emergency generator), I see quite high arc flash values on that bus (>140 cal/cm2), and rightfully so as it goes to the full 120 cycles. What would be the proper sequence of events for this scenario?

Notes:
-The emergency generator and the utility feed are both protected by relay's and circuit breakers. At this time, there is no intention to have them communicate, but it is possible.

Thank you for your help!

You are increasing the amount of available fault current but your incident energy goes up? Can you explain how?

Typically the highest incident energy is during a generator-only scenario because of the low available fault current which drastically increases the opening time of the overcurrent protective devices. Plus as stated it never trips for 2 seconds? Something isn't right here. Is your overcurrent protection even set to do anything? Is this an SKM bug thing where you need to rerun the entire study and not just do a window on a specific area?

Great questions and thank you for pointing them out.

You are increasing the amount of available fault current but your incident energy goes up? Can you explain how?
This doesn't make sense to me either. And by the way, I'm using ETAP. I am assuming that the fault might be getting cleared by one relay/breaker but not the other, and thus the fault is still being fed from the 'other' feed. Thoughts? I will check the TCC's.

Typically the highest incident energy is during a generator-only scenario because of the low available fault current which drastically increases the opening time of the overcurrent protective devices.
This happens as well, but of course that is another topic (or post).

Based on your description you're getting to 2 seconds...effectively the overcurrent protection is not operating at all and could be deleted from the circuit. When you are operating in "definite time" (2 seconds in this case), incident energy will increase with current. When you are operating within the inverse time region of an overcurrent protection device it can go either way. Most of the time incident energy goes down as current increases due to the fact that opening time decreases at a faster rate than the increase in arc power.

I'm less familiar with ETAP but with SKM it has a way of doing a partial update so that rather than solving the entire system with all buses you can manually select just a portion of the overall system and update that portion. Over time it slowly somehow gets corrupted doing this so when doing a lot of "what-if" scenario's after a few rounds of this it becomes necessary to do a fresh complete run.

I did some digging around in their "Help" section last night and based on other PE's suggestions on parallel feeding a bus, I've come to this:

1. ETAP, by default, will choose the longest pickup time for a protective device when there are multiple feeds into a bus. With the generator seeing about 2kA and the utility feed being around 24kA, the fault current is so low (as you've mentioned) that it's not tripping and going 120 cycles. Which brings me to my second conclusion.

2. In reality, the situation in which the emergency generator and the utility are both feeding the same bus occurs so infrequently, and additionally that live work would happen during that time, has led me to simply ignore this scenario while still informing the client of this particular scenario. The majority of the time the bus will be fed from the utility, followed by a blackout scenario where the emergency generator feeds the bus.

Thank you for your help!

Arc Flash is about working on parts 'live'.
When I have normal loads, that are backed up by generators, I just use the 'normal' IE and then put a statement on the label that says, "Warning: No live work while on generator supply".
Do you really need to work on equipment, live, while on generator?

As you mention, ETAP will use the longest clearing time for the results when the system has multiple sources. However, you can change this. It has an option called "subtraction of incident energy for multiple source systems". By default, this is set to False.
If you set this option to true, it will remove each source's contribution as it's PD trips it offline.

The option is located in tools -> options (preferences) -> arc flash.

I'm sure you've scoured the help file, but I've found the "Calculation Methods - Arc Flash" section of the help file most helpful for these issues.