short circuit current and soft loading - for equipment evaluation analysis.

[Needhelp850]

I am doing an arc-flash study. Here is the situation I am coming across and am trying to see how to go by it and understand it.
when analyzing the closed transition scenario (utility parallel to generators), few breakers are failing Equipment Evaluation. However, the client is saying there is soft-loading and the time is limited to 20sec, so not to include this closed transition scenario.

My understanding is that soft loading is for transfer of loads from one source to another. If for 20sec (during soft loading) the system will be in closed transition, I would think we would have to analyze this closed transition scenario for equipment evaluation when doing arc-flash study. Am I correct in saying soft loading will have nothing to do with the short circuit calculation in an event of a fault at 20second or less.
Any suggestions pls..............

 

[RonShap]

Any closed transition (milli-seccond or multi-second) should include both source contributions for short circuit calculations and arc flash if it creates worst case.

Generally worst case for arc flash will be one source (lowest fault current).

 

[Needhelp850]

Thanks Ron. Yes that was my understanding for both source contribution for SCC calculation for closed transition.

 

[waross]

I believe that the initial transient fault current will be the same.
After the initial transient, the type of governor action and control may have an influence.
If the generator has field forcing to support short circuit currents, it will make a difference.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

For equipment evaluation (fault rating) purposes, you would have to consider the systems in parallel. For arc hazard, you could restrict any maintenance activity that could cause an arc flash during the closed transition.