Depending on the complexity of a power system, it may be necessary to model multiple scenarios for the arc flash study.

This may include considering configurations such as a tie being open and closed, operation from alternate sources such as local generation and different utility/source short circuit contributions.

Here is this week's question:

What is the maximum number of scenarios that you have ever used for an arc flash study?

Only 1
2 to 3
4 to 5
more than 5

I have personally ran a study with 27 scenarios. This number will however be smaller than my next study of a facility that has several generator, utility feeds, MTM configurations at switchgear and MCC levels as well....fun! Too bad SKM only allows 8 scenarios at the time.

Marek

Normally one scenario. Our report states everything is is "normal" configuration. That covers most of our work. Off the radar I will check HRC of a transfer switch fed by generator source because that is one item that might be trouble shot during times of generator operation and I would not wish to mislead someone with an 'academically correct" label.

However, in a special case where multiple sources were used routinely, we have modeled approximately 20 scenarios: different combinations of sources with different combinations of tie breakers open and closed on numerous bus'. In that case all labels installed were for highest possible HRC and a reference to particular book of results we collated also included on the labels for more favorable work conditions.

I model different cases from the utility coupled with whether our two transformers are both online, or just one is online with the bus tie closed.

I usually consider maximum calculated available short circuit current including all inductive loads and generators running and contributing to the fault for arc flash analysis (values from column Max Isc3, KA below). However, I use minimum available short circuit current through upstream protection device represented by fault current contributed from single source only assuming other sources and inductive loads are turned off to determine arc duration as a function of arcing current and the device time-current characteristics (values from column Min Isc3, KA below).

11 or 12 is (I think) the largest # of scenarios for arc flash that I have run.
If there are "N" paralleled generators, that is usually N-1 scenarios right there,
then there frequently are large multi module UPS systems, and at least 1 utility, often more.

I have one on again / off again project with a hospital where they are using closed transition ATS units
(on return to normal) and want each transition modelled for I.E.

[SKM scenarios won't begin to handle that.
SKM scenarios are bad enough at handling things.]

I am curious how the incident energy is reported for those cases where the arc flash analysis is run more than once. I have seen installations with multiple arc flash stickers on a piece of equipment and it seems this approach is likely to cause confusion in the field.

Personally, I like to indicate in my report that the arc flash analysis was done with the system in its 'normal operating condition.' As part of the energized work permit the worker needs to verify the system is in its 'normal operating condition' or re-run the arc flash analysis in its current operating condition. This should ensure the worker is wearing the correct PPE for the work to be performed.

Thanks in advance for any input.

We have never run a study with only 5 scenarios. We are a major utility and are doing the arc flash studies inside our power plants. A medium sized gas-fired power plant may have "only" 12 or 13 scenarios. A larger coal-fired plant (lots of extra busses for environmental and coal handling loads) may have greater than 50 and our largest plants have gone over 100 scenarios.