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 Post subject: Iterative Infinite Bus CalculationPosted: Fri May 19, 2023 12:54 pm
I understand iterative infitite Bus must be performed for short circuit calculation to perform arc flash calculation.

SKM modeling department says they use iterative infinite bus for their calculation also.

Is iterative infinite bus required for short circuit calculations to establish circuit breaker settings and relay settings as part of TCC curves?

My friend took Brainfiller short circuit class and said iterative infinite bus never came up. Why not?

I have plans on taking short circuit class also so I can ask this question and participate in the class.

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 Post subject: Re: Iterative Infinite Bus CalculationPosted: Sun May 21, 2023 3:31 pm
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Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1702
Location: Scottsdale, Arizona
mr.underwood.ee@gmail.com wrote:
I understand iterative infitite Bus must be performed for short circuit calculation to perform arc flash calculation.

SKM modeling department says they use iterative infinite bus for their calculation also.

Is iterative infinite bus required for short circuit calculations to establish circuit breaker settings and relay settings as part of TCC curves?

My friend took Brainfiller short circuit class and said iterative infinite bus never came up. Why not?

I have plans on taking short circuit class also so I can ask this question and participate in the class.

I'm not sure which method they mean. Typically an iterative approach for fault current calculations might be used for symmetrical and asymmetrical Short-circuit calculations with converter-based connected renewable energy sources. However using the term with infinite bus and arc flash might mean something different.

I have suggested for years that if the fault current is not available from the utility, to use an infinite bus. It is posted elsewhere at this forum and in the book that I wrote back in 2010. People will immediately state that is not the worst case. The correct statement is it may not be the worst case. If incident energy calculations using infinite bus and lower actual fault currents are compared, if the arc duration/clearing time of the upstream device remains the same for each, then infinite bus would be the worst case.

Here is an example of what I have referred to regarding infinite bus.
A 1500 kVA 5.75 percent 480 V secondary transformer would have an infinite bus secondary fault current of 31,374 Amps.

Begin the arc flash study with that value:31,374A (maybe add motor contribution too)
Note all of the locations minimum PPE ratings based on the first iteration of calculations that would be necessary.
Subtract 5 or 10 percent from 31,374 and re-run the study. The incident energy will likely go down.
Subtract another 5 to 10 percent. Again, the incident energy will likely go down again.
Keep doing this until there is an iteration where the incident energy increases (maybe dramatically)
This means you found the a fault current where devices begin to operate in the time delay region rather than instantaneously. The duration increases the incident energy in the calculation.

This has identified the range of fault current where the PPE is suitable.

Hope that helps clarify (assuming that is what was meant)

By the way, I do discuss this method in my Arc Flash Studies/IEEE 1584 training. (that class is coming up in early June)

Arc Flash / IEEE 1584 Class

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