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Using Integrated vs Momentary fault current values for ArcFlash
2

Using Integrated vs Momentary fault current values for ArcFlash

Using Integrated vs Momentary fault current values for ArcFlash

(OP)

I recently read a paper that suggested an integrated method for determining Incident Enegry levels when performing Arc Flash Calculations.  This method accounted for the fact that the fault current contribution from all non-synchronous sources (induction motors etc..) would begin to decay after the initial inception of the fault. This decaying fault current for different time periods through 30cycles would lead to different IE levels based on the decaying fault current with then the total IE level for the equipment being a summed total of the various IE levels for the periods between 0 and 30 cycles.  

Up until coming across this information I always used the momentary fault current for determining the IE level at a given location.

I would think that the momentary method would produce the worst case IE level however I am now curious to hear from others what method they use and if this integrated method is one that is typically used?

One of my decisions comes in what IE values to use when supplying equipment with Arc Flash labels those IE values calculated from the momentary or integrated methods?

RE: Using Integrated vs Momentary fault current values for ArcFlash

Engineering judgment.  IEEE 1584 does not explicitly state what fault network to use in determining the short circuit current used for determining the incident energy.  

Using the momentary (1/2 cycle) fault current will generally be the more conservative.

But for a 2 second arcing fault, including a large amount of induction motor fault current contribution is obviously not an accurate model.  

The software I am familiar with that has an integrated method is still quite conservative in how the calculation done and I generally use this method without any concerns.

For a system without a lot of large motors or local generation, it doesn't make much difference, in my experience.   

David Castor
www.cvoes.com

RE: Using Integrated vs Momentary fault current values for ArcFlash

We use SKM which uses the integrated method.  Using the momentary current would be unnecessarily conservative if a significant portion of the fault current was from motors and the interrupting time was more than 10 cycles.  The motor contribution will decay to nothing in 3-5 cycles.
 

RE: Using Integrated vs Momentary fault current values for ArcFlash

(OP)

Just so I understand, wouldn't the momentary fault current result in the fastest clearing time for a given protective device?  With that said you guys are saying that if the protecdtive device clearing time is greater than 3-5 cycles then most if not all of the motor contribution will have diminished in this time and therefore not contribute greately to the IE value?

The one thing that I am struggling with is the IE level at the inception of the fault.  Wont the IE level be the greatest or worst at the inception of the fault when the momentary fault current is present?  Would't this be the worst case IE level or Arc Flash?  Or do the Arc-Flash equations for determining IE levels not strictly look at the inception of the fault, but rather the total time between inception and device clearing?

Thanks for the help in understanding.

RE: Using Integrated vs Momentary fault current values for ArcFlash

IE is energy, not power, so it has no meaning "at the inception of the fault".  It has meaning only for the time period of exposure.  If you don't consider the short time period of the motor contribution, then you are assuming that the arc current stays at the initial level until the protective device clears.  This is not the case if the device takes longer than 5 cycles to clear.

 

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