Hi,

I have a feeder that has a greater probability of having a 2 Phase fault because the other phase line is far away from the 2 others. Hence, only 2 Phase fault should be considered in the arc flash calculations.

Can we use IEEE-1584 arcing current calculations? Are the equations only valid for 3phase faults?

Thanks

The 1584 equations are based on three phase faults (see page 1) and do not cover single phase, DC, or the case you mention. That being said, the 1584 equations are still the industry standard for determining the hazard. You may think about using the arcing current equation with a multiplication factor to reduce the current. This will account for the two phases since the 1584 equations are so heavily based on current (trip time might be reduced).

Phase to Phase Faults

John,

I expect what Homer meant to say is that the trip time would probably be increased since the line to line fault current available will generally be 87% of the 3 phase value. Don't know how that would figure into the equations though. The standards use the 3 phase value assuming that with close phase spacings the fault will quickly involve the adjacent phases. I would hesitate to assume that you could extrapolate the energy from a 2 phase fault based on the 3 phase equation. I guess you could use the equations with the lower value of line to line fault current and the increased clearing time, but I am not sure how good the information you get would be. If you take that approach and the energy is higher with the longer times, I would probably use those values, eventhough the results might be higher because it is a 3 phase model. After all, there have already been many assumptions for the published equations. The other option would be to go with the flow and use the 3 phase values for fault current and take what you get. Probably not what you were looking for but at least you would have the standards to back up your calcs. Unless this ends up being a common issue, I would expect that we will never have any test data to work with.

Do we have a physicist in the forum?

Maybe someone else will have some input here?
Alan

oops! Reduced arcing current = Increased trip time. Thanks for the catch