The standard's reliance on the three-phase bolted fault value is partly justified by arc physics: with close phase spacing, an initiating fault is assumed to escalate rapidly and involve the adjacent phases, converging toward a three-phase arcing event. Whether that assumption holds for a given electrode configuration is not test-verified, and no published test data exists for the two-phase case.
Why a Line-to-Line Fault Can Increase Incident Energy
The available line-to-line fault current is generally about 87% of the three-phase value (derived from the sqrt(3)/2 relationship between line-line and three-phase fault magnitudes). The countervailing effect is protective device clearing time: reduced fault current pushes the device onto a slower region of its time-current curve, increasing trip time. If incident energy scales more strongly with duration than with current over that region, the two-phase case can produce higher incident energy than the three-phase case.
Decision Path for Two-Phase Arc Flash Estimates
Two defensible approaches exist, both built on the published three-phase equations:
- Reduced-current method: Apply the arcing current equation using a multiplication factor to reduce the current (approximately 0.87 of the three-phase value for a line-to-line fault), then use the resulting increased clearing time from the protective device curve. If the calculated incident energy with the longer duration exceeds the three-phase result, use those higher values.
- Three-phase fallback: Use the unadjusted three-phase fault current and accept the result. This keeps the calculation fully traceable to the published standard, which matters for documentation and audit defense.
Do not extrapolate incident energy for a two-phase fault by rescaling the three-phase energy result directly — the model structure does not support it, and the current-versus-time tradeoff makes the direction of the error unpredictable. Given the assumptions already embedded in the published equations, selecting the higher of the two calculated results is the conservative engineering choice.
FAQ
No. Two-phase cases must be approximated using the three-phase equations with adjusted inputs.
What percentage of three-phase fault current is a line-to-line fault?
A line-to-line fault generally delivers about 87% of the three-phase fault current, from the sqrt(3)/2 ratio between the two fault magnitudes.
Can a line-to-line fault produce higher arc flash incident energy?
Yes. The lower arcing current (~87% of three-phase) can increase protective device clearing time, and the longer arc duration can raise incident energy above the three-phase result. Calculate both cases and use the higher value.
