If I may expand on Mr. Fuhr's excellent suggestion.
1) look at the direction of the current conductors feeding the arc... the force vector pushing the arc is created by those conductors. If there are clips and or lugs it may be better represented as HCB even if the cables come vertically.
2) Generally HCB vs VCB produces very similar arcing current, so HCB is the most conservative with respect to incident energy and usually arcing current as well.
3) However, if you are setting the protection tightly make sure you identify the lowest possible arcing current and ensure the protection will respond as you wish it to respond to that lower arcing current with margin for safety.
4) Regardless what model you use you want to make sure you have the right "time" in the equation!
5) If in doubt about the gap, use the largest gap that may be the case... larger gap is always more conservative.
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File comment: Some charts showing incident energy at HCB, VCB VCBB, for 4.16 & 12.47 kV. 36 inches, 100ms, Std box at V. NOT INTENDED TO replace a proper Ei calculation, but may help to understand the impact of the decision you are pondering. Ei is roughly proportional to time.
5k 15kv AF charts.docx [122.96 KiB]
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