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Three-phase prospective fault current, worked throughWhy double the highest reading?
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Guidance Note 3 · p.88 · 2.6.16, which covers single and three phase.
Line to neutral, one conductor at a time
L1 – N = 398 A L2 – N = 402 A L3 – N = 405 A <- highest 405 × 2 = 810 A = 0.81 kA
The reason for doubling: the worst case on three phase is a fault across all three lines (about twice the single-phase value), and on many instruments it cannot be measured directly — so the book has you double the highest single-phase reading, erring towards safety.
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