Neutral Grounding Resistor Sizing (IEEE 142)
Sizing the NGR off line-to-neutral, not line-to-line: a resistance-grounded transformer neutral limits a ground fault to a known current -- a few amps (HRG, keep running through the first fault) or 100-400 A (LRG, fast tripping). The resistor sees V_LN = V_LL / sqrt(3), NOT V_LL, so sizing off V_LL makes it sqrt(3) too large and the fault too small to sense. R = V_LN / I_ground; P = I^2 R = V_LN x I. A 480 V system at a 5 A HRG target: the resistor sees 277 V -> 55.4 ohm, 1,386 W continuous. HRG resistors are rated continuous, LRG for the short trip time. A design aid; IEEE 142 and the protection scheme govern.
Formula and source
V_LN = V_LL / sqrt(3); R = V_LN / I_ground; P = I_ground^2 x R (= V_LN x I_ground).
Neutral grounding resistor sizing (IEEE 142 grounding practice), first-principles, by name; IEEE 142 and the protection scheme govern.
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