Neutral Grounding Resistor Sizing (IEEE 142)
Sizing the NGR off line-to-neutral, not line-to-line.
Example
You enter
- System line-to-line voltage (V) 480
- Target ground-fault current (A) 5
- Grounding duty hrg
You get
- Voltage across the resistor (line-to-neutral) 277.1 V
- Resistor value 55.43 ohm
- Power dissipation 1386 W (continuous)
Details, formula, and sources
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.
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.
The NGR sizing relation is first-principles Ohm's law on the line-to-neutral voltage; the system voltage and target fault current come from the design.
Estimate. AHJ and licensed professional govern.
Field names used by the API: system_voltage_ll_v, target_fault_a, duty, v_ln_v, r_ohm, p_watt
- Line-to-neutral the resistor sees V_LL / sqrt(3); sizing off V_LL makes it sqrt(3) too largeIEEE 142
- HRG vs LRG HRG limits to a few amps (continuous rating); LRG to 100-400 A (short-time rating)IEEE 142
- Charging current the HRG resistor current must exceed the system charging current to control overvoltageIEEE 142