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Neutral Grounding Resistor (NGR) Sizing Calculator electrical
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Neutral Grounding Resistor (NGR) Sizing Calculator

Determine resistance (Ω), continuous and short-time thermal power ratings (kW), and charging current thresholds for High (HRG) and Low Resistance Grounding (LRG).

System Voltage & Architecture

Typically 5A for HRG, 100-400A for LRG
Cable capacitance charging current

Thermal Time Duty

NGR Sizing Specifications

NGR Resistance Value (R)
-- Ω
-- V L-N across resistor
Fault Thermal Power
-- kW
-- MJ thermal energy
HRG Criterion (Ig > 3Ico)
--
-- ratio
Recommended CT Ratio: --
Overvoltage Suppression: Transient-Free
Fault Clearing Strategy: Alarm Only
HRG limits ground fault current to 5A so that the initial phase-to-ground fault will not escalate into an arc flash or trigger an immediate process shutdown.

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Frequently Asked Questions

What is High Resistance Grounding (HRG) vs Low Resistance Grounding (LRG)?

HRG limits ground fault current to a very low level (typically 5A or less) such that the electric arc self-extinguishes, permitting critical continuous process facilities to operate without tripping the breaker. LRG allows higher currents (100A to 400A) to reliably actuate standard overcurrent ground protective relays for selective automatic tripping on medium-voltage networks.

Why must HRG let-through current exceed system charging current (Ig > 3Ico)?

In an ungrounded or improperly grounded system, intermittent arcing faults discharge into system phase-to-ground capacitance. Without sufficient resistive damping, the trapped charge causes severe resonance and cumulative voltage escalation (up to 5 to 6 times nominal voltage), puncturing motor and transformer winding insulation.

Can 480V systems with neutral grounding resistors supply line-to-neutral lighting loads?

No. National Electrical Code (NEC 250.36) strictly prohibits connecting line-to-neutral single-phase loads to an impedance-grounded system. Any 120V or 277V lighting/receptacle loads must be fed from separate, solidly grounded isolation step-down transformers.