System grounding methods
Solid / low-Z NGR / high-Z HIRG / ungrounded — sets I_gf, V_rise on healthy phases, detection method (51G vs 59N), and ride-through capability. NEC 250.20 LV default solid; 250.21 HIRG for industrial 480 V+. Ungrounded mostly phased out.
Step 1 — System grounding: how the Y-neutral connects to earth
Reference notes
System grounding refers to how the WYE NEUTRAL of a power system connects to earth. The choice determines ground-fault current magnitude, equipment damage, detection method, voltage rise on healthy phases, and the ability to continue operating through a single ground fault. Use Next → to walk through the four major grounding methods and their trade-offs.
Four grounding methods
| Method | I_gf typical | Detection | V_phase rise | Where used |
|---|---|---|---|---|
| Solid | 1-50 kA | 51G / 50G | normal | LV (NEC default), MV distribution |
| Low-Z (NGR) | 100-1000 A | 51G on NGR | modest | MV industrial (oil/gas, mining) |
| High-Z (HIRG) | 1-10 A | 59N V_neutral | up to V_L-L | Continuous-process (petrochem, fabs, data centers, generators) |
| Ungrounded | few A (capacitive) | 59N / open-Δ VT | up to V_L-L | Phased out (ferroresonance + arcing OV) |
Solidly grounded
- Neutral wired directly to ground with no intentional impedance.
- Ground-fault current is high — typically thousands of amps, comparable to 3-φ fault current.
- Pros: easy 51G/50G detection; healthy phases stay at normal line-to-neutral voltage during ground fault; surge arrester ratings lower.
- Cons: severe equipment damage at fault location; immediate trip on every ground fault; high arc-flash hazard.
- Required by NEC 250.20 for most LV services (50-1000 V). Standard for utility distribution 4-35 kV.
Low-impedance grounded (NGR)
- Neutral connected to ground via Neutral Grounding Resistor (most common) or sometimes a reactor.
- NGR sized to limit I_gf to 100-1000 A (typical 400 A for many MV industrial systems).
- NGR rated short-time duty (e.g., 10 seconds at limited current).
- Pros: dramatically reduced equipment damage and arc-flash energy; standard overcurrent detection still works.
- Cons: still trips on ground fault; NGR is extra component to size, cool, maintain.
- Standard for MV industrial 2.4-13.8 kV (oil and gas, paper mills, mines, large motor installations).
High-impedance grounded (HIRG)
- Neutral connected via high resistance (typically a distribution transformer at the neutral with a low-impedance secondary loaded with a resistor; reflected primary impedance gives the desired high-R).
- Sized to limit I_gf to 1-10 A — too small for overcurrent detection.
- Detection via ANSI 59N — the small ground-fault current causes a measurable voltage rise at the grounding resistor.
- KEY PRO: single ground fault does NOT cause immediate trip. System continues operating with ALARM only. Crew can locate and fix during planned outage. Critical for continuous-process industries.
- CAVEAT: a second ground fault on a different phase becomes a phase-to-phase short with full fault current. First fault must be located and cleared promptly.
- Cons: healthy phases rise to L-L voltage during fault → all equipment must withstand this elevated voltage continuously → higher insulation cost, higher surge arrester rating.
- Permitted by NEC 250.21 for industrial 480 V+ systems with maintenance personnel and fault detector. Standard for: petrochemical, paper mills, semiconductor fabs, large data centers, steel mills, generator stator grounding.
Ungrounded (mostly phased out)
- Floating neutral, no intentional ground connection.
- I_gf is small — only system capacitive charging current flows through a ground fault (few amps total).
- Severe drawbacks:
- Ferroresonance — VT magnetizing reactance resonates with system capacitance, causing sustained overvoltages.
- Arcing ground-fault overvoltages — restriking arcs at current zero produce 2-6 pu transient overvoltages, eventually breaking down insulation system-wide.
- Healthy phase voltage rise + difficult fault location.
- Effectively prohibited by NEC for most new installations; a few legacy systems remain.
Detection method matrix
- Solid / low-Z: 51G time-overcurrent on residual current (3·I_0) or NGR current. Pickup 5-20% FLA. Standard practice.
- High-Z / ungrounded: 59N neutral overvoltage. Pickup ~5% of phase-to-neutral nominal voltage.
Surge arrester ratings
IEEE C62 ties surge-arrester rating to grounding method. Effectively-grounded systems (where ground-fault current ≥ 60 % of three-phase fault) allow lower arrester ratings. Ungrounded and HIRG systems require higher ratings because healthy-phase voltage rises to L-L during ground fault.
NEC requirements
- NEC 250.20: services 50-1000 V generally required to be solidly grounded (with specific exceptions).
- NEC 250.21: high-impedance grounded systems permitted only for industrial 480 V+ with maintenance personnel and a fault detector.
- NEC 250.32: separate buildings or structures must be properly grounded.
- International practice varies — Europe uses different IEC 60364 grounding system classifications (TN-S, TN-C, TN-C-S, TT, IT).
Generator stator grounding (special case)
Large synchronous generators typically use HIRG specifically for stator ground-fault protection — limit I_gf to 5-10 A to prevent iron damage at the fault location during stator-to-ground fault. Iron is the most expensive part of the generator; melted iron requires factory rebuild. Detection via 64G neutral overvoltage + 27TN third-harmonic neutral undervoltage (for the last ~10% near the neutral). See the generator-protection lesson.