Neutral Earthing Resistor Calculator
Resistance and short-time power of a neutral earthing resistor for a chosen earth-fault current. With the formula, a worked example and typical values.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
This page calculates resistance, power during fault and energy during fault from system voltage, earth-fault current to allow and fault duration. Change any value or its unit and the result updates. The steps below show the working for the default values, so you can check each line against your own method.
Limits of this calculator
Choose the current from relay sensitivity and damage limits. The resistor is rated for its short-time duty, not for continuous use.
Cross-check: 8 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Indicative
Indicative. Estimation only; further engineering checks are required. Basis: Source impedance neglected, resistive fault.
What this calculator checks
- Resistor value for the earth-fault current you choose, its power and energy for the stated time.
What remains unverified
- Source impedance and the relay settings.
- The resistor's short-time and thermal rating from the manufacturer.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | IEC 60076-25, 2023 Ed 1.0 |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Neutral grounding resistors for power transformers are covered. The calculator derives resistance, duty power and energy from selected voltage/current/time; check system grounding philosophy, duty cycle, tolerances and thermal rating. |
| Cross-check test cases | 8 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Indicative |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; licensed clause-by-clause standard review; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| System voltage (L-L) | 11 kV |
| Earth-fault current to allow | 400 A |
| Fault duration | 10 s |
- R = (11 kV / √3) / 400 A = 15.877 Ω
- P = 400² × 15.877 = 2,540.3 kW for 10 s
Assumptions
- Single line-to-earth fault, source impedance neglected
- Fault current is purely resistive
Choose the current from relay sensitivity and damage limits. The resistor is rated for its short-time duty, not for continuous use.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| System voltage (L-L) | kV | greater than 0 kV | |
| Earth-fault current to allow | A | greater than 0 A | |
| Fault duration | s | greater than 0 s |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Resistance | Ω | Shown to 2 decimal places | |
| Power during fault | kW | Shown to 1 decimal place | |
| Energy during fault | kWh | Shown to 3 decimal places |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
How the result changes with system voltage (l-l)
| System voltage (L-L) (kV) | Resistance (Ω) | Power during fault (kW) | Energy during fault (kWh) |
|---|---|---|---|
| 1.1 | 1.59 | 254 | 0.706 |
| 2.75 | 3.97 | 635.1 | 1.764 |
| 5.5 | 7.94 | 1,270.2 | 3.528 |
| 11 | 15.88 | 2,540.3 | 7.057 |
| 22 | 31.75 | 5,080.7 | 14.113 |
| 44 | 63.51 | 10,161.4 | 28.226 |
| 110 | 158.77 | 25,403.4 | 70.565 |
Other inputs are the same as in the worked example above.
Standards and further reading
- IEEE Std 80 (safety in AC substation grounding)
- IEC 60364-5-54 (earthing arrangements and protective conductors)
Soil resistivity varies with season and depth; use measured values and a site-specific design for any installation. These are named as background reading. Elecmetric does not embed clause text or rating tables, and the reviewer record for this calculator is separate from this list.
Frequently asked questions
What do I need to enter?
Enter system voltage (L-L), earth-fault current to allow and fault duration. Change the unit of a value with its selector and the result stays the same.
What does this calculator assume?
Single line-to-earth fault, source impedance neglected. Fault current is purely resistive.
Which calculators should I use next?
Common next steps are Fault Current by Fault Type and Earth Rod Resistance. The related links at the bottom of this page open them.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 8 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- Neutral Earthing Resistor Formulas: Neutral earthing resistor formulas: the resistance for a ground fault current limit
- Earthing Formulas: Earth Rod and Fault Loop: Earthing formulas for the resistance of a vertical earth rod, rods in parallel
- Short-Circuit Formulas: Fault Level by MVA: Short-circuit formulas using the MVA method: transformer fault level, adding the source, three-phase fault current
More Earthing and Grounding calculatorsAll engineering calculatorsNeutral Earthing Resistor in Elecmetric