Fault Current by Fault Type Calculator
Three-phase, line-to-line and line-to-earth fault currents from positive-sequence impedance and the zero-sequence ratio.
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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 three-phase fault, line-to-line fault and line-to-earth fault from system voltage, positive-sequence impedance to the fault and zero-sequence ÷ positive-sequence impedance. 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
The largest value sets the breaking capacity; the smallest earth-fault value must still trip the protective device in time.
Cross-check: 6 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: Z2 = Z1, magnitude addition, bolted fault.
What this calculator checks
- Three-phase, line-to-line and line-to-earth bolted-fault currents from Z1, Z0 and voltage.
What remains unverified
- Arc resistance and the angle of the impedances (they are combined as magnitudes).
- Z2 = Z1 is assumed.
- Maximum versus minimum fault-level cases and the IEC 60909 factors.
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 60909-0, 2026 Ed 3.0 |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Short-circuit calculations for three-phase AC systems, including fault types and sequence networks, are relevant. The calculator is a simplified symmetrical-component estimate and assumes the sequence impedances entered represent the complete source path. |
| Cross-check test cases | 6 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) | 415 V |
| Positive-sequence impedance to the fault | 20 mΩ |
| Zero-sequence ÷ positive-sequence impedance | 1 |
- I3φ = 415 / (√3 × 20 mΩ) = 11.98 kA
- ILL = 0.866 × I3φ = 10.375 kA
- ILE = √3 × 415 / ((2 + 1) × 20 mΩ) = 11.98 kA
Assumptions
- Z2 = Z1
- Impedances combined as magnitudes (no angle), slightly conservative
- Bolted fault, no arc resistance
The largest value sets the breaking capacity; the smallest earth-fault value must still trip the protective device in time.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| System voltage (L-L) | V | greater than 0 V | |
| Positive-sequence impedance to the fault | mΩ | greater than 0 mΩ | |
| Zero-sequence ÷ positive-sequence impedance | (number) | 0.1 or more, up to 20 | About 1 for a solidly earthed transformer secondary, often 2 to 5 at the end of a cable run |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Three-phase fault | kA | Shown to 2 decimal places | |
| Line-to-line fault | kA | Shown to 2 decimal places | |
| Line-to-earth fault | kA | Shown to 2 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) (V) | Three-phase fault (kA) | Line-to-line fault (kA) | Line-to-earth fault (kA) |
|---|---|---|---|
| 41.5 | 1.2 | 1.04 | 1.2 |
| 104 | 3 | 2.6 | 3 |
| 208 | 6 | 5.2 | 6 |
| 415 | 11.98 | 10.38 | 11.98 |
| 830 | 23.96 | 20.75 | 23.96 |
| 1,660 | 47.92 | 41.5 | 47.92 |
| 4,150 | 119.8 | 103.75 | 119.8 |
Other inputs are the same as in the worked example above.
Standards and further reading
- IEC 60909-0 (short-circuit currents in three-phase AC systems)
- IEC 60947-2 (low-voltage circuit-breakers)
- IEC 60898-1 (circuit-breakers for household and similar use)
This is a first estimate. A protection study and the manufacturer's ratings and time-current curves decide the final selection. 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), positive-sequence impedance to the fault and zero-sequence ÷ positive-sequence impedance. Change the unit of a value with its selector and the result stays the same.
What does this calculator assume?
Z2 = Z1. Impedances combined as magnitudes (no angle), slightly conservative. Bolted fault, no arc resistance.
Which calculators should I use next?
Common next steps are Cable Short-Circuit Withstand and Fault Level (kA to MVA). 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. 6 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.
- Short-Circuit Formulas: Fault Level by MVA: Short-circuit formulas using the MVA method: transformer fault level, adding the source, three-phase fault current
- Earth Fault Loop and RCD Formulas: Earth fault loop formulas: the loop impedance from the supply and the cable, the fault current
More Electrical Protection calculatorsAll engineering calculatorsFault Current by Fault Type in Elecmetric