Inputs
Choose an input method, then enter impedances, grounding, and clearing time. Values may come from equipment data, studies, or site measurements.
Formula used
- VLG = VLL / √3 (three-phase).
- Zfeeder = √(R² + X²).
- Ztotal = Zsource + Zfeeder + Rfault.
- If = V / Ztotal.
- GPR = If × Rg.
- Vtouch = ktouch × GPR.
- Vstep = kstep × GPR.
- I²t = If² × t.
Transformer method converts %Z into an equivalent source impedance using rated voltage and kVA.
How to use this calculator
- Select the system type and input method.
- Enter voltage, feeder R and X, and fault resistance.
- Provide grounding resistance and clearing time.
- Adjust touch and step fractions for sensitivity checks.
- Press Calculate to view results above the form.
- Use CSV or PDF downloads for reporting.
Example data table
| Scenario | System | Voltage used (V) | Ztotal (Ω) | Fault current (A) | GPR (V) | I²t (A²·s) |
|---|---|---|---|---|---|---|
| Typical site distribution | Three-phase | 231 | 0.110 | 2100 | 3150 | 882000 |
| Long feeder run | Three-phase | 231 | 0.210 | 1100 | 1650 | 242000 |
| Small single-phase panel | Single-phase | 230 | 0.180 | 1278 | 1917 | 327000 |
Example values are illustrative and depend on actual system data.
Professional article
1) Why ground fault calculations matter
Ground faults can elevate exposed metalwork, change protection response, and create hazardous touch and step potentials. A fast estimate of fault current and ground potential rise helps teams verify that protective devices can clear the fault quickly and that temporary works do not introduce unsafe conditions.
2) What this tool estimates
This calculator estimates fault current from an equivalent loop impedance and converts that current into grounding stress indicators. Outputs include current (A and kA), total loop impedance, ground potential rise, estimated touch and step voltages, touch current using a selectable body resistance, and clearing stress using I²t.
3) Input methods and when to use them
Use the Thevenin impedance method when you have study results, utility data, or a measured source equivalent. Use the transformer method when nameplate kVA and percent impedance are available. The tool converts percent impedance to an equivalent source impedance at the chosen voltage level.
4) Feeder effects and fault resistance
Longer feeders increase resistance and reactance, which reduces fault current and may slow protective clearing. Fault resistance accounts for imperfect contact or arcing and often lowers current further. If your device settings rely on high current for instantaneous pickup, conservative resistance assumptions are important.
5) Ground resistance and potential rise
Ground potential rise is approximated as fault current multiplied by ground resistance. This provides a practical planning metric for temporary electrodes, generator skids, and site earthing arrangements. The touch and step fractions represent how much of that rise could appear across a person’s reach or stance.
6) Interpreting I²t and energy
I²t scales with the square of current and with clearing time, making time coordination a strong lever for reducing thermal stress. The tool also estimates energy dissipated in fault resistance over clearing time. Use these indicators to compare scenarios, not as a substitute for a full protection study.
7) Example data and quick check workflow
The example table shows three common situations: typical site distribution, a long feeder run, and a small single-phase panel. Try the default inputs first, then adjust feeder R/X and clearing time to match your project. If calculated current seems low, verify conductor sizes, lengths, and connection quality.
8) Good practice notes
Always validate assumptions with site drawings, equipment data, and tested grounding values. Where required, follow the applicable electrical codes and engineering standards for fault studies and touch/step criteria. For high-risk locations, obtain a formal study and document settings, test results, and inspection steps.
FAQs
1) What is a ground fault?
A ground fault is an unintended connection between an energized conductor and earth or grounded metal, creating current flow outside the normal circuit path.
2) Why does feeder impedance reduce fault current?
Feeder resistance and reactance add to the loop impedance. Higher impedance limits current for a given voltage, which can reduce instantaneous tripping and increase clearing time.
3) What does ground potential rise mean on site?
It is the voltage rise of the grounding system relative to remote earth during a fault. Large rises can create dangerous touch and step voltages nearby.
4) How should I choose touch and step fractions?
Use conservative fractions for early planning, then refine using grounding layout, soil conditions, and spacing. A detailed grounding analysis can provide more accurate distributions.
5) Is the transformer method accurate for all cases?
It is a reasonable first estimate using nameplate kVA and %Z. For complex systems, include upstream impedances, motor contribution, and utility data from a full study.
6) What clearing time should I use?
Use the protective device’s total clearing time at the expected fault current. If unknown, use a conservative time and compare with device curves once available.
7) Can I rely on these values for final protection settings?
Use them for screening and comparisons. Final settings should be based on a coordination study, verified field conditions, and required safety criteria for the project.