Fault-current relationship
Relationship chart for voltage, source impedance, transformer impedance and prospective fault current.
Fault-current relationships before protection review
Use these row summaries to read the calculation relationship before checking the value table.
Ohm-law fault current
Estimate current from voltage and loop/source impedance.
I = V / ZkA conversion
Display prospective current in kA.
Isc_kA = Isc / 1000Transformer full-load current
Support transformer-based fault estimates.
FLC = kVA x 1000 / (sqrt(3) x VLL)Transformer short-circuit estimate
Estimate prospective current from transformer impedance.
Isc = FLC x 100 / Z%Breaking-capacity margin
Record an optional comparison margin before formal review.
margin = I_breaking - Isc
- Ohm-law and transformer-impedance relationships used by released calculators.
- Show the relationships behind fault-current worksheets without replacing protection studies.
- AS/NZS 3000, AS/NZS 3017 testing context and protection-device data noted for project review.
- Use AS/NZS 3017 measurement context, manufacturer device data, breaking capacity, loop impedance, source impedance and threshold margin records for formal review.
Impedance and current rows for fault records
Show the relationships behind fault-current worksheets without replacing protection studies.
| Relationship | Formula | Use | Boundary |
|---|---|---|---|
| Ohm-law fault current | I = V / Z | Estimate current from voltage and loop/source impedance. | Impedance source and voltage basis must be recorded. |
| kA conversion | Isc_kA = Isc / 1000 | Display prospective current in kA. | Does not set equipment breaking capacity. |
| Transformer full-load current | FLC = kVA x 1000 / (sqrt(3) x VLL) | Support transformer-based fault estimates. | Voltage source must be line-to-line for three-phase project review. |
| Transformer short-circuit estimate | Isc = FLC x 100 / Z% | Estimate prospective current from transformer impedance. | Network source and upstream impedance may change the actual value. |
| Breaking-capacity margin | margin = I_breaking - Isc | Record an optional comparison margin before formal review. | Manufacturer device data, authority requirements and project study control the decision. |
- Use the calculators for project-specific values.
- Protection selection needs device data and competent review.
Formula variables
Use these symbols and units when reading the chart rows.
| Variable | Meaning | Unit | Use |
|---|---|---|---|
| V | Voltage basis for fault-current estimate. | V | Record phase-to-neutral, phase-to-phase or three-phase basis. |
| VLL | Line-to-line voltage used for three-phase transformer current. | V | |
| Z | Loop impedance or source impedance. | ohm | Record whether the value came from measurement, DNSP data or study output. |
| Zloop | Measured or entered loop impedance. | ohm | AS/NZS 3017 testing context can affect the record. |
| Zsource | Source or upstream impedance used by a study. | ohm | Do not mix with loop measurement without review. |
| I | Fault current estimate. | A or kA | |
| Isc | Prospective short-circuit current estimate. | A or kA | Compare against breaking capacity only with device data. |
| Isc_kA | Prospective short-circuit current expressed in kiloamps. | kA | |
| kVA | Transformer apparent power used for a full-load-current estimate. | kVA | |
| sqrt(3) | Three-phase line-current factor. | Used with a line-to-line voltage basis. | |
| Zpct | Transformer impedance percent. | % | Use transformer data from the current source record. |
| FLC | Transformer full-load current. | A | |
| I_breaking | Protective-device breaking capacity used for comparison. | A or kA | Use current manufacturer data and the same current unit as Isc. |
| margin | Optional margin between device breaking capacity and estimated current. | kA or % | A review aid, not a protection study. |
Worked relationship example
This example shows how one set of entered values reads through the relationship.
Loop-impedance fault-current reading
A 230 V phase basis and recorded loop impedance are used to read the Ohm-law relationship.
- Voltage basis
- 230 Vphase-to-neutral example
- Recorded impedance
- 0.35 ohmmeasurement/source record required
- Apply Ohm-law relationshipI = V / Z = 230 / 0.35 = 657 A.
- Convert for records657 A = 0.66 kA.
- Keep review separateBreaking capacity and protection decisions need manufacturer data and competent project review.
Prospective current estimateabout 657 A, or 0.66 kA
The estimate depends on the recorded impedance source and voltage basis.
Use the example as a relationship review record before comparing against device data or protection-study outputs.
- Impedance source and phase basis are recorded.
- AS/NZS 3017 testing context and instrument records may affect measured values.
Where the fault-current chart belongs
Use the chart to understand the formula basis; calculators handle project-specific inputs, warnings and results.
| Item | Value |
|---|---|
| Purpose | Show the relationships behind fault-current worksheets without replacing protection studies. |
| How to use the source | Ohm-law and transformer-impedance relationships used by released calculators. |
| Standards and project context | AS/NZS 3000, AS/NZS 3017 testing context and protection-device data noted for project review. |
| Where judgement is still needed | Use this chart to understand the relationship behind a calculation; do not treat it as a substitute for entered project values. |
| Before relying on it | Confirm current standards, local authority requirements, DNSP conditions, project documents and manufacturer data before relying on a decision. |
| Context | Applies to | Public boundary |
|---|---|---|
| AS/NZS 3017 testing context | Loop impedance records and prospective-current estimates | Instrument setup, test method and recorded conditions belong with the measurement record. |
| Protection-device data | Breaking-capacity comparison, threshold margin and coordination review | Manufacturer data, authority requirements and engineering review govern formal protection decisions. |
| Source and loop impedance | Measured loop impedance, DNSP/source data and transformer impedance | Do not mix impedance sources without a documented project basis. |
Formula basis for fault-current relationships
Confirm the cited calculator source, impedance basis, device-data boundary and Australian testing context before relying on these fault-current relationships.
| Item | Value |
|---|---|
| Source | AUWiring fault-loop and short-circuit current calculator formulas. |
| Source type | Measurement record with standards context |
| Derivation basis | Derived from AUWiring calculator formulas; no protection coordination tables are reproduced. |
| Last checked | 2026-07-13 |
| Review interval | Annual or when protection calculator formulas or standards context changes. |
| Review trigger | Calculator formula change, protection data practice change, standards update or project feedback. |
| Version used | R17-2026-07-13 |
| Australian application | Australia; instrument data, protective-device data, authority and engineering review govern formal studies. |
Use a calculator for project values
Use the chart to read the formula relationship, then open a calculator when you need entered project values, warnings and result context.