Fault loop impedance in Australian protection checks
How fault loop impedance relates to fault current, active-to-earth paths, thresholds and protective-device review.
What Fault Loop Impedance Means
Fault loop impedance is the impedance in the fault path used for protection review. In Australian low-voltage language the value is often discussed around an active-to-earth fault path, MEN context, protective-device operation and test records.
The calculator relationship is simple: Ifault = V / Zs. A 230 V basis with Zs = 0.46 ohm gives 230 / 0.46 = 500 A. The difficulty is not the arithmetic; it is keeping the measured or entered value tied to the circuit, device and criterion.
Voltage, Zs And Thresholds
Fault-loop work should keep four things visible: voltage basis, loop impedance, estimated fault current and the threshold being used for review. The threshold might be an entered minimum fault current, an entered maximum loop impedance or no threshold when the page is used only to understand the relationship.
When a protective-device rating is entered, the current multiple can also be shown. That multiple is not a pass/fail rule by itself; it is a ratio for review.
| Value | Example | Why it matters |
|---|---|---|
| Circuit or equipment label | DB-2 final subcircuit C7. | Zs is not portable between circuits. |
| Voltage basis | 230 V a.c. active-to-earth basis, or project-entered value. | Estimated fault current changes with voltage. |
| Loop impedance | Measured or entered Zs in ohms. | Fault current is directly tied to impedance. |
| Threshold | Minimum current or maximum impedance entered by the reviewer. | It explains what the comparison means. |
| Device context | Protective-device type and rating where relevant. | Device data controls the real review, not the formula alone. |
Worked 230 V Example
For an entered 230 V basis and Zs = 0.46 ohm:
Ifault = 230 / 0.46 = 500 A, or 0.50 kA.
If the entered minimum fault current threshold is 400 A, the current margin is 100 A. If the entered maximum loop impedance is 0.50 ohm, the impedance margin is 0.04 ohm. Those margins are worksheet values, not a test method or approval.
| Field | Example entry | Reading |
|---|---|---|
| Board and circuit | DB-2, final subcircuit C7. | The value belongs to one measured point. |
| Entered value | 0.46 ohm loop impedance, source recorded. | The calculation needs value, unit and source. |
| Voltage basis | 230 V a.c. active-to-earth basis. | Fault-current estimate changes with voltage basis. |
| Fault current | 500 A estimated from entered values. | The result can feed a threshold comparison. |
| Criterion | 400 A minimum current or 0.50 ohm maximum Zs entered by the reviewer. | The comparison is meaningful only with its criterion. |
Measurement Context And MEN Wording
If a value comes from a test record, keep the test date, circuit label, instrument/source context and competent verification note close to the result. If the value is an estimate, label it as an entered assumption.
For Australian low-voltage work, the active-to-earth path, MEN context and protective-device label should remain visible. That helps distinguish a measured loop value from source-impedance or PSCC values used elsewhere.
Next checks
- Use the fault-loop impedance calculator when voltage, Zs and a review threshold are known.
- Use the fault-current relationship chart when comparing voltage, impedance and current relationships.
- Use testing record fields when measured values need circuit, instrument and criterion columns.
Boundaries
- This page does not provide a test method or verification procedure.
- It does not approve protective-device operation or replace device curves.
- Current standards, test instruments, site conditions, product data, entered criteria and competent verification remain controlling inputs.