Transformer impedance and fault current
How transformer kVA and percent impedance feed prospective fault-current worksheets for Australian electrical projects.
Fault-current purpose
Transformer percent impedance is one way to estimate prospective current near the transformer secondary. It is useful for early protection records when the transformer data is known, but it should not be treated as the whole source record.
The result needs the transformer data source beside it.
Workflow
- Confirm the transformer kVA, voltage and percent impedance source.
- Check that the transformer is the source being reviewed.
- Use the short-circuit current calculator with the transformer-data path.
- Record the prospective-current estimate and location.
- Continue into breaking-capacity, withstand or engineering review only with the data source attached.
Transformer fault-current record
| Field | Record | Review concern |
|---|---|---|
| Transformer kVA | Nameplate, utility data or design record | Sets the source scale |
| Percent impedance | Transformer data value | Strongly affects current estimate |
| Voltage basis | Secondary or reviewed-side voltage | Must match calculation location |
| Location | Transformer terminals or downstream point | Downstream impedance changes current |
| Protection review | Device and cable context | Requires product and project data |
Boundaries
- Do not use transformer percent impedance from another transformer.
- Do not move a transformer-terminal result downstream without considering circuit impedance.
- Do not infer selectivity or let-through performance from prospective current alone.
- Do not hide utility or transformer-data uncertainty.