Prospective short-circuit current in Australia
How PSCC is read in Australian protection work, including source impedance, transformer data, kA values and breaking-capacity checks.
What PSCC Means
Prospective short-circuit current, often shortened to PSCC, is the current that could flow under a short-circuit condition at a point in an electrical system. In Australian low-voltage work it is usually handled in kA, with the location and source basis kept beside the value.
The value can come from DNSP information, project models, transformer data, source impedance or an engineering calculation. A PSCC value is not portable across a site unless the supply path and calculation point are the same.
Source Impedance And Transformer Inputs
Two common calculator inputs are source impedance and transformer percent impedance.
- Source-impedance mode uses
I = V / Zfor single-phase, orI = V / (sqrt(3) x Z)for a balanced three-phase line-to-line basis. - Transformer mode estimates full-load current from
kVA x 1000 / (sqrt(3) x V), then multiplies by100 / transformer %Z. - The result is converted from amps to kA for breaking-capacity and switchboard comparisons.
| Input | Typical use | Why it matters |
|---|---|---|
| Calculation point | Main switchboard, distribution board, equipment terminals or incoming supply location in the project model. | Fault current changes with location and upstream impedance. |
| Source impedance | Entered ohm value for the selected voltage and phase basis. | Lower impedance gives higher prospective current. |
| Transformer kVA and %Z | Common shortcut when transformer data is the source. | Full-load current and percent impedance drive the estimate. |
| Voltage basis | 230 V single-phase or 400 V three-phase where appropriate. | The formula changes with phase arrangement. |
| Device comparison | Entered breaking capacity in kA. | This is a screening comparison, not a device approval. |
Worked 400 V Example
A three-phase 400 V source-impedance estimate with Z = 0.02 ohm gives:
I = 400 / (sqrt(3) x 0.02) = 11547 A, or about 11.55 kA.
If the entered device breaking capacity is 25 kA, the worksheet margin is about 13.45 kA and the utilisation is about 46%. That comparison still needs the actual switchboard location, product data and project review.
| Field | Example entry | Reading |
|---|---|---|
| Location | Main switchboard incoming section. | PSCC belongs to the point being reviewed. |
| Source basis | 400 V three-phase, 0.02 ohm source impedance. | The calculation can be reviewed later. |
| Result | 11.55 kA prospective short-circuit current. | Downstream comparisons use kA. |
| Device comparison | 25 kA entered breaking capacity. | Product data and conditions still control the real suitability question. |
Where PSCC Is Used
PSCC feeds breaking-capacity checks, switchboard fault-level notes, coordination discussions and cable withstand review. The same value can also sit near fault-loop or earth-fault work, but those are not the same calculation.
Keep the DNSP, transformer label, project model, source impedance or manufacturer source close to the value. When the value moves into a protective-device comparison, the location and source basis should move with it.
Next checks
- Use the short-circuit current calculator when the source impedance or transformer kVA and %Z are known.
- Use the breaking-capacity page or protection device terms when the next question is device fault-interruption rating.
- Use I2t cable withstand only after the fault-current point, clearing time and conductor data are ready.
Boundaries
- This page does not select a protective device or approve coordination.
- It does not replace DNSP data, transformer data, manufacturer ratings or switchboard documentation.
- Current standards context, supply data, equipment ratings, project requirements and competent engineering review remain controlling inputs.