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How this is calculated#

The tool finds the fault current at a board. The source is a transformer or a stated supply fault level. A submain then adds impedance. The tool compares the result with the breaking capacity of your device.

1. Source impedance. For a transformer, the tool works Z = U² ÷ S × Z% per phase. It takes the source as all reactance. That gives the highest current for the size.
2. Source current. It gives the fault current at the transformer terminals.
3. Submain impedance. It reads the resistance and reactance of the submain for your size, length and material. The tables are Table 30 for reactance and Tables 34 and 35 for resistance. It scales the resistance to 20 °C for the maximum figure.
4. Protective conductor. It reads the earth conductor size from Table 5.1 of AS/NZS 3000:2018. It adds that resistance to the earth loop.
5. Fault currents. It gives the three‑phase current as U0 ÷ |Z|. It also gives the phase-earth current, maximum and with the cable hot.
6. Breaking capacity. It compares the three‑phase current with the rating you enter.

Worked example#

• 500 kVA transformer with a 30 m 35 mm² submain. The transformer has 4.5 % impedance. The three‑phase fault current at the board is 10.01 kA. The maximum phase-earth current is 3.18 kA. A 15 kA device passes.

Limits#

• The tool works at nominal voltage. It does not apply the IEC 60909 voltage factors.
• It takes the network above the transformer as infinite. A real network lowers the current a little.
• It takes the phase-earth loop at the transformer as its phase impedance, as for a Dyn transformer.
• It does not add motor contribution. It does not grade devices. Use the IDMT tool for grading.
• The result is an estimate for sizing. Ask the network company for the supply fault level at the point of supply.

See also: Prospective fault current at a board, Fault level at a sub‑board on a 300 kVA transformer and Fault rating.