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Calculating the fault current of a power transformer is a critical step in electrical system design, protection coordination, and safety analysis. When a short circuit occurs, the current flowing through the transformer is primarily limited by its internal impedance and the impedance of the upstream network. Engineers use these computed values to specify the breaking capacity of circuit breakers, fuses, and switchgear components to prevent catastrophic equipment failure and protect personnel.
The primary formula determining the symmetrical secondary fault current ($I_{sc}$) based on the full load current ($FLC$) and percentage impedance ($Z$) is expressed as follows:
$$I_{sc} = \frac{FLC}{\left(\frac{Z}{100}\right)}$$
Where the full load current for a three-phase transformer is calculated using the kVA rating and line-to-line secondary voltage ($V_{LL}$):
$$FLC = \frac{kVA \times 1000}{\sqrt{3} \times V_{LL}}$$
Follow these simple steps to perform a comprehensive fault current analysis for your transformer setup:
Q: Why is system impedance included in the calculation?
A: Upstream network impedance reduces the overall available fault current at the transformer terminals, making the calculation realistic.
Q: What is asymmetrical fault current?
A: It represents the maximum peak momentary current during the initial transient period of a short circuit event.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.