How to Calculate a Fault Current for Transformers

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A fault current is an electrical current in a circuit caused by a fault in the transformer, such as a short circuit. It could be caused by conductors losing insulation between each other, or by a conductor losing insulation with the ground. Calculating the total possible fault current for a transformer is important so that you know what equipment to buy to protect the system from such a current. Without proper protection, a fault current could cause a fire or even explode.

  • Determine the transformer rating in Kilovolt-amperes, or Megavolt-amperes (kVA or mVA, respectively). You can find the rating on a label on the transformer itself, or you can consult your transformer manufacturer for the information. For example, your transformer might be rated for 500kVA.

  • Determine the %p.u. or per unit impedance of the transformer. This information will also be available on the unit's label or from the manufacturer. Your unit might have a per unit impedance of 5%.

  • Determine the circuit voltage. The voltage depends on how you design the circuit, so you may already know the voltage, but if you don't, you can just use a voltage meter to test it. There is the line voltage (VL) and the phase voltage (VP). If your transformer is a 3-phase, use the phase voltage. If it's a 1-phase, use the line voltage. For example, your circuit voltage could be 11kVA/400V (VL/VP), and your transformer might be 3-phase, so you would use 400V.

  • Measure the distance in feet from the power source to the point where you want to measure the fault current (often at the connection point).

  • Determine the resistance per 1000 feet, in ohms, of the cable you're using by consulting the manufacturer.

  • Determine the reactance per 1000 feet, in ohms, of the cable you're using by consulting the manufacturer.

  • Find the available fault current at the source of the feeder cable. This information is available from the manufacturer of the power source providing the current.

  • Multiply the distance from Step 4 by the resistance per 1000 feet, then divide that by 1000. Multiply the result by itself.

  • Multiply the distance from Step 4 by the reactance per 1000 feet, then divide by 1000.

  • Find the square root of the number of the phases (1 or 3), then multiply that by the available fault current at the source of the feeder.

  • Add the result of Step 9 to the result of Step 10, then square the result.

  • Add the result of Step 8 to the result of Step 11, and raise the result to the power of 0.5.

  • Divide the circuit voltage by the square root of the phases (1 or 3), then divide that by the result in Step 12. This is the fault current for the transformer measured in amps.

Tips & Warnings

  • Miscalculating the fault current could lead to injury or death. Please consult a professional.

References

  • Photo Credit Hemera Technologies/Photos.com/Getty Images
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