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How to Estimate and Measure Prospective Short-Circuit Fault Current in Three-Phase Systems

Regulation Guidance Note 3 provides methods for estimating and accurately measuring prospective short-circuit fault currents in three-phase systems, particularly when direct Line-to-Line measurements are not possible.

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OW London Electrician and Home Automation Engineers Team

OW London Electrician and Home Automation Engineers Team

How to Estimate and Measure Prospective Short-Circuit Fault Current in Three-Phase Systems

When working on three-phase electrical installations, it is crucial to accurately calculate the prospective short-circuit (SC) fault current. However, there are cases where your measuring instrument might only allow Line-to-Neutral (L-N) measurements and not Line-to-Line (L-L) measurements.

Guidance Note 3 (Page 89) states:

"With some instruments, the voltage between line conductors cannot be measured directly. Where this is the case, it can be assumed that for three-phase supplies, the maximum balanced prospective short-circuit level will be, as a rule of thumb, approximately twice the single-phase value. This figure errs on the side of safety.

Where an instrument is rated for the higher voltage, a more accurate prospective fault current measurement, for a three-phase installation, can be obtained by measuring the line-to-line fault current and dividing the measured result by 0.87."

Simplified Explanation with Example

Scenario:

You are working on a three-phase electrical installation and need to calculate the prospective short-circuit (SC) fault current. However, your measuring instrument only allows a Line-to-Neutral (L-N) measurement, not a Line-to-Line (L-L) measurement.

Step 1: Estimating Using the Rule of Thumb (L-N Measurement)

First, measure the prospective fault current on a single-phase supply, which is a Line-to-Neutral (L-N) measurement.

  • Example: Suppose the measured fault current between Line and Neutral (L-N) is 5 kA (kiloamps).
  • According to the guidance note:

Estimated Three-Phase Fault Current: The rule of thumb suggests that for a three-phase system, the fault current will be approximately twice that of a single-phase (L-N) system.

So, the estimated three-phase fault current would be:

Estimated Three-Phase Fault Current = 2 × 5 kA = 10 kA

This estimation errs on the side of safety, ensuring that your design accommodates potential fault conditions.

Step 2: More Accurate Measurement (L-L Measurement)

If your instrument supports higher voltage, you can measure the fault current between Line-to-Line (L-L).

  • Example: Suppose the measured fault current between Line-to-Line (L-L) is 8 kA.
  • To get a more accurate three-phase fault current value:

Accurate Three-Phase Fault Current: Use the method provided in the guidance note by dividing the L-L measurement by 0.87.

Calculation:

Accurate Three-Phase Fault Current = 8 kA / 0.87 ≈ 9.2 kA

This approach gives you a more precise fault current value, improving the accuracy of your calculations.

Author

Frequently Asked Questions

Q: How can you estimate the prospective fault current in a three-phase system if your instrument only allows Line-to-Neutral (L-N) measurements?

A: According to Guidance Note 3 (Page 89), if your instrument only allows L-N measurements, you can estimate the prospective fault current in a three-phase system by doubling the single-phase (L-N) measurement. This estimate errs on the side of safety.

Q: How can you obtain a more accurate prospective fault current measurement in a three-phase system?

A: To obtain a more accurate measurement, if your instrument supports higher voltage, you should measure the fault current between Line-to-Line (L-L) and then divide the result by 0.87, as stated in Guidance Note 3 (Page 89).

Q: What does the rule of thumb suggest about the relationship between single-phase and three-phase fault currents?

A: The rule of thumb mentioned in Guidance Note 3 (Page 89) suggests that the three-phase fault current is approximately twice the single-phase (L-N) fault current. This is an estimation that errs on the side of safety.

Q: What should you do if direct Line-to-Line measurement is not possible?

A: If direct L-L measurement is not possible, you can use the estimated value by applying the rule of thumb, which doubles the L-N measurement, as advised by Guidance Note 3 (Page 89).

Q: Why is dividing by 0.87 necessary when measuring L-L fault current?

A: Dividing by 0.87 when measuring L-L fault current provides a more accurate estimate of

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