# System Impedance Ratio

**Definition:** The ratio of the power system source impedance to the impedance of the protected zone.

## Related Links

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Source Impedance Ratio (SIR) Calculation – Electric power & transmission & distribution – Eng-Tips

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A tutorial on calculating source impedance ratios for determining line length – IEEE Conference Publication

Impedance matching – Wikipedia

The Importance of the X/R Ratio in Short Circuit Calculations

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IEC 60050 – International Electrotechnical Vocabulary – Details for IEV number 448-14-14: “system impedance ratio”

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