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IEC 61660-1 DC Short Circuit Current Correction Factor

IEC 61660-1 DC Short Circuit Current Correction Factor

IEC 61660-1 DC Short Circuit Current Correction Factor

(OP)
For a DC fault involving multiple current sources and a common branch, equation 50 of IEC 61660-1 describes a correction factor to be used in determining the short circuit current delivered by a source to the fault location.  For source j, the correction factor is given as

σ_j = R_resj * (R_ij + R_Y) / (R_resj*R_ij + R_ij*R_Y + R_resj*R_Y),

where R_resj is the equivalent resistance of the other parallel source branches that contribute to the short circuit current through the common branch; R_ij is the resistance of the source up to the common branch; and R_Y is the resistance of the common branch.

In my case, I'm analyzing a system consisting of a battery and a battery charger delivering short circuit current through a common branch.  Using the above equation, the correction factor for the battery charger would be

σ = R_bat * (R_chg + R_Y) / (R_bat*R_chg + R_chg*R_Y + R_bat*R_Y),

where R_bat is the resistance of the battery circuit up to the common branch, and R_chg is the resistance of the battery charger circuit up to the common branch.

Before considering the IEC 61660-1 correction factor, I had been planning to correct the battery charger current using the following equation:

σ = R_bat / (R_bat + R_Y),

which simply describes how much of the battery charger current flows to though the common branch to the fault as opposed to flowing towards the battery.

I am interested in determining why these two equations are different.  Am I blatantly misinterpreting something, or does the IEC equation take into account addition factors that I am not considering?  I was wondering if someone could explain how the IEC 61660-1 correction factor is derived or otherwise direct me to a helpful reference.

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