Source Impedance missing in the equation

[NickParker]

The below snapshot is from Cahier technique no. 172, "System earthings in LV"

Since the equation is to determine the fault current at the end of line (before the downstream circuit breaker); In other words, minimum short circuit current (at the end of line) is used to verify whether the circuit breaker can be tripped for the magnitude of short circuit current at the end of line.

I have a question that, Why no source impedance has been included in the below equation from the Schneider cahier technique? If the source impedance is included, it would further reduce the short circuit current. So, I think source impedance to be included in the equation to determine the end of fault current.

 

[7anoter4]

TN power systems have one point directly earthed, the exposed-conductive-parts of the installation being connected to that point by protective conductors.
That means, in this case, the short-circuit current phase-to-ground is elevated and the supply system impedance may reduce the current.
Then reducing the supply voltage by 20% the system impedance was taken in consideration.
For TT and IT the system impedance has a small influence.

 

[Kiribanda]

The given equations are dealing with earth fault loop impedances for TN & TT systems.
1) For a TN system, it is the earth fault loop impedance which includes phase conductors, arcing resistance (if any) and the resistance of PE conductor.
Therefore, source impedance does not come into the equation. It is a metallic path.
2) For a TT system, it is the earth fault loop impedance which includes phase conductors, arcing resistance (if any), resistance of PE conductor,
installation earth electrode resistance, soil path resistance and finally the neutral grounding electrode at the source.
Therefore, the source impedance "neutral grounding electrode at the source" is in the equation. Hence we are measuring the total earth fault loop impedance Zs.