EARTH FAULT CALCULATION

[Padlock01]

Hi Guys,
Does the earth fault calculation follow the same sequence as a short circuit calc. for example I have a 22kV/11kV 4000kVA 6.4% Z Dyn11 Transf. 300amp NER and a 300metre length of cable 3c/70mm2 XLPESWA Z = 0.356 Main Intake Earth Mat = 5 Ohms should one take the primary impedance of Transformer into consideration.

 

[jghrist]

The earth fault calculation is made with the positive-, negative-, and zero-sequence circuits in series. The impedance of the transformer is the total impedance and is not broken into primary and secondary components. The impedance of the 22kV system is not included in the zero-sequence circuit. The earth fault returns through the cable ground, not the earth, so the earth mat resistance is not included. Three times the NER resistance is included in calculating I0. Note that Ifault = 3·I0.

 

[JensenDrive]

The question seems to say you need a bit more background in fault analysis than can be mentioned here. I am tempted to leave a reference to someone (me) that has experience and you can see how it is done, but I this needs to be non-commercial answers.