Earth conductor sizing......

[simonfr]

Yes, i know, its been done millions of times before! So sorry in advance.

So, im overthinking this so doubting myself. Opinions please.....

Cable from X to Y - 5km long. Secondary Voltage 11kV (Primary is 66kV, Z=6%). Power 6MW. System is TN-C. The cable is 3 core and earth. Cable conductor size is 240mm2 and copper. I dont have a protection device trip time so assuming 0.7s.
The cable is on the output (secondary) of a Dyn11 transformer. Star point is grounded via a 335ohm resistor. The NER is inbetween the star point and the PEN.

Fault current calculated at 18.9A. NER resistor selected is 25A rating.

Using the adiabiatic equation, the earth conductor is calculated as very small, so referencing IEC 60364-5-54, im going to select the earth core is 10mm2 in the 5km cable.

Comments please.......

 

[7anoter4]

For earthing -protective- conductor cross-section area see BS7671 Regulation 543.1.3.
S=sqrt(I^2*t)/k k=143 see Table 54.3 then S=sqrt(18.9^2*0.7)/143=0.11 mm^2.
But, according to table 54.1 the minimum cross section area of protective conductor has to be 2.5 mm^2[protected against damage] or 16 mm^2 if it is not protected.
In my opinion 2.5 is enough.

 

[7anoter4]

See 54.1