NGR Question

Do you use the neutral in your system? If you do you should not use a NGR

NGR Ohmic Value = System Phase Voltage / Desired Earth fault current. In your case, the System Phase Voltage for the LV side would be 240V. The desired earth fault current is (assumed to be) the short time withstand rating of the LV bus (i.e.) 50kA in your case. Hence, the NGR sizing shall be: 240 / 50000 = 4.8 milliOhms. And, the voltage rating of the NGR shall be 240V. But, I feel that for a ground fault in the bus bars, the bus bar impedance itself would be much more than 4.8 milli-ohms to limit the earth fault current to 50kA. So, you need not go for any external NGR for limiting the fault current from 55kA to 50kA. Pl. remember 55kA fault current is arrived by considering only the transformer impedance.

Moreover, here in India, one is not permitted by Law to go for any other method of earthing than Solid Earthing for the LV side. Pl. check what is the law in your country of operation.

Pl. see revised posting above.