At one of our substation, we have two 40MVA, 88/33kV, star/delta transformers with 8.4% impedance voltage. These units are earthed through zigzag earthing transformers on 33kV side. Recently, we had a fault on 33kV side of one of these transformers (33kV surge diverter shuttering in the process). Investigations and tests revealed that the earthing transformer on this bay got burnt during the fault. The earthing transformers are rated for 6A continous and 300A short time for 10s. Our feeling is that these transformers could have been underspecified. We want to do fresh specifications for replacement transformers. Could anyone with experience in this subject please help me with information on how to specify an earthing transformer especially with regard to current ratings and KVA rating?