Unfortunately, nowadays with our world information transmission system (internet), we usually neglect the basic conceptions and deep thinking about new problems.
With more deep thinking, we can find that the mentioned evident equation (S = 1.73 VI) is not correct for zig-zag earthing transformers.
Indeed, each leg of a grounding transformer carries one-third of the neutral current and has line-to-neutral voltage. So in a grounded wye – delta transformer, the total power rating including all three phases is the neutral current times
The line-to-ground voltage:
S= 1.73 VI
But a zig-zag transformer is more efficient than a grounded wye – delta transformer.
In a zig-zag, each winding has less than the line-to-ground voltage,
by a factor of 1.73, so the bank may be rated lower:
S = 1.73/1.7.3 VI = VI
Therefore, the continuous rating current of E/T is 5 A and rating neutral current is 15A, and we can choose 12.5A as correct earth fault primary current setting .