reading through this, I just wanted to add: by definition, S_sc = S_base / Zpu.
When they give us "fault duty" of 40MVA, that means they gave us S_sc, using 40MVA as S_base:
S_sc = S_base / Zpu
40MVA = 40MVA / Zpu
Zpu should be assumed to be 1pu, as Flyer has said, but it didn't click with...