Good question!
The puzzler in this case is the fact that the secondary is actually earthed. If it weren't, a secondary earth fault would simply cause one of the secondary conductors to assume 0 V (earth) potential and the other would shift to the transformer's regular output voltage. However with an earthed secondary I think a primary RCD COULD trip, but I'm not sure. Electrical theory isn't exactly my main field these days and centre-tapped transformers are quite special beasts anyway.
