Dont know what type of on/off switch you have but my transformer has rotary cam switch that when i switch like you are supposed to it trips the breaker but when I go slow it doesent trip and welds fine. If you are not sure are coils fried by teand touching the ground, try attaching the car battery charger or battery to input or output terminal and try scratching the case to see if it sparks. But since it triped the rcd its most likely that the pirmary touching the earth.
EDIT: One more reason why it trips the breaker is because the primary windings have burnt and now ratio is changed and that trips it