The protection on a 50-0-50 supply for control and low energy actuators is not normally for protection from electric shock but more for the protection of the equipment during fault conditions.Protection-wise, if you wanted/needed RCD protection on the secondary side, you would have to have one end of secondary connected to earth and install an RCD on the secondary side,
Some systems have used an RCD with the centre tap of the 50-0-50 bonded to the metal work. Then any single short to metal work that would otherwise go un-noticed will trip the RCD and invoke an inspection / repair of the equipment. A lot of equipment operating on ELV will tolerate a single short to chassis / framework until a second fault occurs.