You can still use a 45A fuse or MCB on any of the three systems.
The Regs. specify that in the event of an earth fault a protective device must open the circuit within a certain length of time (most commonly 0.4 sec. for general-purpose outlets and 5 sec. for fixed appliances).
If the loop impedance is not sufficiently low for the OCPD to meet this disconnect time, then you need to add earth-fault protection in the form of an RCD.
In the case of the quoted TT system at around 21 ohms, the fault current on a dead short to earth would be only a little over 11A, so the 45A fuse alone would offer no protection whatsoever.
