We normally put a fuse or cb for the inputs and outputs.
Depending on the application, I have even seen a fuse for (4) inputs on a card for a total of four fuses per input card.
We sometimes put a fuse on each output card and even on each group of 8 outputs. Again, this depends on the application and the risk analysis done if we loose all the inputs / outputs.
The reason behind fusing is to not loose the main power.
If the main power fuse blows, you may loose the plc, hmi, mmi, and damage the machine and produce versus loosing the power to all inputs or even a single card. Less chance of machine damage / product damage.
regards,
james