It seems that you are assuming all PLC'S are connected to field wiring. Very many are part of machines ( our case ) where all the wiring is on the machine. There are no conduit runs all over the place, just a few wire ways on the machine with lots of devices like photocells, solenoid valves etc. In our case if even one device is not available you do not want to run the machine since it there is a risk of making defective product. In our case the machine most likely stopped when the fuse blew or the operator stopped it because it was making scrap or not behaving properly.

James Bouchard

 

It is a fact that most manufacturers of I/o cards are adding overcurrent protection to their individual I/O. Most Dig Output modules have high impedance at the channel to limit the amount of current to the module. And most Analog output module provide loop power or electronically monitor the current to avoid overcurrent.

However, during equipment debug and startup, output fusing can be of great help. The question I always ask is: Do I have to separate the outputs that are powered from the estop circuit? Do I have an estop circuit? For example, pilot lights do not need to be in the estop power circuit. But solenoid valves and control valves might be. If I have an estop circuit, I do not consider necessary to fuse the outputs because I will segregate the outputs that need to be powered from the estop circuit. But if I do not have an estop, I install fuses on the individual outputs. I do this because I can flip the fuse out during point-to-point checkout and avoid initiating an action unintentionaly. If I have an estop circuitry, I just conduct the point-to-point with the estop pressed. No need for fusing (if the estop is correctly designed).

It depends on your equipment, if you need overcurrent protection, you need the fuses. I find value on being able to "disable" an output too.