Kenton, in the first image, that disconnect is a 600 volt rated, 3 pole disconnect, typically installed on three phase systems where the service is either a 480 volt three phase, or an uncommon 575 volt three phase. Both are voltages used in industry. It is completely possible that the actual connected voltage is 120/240 volt single phase, but we can not tell that without actually measuring it. Mind you, we are speaking in terms of voltage and not in current.
Disconnects are rated in general for two voltages when dealing with low voltage systems. They are normally rated at 250 volt, or 600 volt. This is a distinction that follows the NEC, where only 2 types of system voltages are identified. They are systems “operating at less than 600 volts”, and systems “operating at over 600 volts”. I am attaching a PDF chart that actually details system voltage definitions according to the IEEE, just as a reference. In industry and engineering, we actually consider 601 volts to 69,000 volts medium voltage.
Anyways, back to your post. Since fuses trip on current, a fuse rated at 600 volts, 100 amps will still open an overcurrent event if it is indeed installed on a 240 volt system where the current exceeds the current rating on the fuse. If you compare a fused 600 volt 100 amp disconnect to a fused 250 volt 100 amp disconnect, the physical size difference is obvious. The 600 will be roughly twice the size.
BTW, it is not a code violation to install a 600 volt rated disconnect on a 240 volt system. However, it *is *a violation to install a 250 volt rated disconnect on either a 480 volt or 575 volt system, since the applied voltage exceeds the voltage rating.
I hope this gives you a little more information regarding disconnects and their application.
Edited to add: By code, the actual knife blades of the disconnect (the part that completes the connection and actually protrudes out when the disconnect is in the “off/open” position, are required to be deenergized. If not, the disconnect would be back fed and the fuses would be energized at all times. This is perhaps the most serious safety issue regarding improper installation. I have ran into it many times in my career and is the main reason for always verifying with a meter even after you turn something off. Back feeds are unrecognized killers!
