I know this is a pretty dumb question but every time I search on anything to do with Separately Derived Systems ALL I ever get is info on grounding and bonding them. What I want to know is: What exactly is the purpose when the system is a transformer? I understand that there are many times you want or even must use power not from the utility and these "off grid" systems are essential for those times as either main power or standby power. PV, generators, UPS, wind, inverters etc can all provide off grid power ON THEIR OWN when needed but doesn't a transformer get it's primary power from the utility Co? If so then, other than increasing or decreasing the voltage for a special purpose, what is the point as a SDS? If the utility "service" goes down the transformer goes down. So what am I missing or am I misunderstanding what a SDS is used for?