Turn off your natural gas line at the meter.
(Unless the house has sprinklers, of course, or other fire suppression, obviously.)
Not that this helps, I just figure it is interesting and one less thing to worry about at a stressful time.
They usually don't require power or external water because those can be disrupted. I only know this because I've been involved in having a few systems installed and am not an expert.
The systems I'm familiar with were all commercial, so that may be why I'm unfamiliar with them being fed from the main. I had to deal with getting a couple of commercial buildings done. I am not completely certain, but I think both of those systems were completely independent of the municipal water supply. I'm 99% certain, at least.
Not that I'm aware of, which means I'm pretty sure not. It might be that at the house level, a single house isn't expected to draw so much water, while in a larger structure it's probably possible to overwhelm any feasible main that could be expected if a bunch of sprinklers go off. Even if the main can handle it, maybe there isn't a large enough connections to specific portions of the building from the main which causes water starvation. I imagine there's lots of insidious details when scaling to a large building which aren't immediately obvious.
I can see why there would be different systems and code requirements for residential construction. Come to think of it, whilst you could put the tank in an attic space - or use a bladder system, it would occupy a lot of space and I'm not sure residential construction would support the weight of the system.