I imagine the fire department wants to cut power to the house before they go in spraying water all over the place where there is live circuitry. If there is a standby generator, there ought to be a sticker by the meter/main switch saying so and where it is. I'd guess that standard procedure would be to do a walk around anyway looking for one, or they might hear it start up, although some are fairly quiet and might not be heard over the noise of a fire engine. They might also look for a propane tank and turn off the valve at the tank without knowing if supplies only the generator or also a heating system. Perhaps someone here who works for a fire dept will chime in on this.
A standby generator wouldn't feed into the system upstream of a main shutoff, as that would backfeed into the line from the pole. It goes to the transfer switch, in parallel with line power. On failure of line power (or throwing the external shutoff switch, as for testing the transfer switch/generator operation), the transfer switch circuitry waits briefly, then signals the generator to start. On receiving ready status from the generator, the transfer switch is thrown to disconnect line input and connect generator input. It has to work this way to protect power company folks doing line repairs.
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