181106-1533 EDT
sii:
It should be self-evident and obvious.
First, safety. The likely reason you want to use a generator is that power has been lost or is messed up from the power company. You want to completely disconnect from the power company lines so anyone working on those lines won't get hurt from you applying power to the lines.
Second, load. Looking back toward the power company lines there is likely considerable load on your circuit, meaning low impedance. The size of this load depends upon where the break or other problem is. If you actually connected your generator to the power company lines you would likely overload the generator, and stall it out.
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