While we are talking about thou shall nots, backfeeding while an all too common practice is generally frowned upon, the level of frowning may range from a warning against it, to being disconnected from the grid and having to beg and promise to never do it again to be reconnected, and may even be a criminal offense in some places (I have seen comments to this regard online, but have never seen references to specific laws, it is certainly an NEC code violation tough.
There are all too many stories that have been posted online about backfeeding killing lineman, etc. As well as close call accidents involving people that "knew what they were doing" most of these involved relatives of said person doing something stupid even after being told by the person that "knew what they were doing" saying not to touch it. For those that may not understand the issue, if the main breaker is closed back feeding can potentially energize the distribution lines momentarily with thousands of volts of electricity as it is stepped up when fed backwards through the pole top transformer, thereby potentially killing a lineman working on a "dead" line even miles away.
Here is a fictional amalgamation story or two based on a combination of reported incidents;
During a prolonged outage an electrically experienced husband turns off the main breaker and back feeds through a dryer outlet suicide cord using a small generator that will run only limited loads in the house. Tells rest of family not to touch the breaker box or the rigged suicide (2 male end) cord. 4 days later the power comes back on while the husband is across town helping his brother in law with a fallen tree, the wife wants gird power and wants it NOW!!! so she can do laundry, dishes, etc that the small generator would not power, so so has the teen age son go flip the breaker back on. The son ignores the big DON"T TOUCH ME sign and proceeds to flip the main breaker back on without first turning off and disconnecting the generator. This was an older breaker box, and the cycles of the line and generator happened to magnify the peak, welding closed the contacts on the breaker, and a moment later the breaker explodes injuring or killing the son.
Now same story with a different variation
During a prolonged outage an electrically experienced husband turns off the main breaker and back feeds through a dryer outlet suicide cord using a small generator that will run only limited loads in the house. Tells rest of family not to touch the breaker box or the rigged suicide (2 male end) cord. 4 days later the wife sees the people across the street have power while the husband is across town helping his brother in law with a fallen tree, the wife wants gird power and wants it NOW!!! so she can do laundry, dishes, etc that the small generator would not power, so so has the teen age son go flip the breaker back on to see if they have power too. And they don't but would have had power in a couple of more minutes except for the fact that when the son flipped the breaker without first turning off and disconnecting the generator, he backfeeds the pole top transformer while the lineman was in the process of reconnecting the line that had broken due to a downed limb 2 doors down the street, killing the lineman in the process.
