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Backfeed is the process of feeding the generators output through breakers installed in the area where normally individual circuit breakers would be in the panel. If the main breaker is not shut off, or is turned back on while the generator is still connected major problems are caused including the possibility of causing the injury or death of a utility worker. The mechanical lockout is a simple device that physically prevents the main breaker and backfeed breakers from being on at the same time. (Cost runs about $150)

Here is another option for wireing a generator into the house circuit, this one uses a MECHANICAL lockout and will meet utilities saftey requirements. The difference with this method is you can then select and activate the circuits you want powered by the generator (within its capacity). I think this method is also somewhat cheaper. Here is the link: http://www.interlockkit.com/intro2.htm