A generator interlock kit (or just interlock kit) is a device designed to allow safe powering of a home by a portable generator during a power outage. It is a less-expensive alternative to purchasing and installing a dedicated transfer switch. The kit achieves the same function by adding an external interlock onto an existing breaker panel that allows the main breaker to be turned on or one designated load breaker to be turned on, but not both at the same time. The interlocked load breaker is repurposed as the "backfeed" breaker, and a generator is connected to it (wired directly or through a power inlet).
Under normal conditions, the main breaker is on, accepting power from the external mains into the panel, and the backfeed breaker is off, isolating the generator. The external mains feeds the panel, but the panel cannot backfeed the generator. Backfeeding the generator should never be done, because it serves no purpose and risks damaging the generator.
In generator mode, the backfeed breaker is on, accepting power from the generator into the panel, and the main breaker is off, isolating the external mains. The generator feeds the home by backfeeding the panel, but the panel cannot backfeed the external mains. Backfeeding the external mains is unsafe and illegal, because it can potentially electrocute lineworkers, start fires, and overload or damage the generator.[1]