Generator interlock kit
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. Installation and Operation. Early generator interlock kits consisted of two sliding steel or plastic (depending on the brand) plates held together by three bolts and installed on the front cover of the home's breaker panel, however, some models made by Eaton (formerly Cutler-Hammer) and Siemens for panels manufactured by them install on the adjacent circuit breakers themselves and consist of a sliding arm for breakers installed  back-to-back or a pivoting arm for breakers that are installed side-by-side or one above the other, these arrangements remain on the breakers even if the panel cover is removed as opposed to the mechanisms installed on the panel covers. When the main breaker is turned on, the plate or pivot arm blocks the generator backfeed circuit breaker and allows the main breaker to remain on. Conversely, when the generator backfeed circuit breaker is turned on, the main circuit breaker is blocked and will remain off. The generator backfeed circuit breaker is connected to a generator inlet  installed (preferably) on the outside of the structure. A short cord connects the generator to the house inlet, usually through a twistlock plug and socket.