A generator interlock kit (or just interlock kit) is a device designed to allow safe backfeeding of a home through a portable generator during power outages, thereby eliminating illegal and/or unsafe generator backfeeding situations that could potentially electrocute power linemen or destroy the generator.
A generator interlock kit is installed on the front cover of the home's breaker panel. It consists of two sliding steel or plastic (depending on the brand) plates held together by three bolts. When moved down, the plate blocks the generator backfeed circuit breaker and allows the main breaker to remain on; when moved up, the generator backfeed circuit breaker may be turned on, but 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 the use of twistlock plugs and sockets.
Using an interlock kit allows for all of a panel's circuits to be energized, up to the load capacity of the generator; whereas using a generator transfer switch only allows a few circuits to be operated.
An interlock kit does not require a separate panel to operate; it can be installed directly on the home's breaker panel.
Interlock kits eliminate the need for extension cords to power appliances.
Even though many are tested by an independent lab to a UL standard, they do not carry the UL Listing mark. Without that mark some local electrical inspectors will not allow the use of these interlock kits.
Removing the electrical panel's cover could allow the generator backfeed breaker to be turned on while the main is on, potentially energizing power lines or overloading the generator, if the generator is connected.
In some applications, installing an interlock kit requires drilling holes in the panel cover; once it is installed, it cannot be removed.
Because the main breaker is shut off, homeowners have no way of knowing when utility power is restored, except by manually turning the generator off and the main back on. However there is at least one product that will produce an audible alert indicating that utility power is restored.
Installing an interlock kit requires anywhere from 2 to 3 spaces in the existing breaker panel.
