A Hot Food Cabinet is used as its name implies – to keep cooked food warm. Equipment of this type is often used in such institutions as hotels, yeshivot, and other public dining rooms.
Such equipment does not violate the Shabbat prohibition of hatmanah (covering food to keep it hot), because the pots do not touch the walls of the cabinet. And since the food is hot when it is placed in the cabinet, there is also no problem of cooking. Sometimes cold food is put into a heating cabinet to warm up, but this is also no problem, since solid food cannot be "cooked" again once it has already been fully cooked.
There is one problem related to opening the door of the cabinet, which in general switches the heat off, the same as in a microwave oven. In addition, typical food cabinets have a thermostat in order to maintain a specific temperature. Opening the door and putting food into the cabinet can directly influence the thermostat and cause the heating elements to be turned on (this is a Torah violation).
The Zomet Institute provides "technical kashrut" certification for such food cabinets, guaranteeing that the sensors in the doors are disconnected on Shabbat. In addition, the thermostat is modified to work on the permitted principle of continuing the existing status, so that opening or closing the doors and putting food in or removing it has no effect on the heating inside the cabinet.
For details and more information, contact The Zomet Institute.