The first major problem with refrigerators on Shabbos came with the introduction of the defrost system. When warm air comes into contact with a cold surface it will condense and stick to the surface- much like a window pane in winter. In freezing temperatures the frost will accumulate and turn into a thick layer of ice.
At first, this ice buildup in the freezer needed melting every so often by unplugging the freezer for a few hours.
Consequently, manufacturers introduced the defrost mechanism. Inside the freezer walls, they installed a heating coil –like the coils found in a toaster oven- that burns red-hot and melts any frost buildup. The small amount of excess water is drained into a tray underneath the refrigerator where it evaporates.