In Fig. 10-1 an electric strip heater assembly is shown. The limit switch is found in the electric furnace as it is in the fossil fuel furnaces. Its main purpose is to protect the unit from reaching excessively high temperatures. The electric furnace can overheat if there is a fan failure or if not enough air is being blown across the resistive elements. Figure 10-2 shows a typical limit switch. Some of these are dual-purpose switches. The first stage of heat rise is wired to control the fan. In this way a blast of cold air is not felt when the electric furnace begins to operate. When the air in the plenum reaches a comfortable, heating temperature, the fan will start operating. If the heat continues to climb, a second set of contacts contained in the limit switch opens. Figure 10-2 shows a surface mount limit switch. On some applications an insertion type, as shown in Fig. 10-3, is used.