Your gas forced air furnace has a power feed that sometimes runs through a fused disconnect switch. The fuse not only protects the furnace from overloads, but can be unscrewed to disconnect the power from the furnace. By using the nameplate rating, you can size the fuse to provide the maximum protection. The switch also disconnects the circuit from the furnace. This device allows a serviceman the ability to disconnect power from the furnace to do his work without trying to find the circuit breaker or fuse panel.
To connect the fused switch disconnect, connect the hot feed to one side of the fuse on a terminal. From the other terminal, run a wire to one side of the switch. The other side of the switch feeds the furnace with power. Tie the white neutral wires together with a wire nut. Connect the green ground wires together and pigtail the connection to the box.