As described above, if there is only a tiny amount of resistance in a circuit the current is very large and the wires get extremely hot. If, for example, the insulation on the wires in an appliance fails and the wires touch each other, the resistance drops and current rises. The wires, including those in the walls, can get hot enough to cause a fire. Household circuits are protected by fuses or circuit breakers. They are designed to open when current exceeds a predetermined limit. With the circuit now open, current stops and the wires will cool.