Metal oxide varistor, a type of overvoltage protection device is often used to protect a circuit from a high voltage spike caused by a lightning strike or the accidental application of the wrong voltage, such as when a 120V fixture is connected to a 208V supply. This contains a ceramic mass of zinc oxide grains, in a matrix of other metal oxides such as small amounts of bismuth, cobalt, manganese sandwiched between two metal plates (the electrodes). If the threshold voltage is exceeded, it becomes a linear resistor and conducts current in direct proportion to the voltage. It can protect a circuit by shorting it at the input and blowing a fuse or tripping a circuit breaker. A MOV is a type of diode called a tunnel diode. When low to moderate voltage is applied to it with proper polarity (reverse biased), then it conducts only a small amount of current due to the leakage across the diode junction. But when the threshold voltage is exceeded, the diode breaks down because of something known as the avalanche effect, and it passes a very large current. It acts a very nonlinear resistor exhibiting very high resistance below the threshold voltage and low resistance above the threshold voltage. In a power supply circuit, if it is placed in parallel across the input of the circuit, then if the threshold voltage is exceeded it will short the conductors and blow a fuse or trip a breaker, thereby sparing all the components downstream of the MOV. When an MOV experiences a high voltage spike, it is often destroyed in the process of protecting the circuit. If it is not destroyed, it should be replaced after it experiences high voltage because it will become stressed.