Varistors

The varistor is a safety component that protects the underlying electronics by absorbing and conducting high transients to earth or between phase/phase and phase/neutral.  This prevents fire and short circuits. This is achieved with a semiconductor, much like diodes, although with a significantly higher reverse voltage drop. The operating case is that the high voltage opens up the varistor to earth, neutral or other phase, causing the product’s fuse to blow.
There is also a type of varistor with three legs and a built-in fuse. This burns with the varistor, which then breaks the circuit. A varistor has no polarity, i.e. it works in both directions, unlike a diode, which means it can be used in certain damping circuits.