PURPOSE: To prevent short-circuiting by the breakdown of a voltage nonlinear resistor, and enable recovering after circuit breaking when there is no breakdown by connecting a positive characteristics thermistor between an electrode of the the resistor and a terminal, and connecting the electrode, at a normal temperature, with the terminal via a bi-directional shape memory element.

CONSTITUTION: A voltage nonlinear resistor 1 has electrodes on both surfaces. An end surface of a ring type positive characteristic thermistor 2 is fixed to the electrode 11. A coil 3 composed of bi-directional shape memory alloy is accommodated in the central space of the thermistor 2. One end of the coil 3 is fixed to the electrode 11 of the resistor 1, and the other end is fixed to a contact member 4. When the voltage nonlinear resistor 1 generates characteristics deterioration or short-circuiting, a large current flows through the positive characteristics thermistor 2, which generates heat. The shape memory alloy 3 turns to a linear state and keeps the shape. The resistance value of the thermistor 2 increases, and the current is limited at a temperature. When the resistor 1 generates heat as the result of simple surge absorption, the temperature of the shape memory alloy 3 quickly approaches the normal temperature. The coil shape is again recovered by the bi-directional shape memory effect of the shape mem ory alloy.