Abstract: An insulating substrate has a heat generating resistor and an electrical conductor on its surface. A pair of conductors, such as power or communication leads, are connected to a protected circuit. A rated voltage element and the heat generating resistor is connected in series between the lines. The electrical conductor is connected in series between one of the lines and the protected circuit. When a continuous overvoltage greater than a rated voltage of the rated voltage element develops in the lines, the rated voltage element acts to draw excess current through it and the heat generating resistor. When this current reaches a certain point, heat from the overcurrent in the heat generating resistor ruptures the substrate and the electrical conductor so that both the rated voltage element and the circuit are isolated from the conductors. This protects the protected circuit from destruction of, and consequent loss of protection by, the rated voltage element.