Electrical isolation device

Abstract

An electrical isolation device is connected at input terminals in lines between a structure to be cathodically protected and a ground, or between two cathodically protected structures, to block selected low levels of DC voltage while freely conducting normal AC currents and all fault currents. The device has two diode stacks, formed of layers, and the stacks are connected in parallel with one another in opposite polarity and in parallel with a capacitor across the input terminals of the device. Each diode layer include a junction wafer and a conducting metal disk. The layers in each stack are preferably pressed tightly together and to metal bus conductors by a clamp to maximize the contact of the diode layers to each other and to the conductors to minimize contact resistance. AC currents are conducted (e.g., to ground) through the capacitor while low level DC voltages are blocked by the cumulative threshold voltages of the plural diode layers connected in series in each of the stacks. When a large AC or DC fault occurs, the voltage across a stack exceeds the turn-on voltage of the stack so that fault currents are conducted to ground through the diode stacks. The number of diodes in each stack may be selected to provide a continued cathodic protection voltage in the presence of large AC fault currents. The diode stacks may also be connected together in a bridge structure to reduce the number of diodes required.