ISOLATOR SURGE PROTECTOR FOR DC ISOLATION AND AC GROUNDING OF CATHODICALLY PROTECTED SYSTEMS

An isolator surge protector for use with corrosion protection systems which are an inherent part of or coupled to an electrical power system, for power transformers, and for DC transmission systems, where it is necessary to present a high impedance to DC and a low impedance to AC up to a predetermined voltage level. Anti- paralleled thyristors in the isolator surge protector turn on during power surges. A bypass circuit insures turn off of the thyristors after a triggering event such as lightning or a 60 Hz surge has ended. The bypass circuit shunts DC bias currents, such as that which may be provided by a DC bias voltage source for cathodic protection, from the thyristors to ensure that the voltage across each thyristor falls below its holding voltage or current, and thus prevents the thyristor from being stuck in a continuous conduction mode. Power to operate the bypass circuit may be tapped from the surge which caused the thyristor to trigger so that the bypass circuit operates autonomously. The circuit may also operate with an auxiliary power supply to provide the required operating power and to turn off the thyristors.