Voltage surge protector

Abstract

Much electrical equipment, such as telephone station apparatus, exposed to occasional, destructively high, voltage surges (e.g., lightning strikes) is protected by a device, placed in parallel with the equipment. This device includes two electrodes defining a fixed narrow spark gap. Such a device is designed to arc over with each surge, shorting the destructive energy to ground, and to recover afterward, restoring the line to its original condition. The predominant failure mode of such devices is the occurrence of a permanant short across the narrow gap, due to electrode damage produced during the protective arcing mode. In the disclosed devices, the electrodes bound an annular narrow gap region, determining the protective breakdown voltage, and a central wider gap region, sustaining the major part of the electrode damage. Shortly after the initiation of the protective discharge in the narrow gap region, the discharge is forced into the wider gap region by the provision, in at least one of the electrodes, of a high resistance carbonaceous material at the annular narrow gap region and a lower resistance metallic material at the central wider gap region. Since the major portion of electrode damage is sustained by the wider gap region of the electrodes, the incidence of shorting failure is suppressed.