Modern communication systems are often connected by long differential transmission lines. These transmission lines are vulnerable to lightning strikes. The voltage levels involved with lightning strikes on the long differential transmission lines can easily damage expensive communications equipment coupled to the transmission lines.
Bellcore Spec GR-1089-CORE was developed to address the problem of protecting equipment from lightning strikes. The Bellcore Spec defines two levels of lightning strike. After a level one lightning strike the system must be capable of operating and must not be a safety hazard. After a level two strike the system is not required to continue operating. The level one strike typically involves lower voltage levels than the level two strike.
One method for meeting the Bellcore Spec's requirements is to use at least one non-resettable fuse. The non-resettable fuse is positioned between the equipment to be protected and the transmission lines so that the nonresettable fuse will trip in response to a level two lightning strike on the transmission line before the equipment is damaged. The fuse will not trip in response to the level one strike. After the fuse trips, the fuse must be manually replaced. One such circuit includes two nonresettable fuses which each have one end coupled to a respective line of a differential line pair, and a voltage regulating circuit coupled between the opposite ends of these two fuses.
While this known system has been generally adequate for its intended purpose, it has not been entirely satisfactory in all respects. For example, after a level two strike the non-resettable fuse will have been tripped and the equipment will have been protected, however, the equipment will be non-operable until the fuse is manually replaced. A complete loss of service occurs until the tripped fuse can be manually replaced. The complete loss of telecommunications service to an area may have serious consequences, for example, if a disaster occurs in the affected area. Additionally, customer confidence and satisfaction in the telecommunications service provider is damaged when the provider can not provide phone service for a period of time, even if the period of time is a relatively short period, such as two or three hours. Given the number of systems deployed using the Bellcore standard for lightning strike protection, an enhanced solution that provides continued service after a level two strike, and that can be easily retrofit into existing systems, would be advantageous. Also, because of the number of systems deployed, a cost effective solution requiring minimal parts and cost would also be advantageous.