Grounding Techniques in Bell System Central Offices

For decades, the field of grounding in central office installation was a neglected art. Since 1960, as more sophisticated circuitry is applied to telephone switching and supporting operations, grounding is being increasingly recognized as an important consideration. Effective grounding is necessary to maintain equalization of potential throughout the ground plane of the system, to minimize impedance produced electrical noise and to afford adequate protection to equipment and personnel. In the Bell System, grounding design has evolved from a rudimentary system that depended greatly on incidental conductivity through cable racks, frame metal and other metallic objects. In modern central offices that rudimentary system has been augmented by a Central Office Grounding (CO GRD) system that effectively extends the office ground electrode to every floor of the building to provide a convenient low impedance point of grounding reference. The CO GRD system is horizontally extended on each floor to every frame. The system is designed to limit normal difference of potential between any two point on the ground plane of the building to less than one-half volt difference. Electronic Switching Systems (ESS) in the Bell System have employed a single point ground system to protect the solid-state circuitry from transient-voltage damage since the inception of ESS. Experience has indicated that the relatively modest-cost system has proved to be reliable and effective in keeping disruption of service and equipment damage to an acceptable minimum. This paper describes the techniques employed to establish a CO GRD system and an ESS single point ground system.