Lightning Surges in Paired Telephone Cable Facilities

The 3-mil air gap carbon block protector, which has a maximum sparkover value of (iOO peak volts, is the basic protection device employed by the Bell System for the protection of communication a p p a r a t u s against extraneous potentials. Prior to the introduction of transistors and miniaturized circuitry, it was the general practice in a p p a r a t u s design to provide a withstand level for both metallic* and longitudinalf potentials greater than (iOO peak volts so as to coordinate directly with 3-mil protector gaps. This customary design objective of providing an inherent withstand capability exceeding the operating value of s t a n d a r d protector gaps is not always feasible in the case of a p p a r a t u s employing * Voltage appearing between t h e conductors of a pair. t Voltage a p p e a r i n g between a c o n d u c t o r a n d ground. 547 548 T H E B E L L SYSTEM TECHNICAL J O U R N A L , MARCH 1961 solid state devices. Furthermore, lower voltage protection cannot be attained satisfactorily by simply reducing protector-gap spacing below 3 mils, since excessive protector maintenance would be introduced. To meet the lower voltage requirements of a p p a r a t u s employing transistors, therefore, it is necessary at present either to modify the circuitry so t h a t surge currents appearing in the more susceptible components are limited in magnitude, or to introduce an additional stage of protection employing semiconductor diodes supplementing the gaps. These protection measures m a y introduce significant additional expense and, in some cases, produce adverse effects on transmission characteristics.