236 BRLL SYSTEM TECHNICAL JOURNAL 



tises under various conditions, they have recommended the National 

 Electrical Safety Code to be used as a guide to practise. 



Protective Devices 



Both telephone and supply circuits are eciuipped with protective 

 de\ices which arc fundamentally the same in principle. They may he 

 divided into two general classes: 



1. Those which provide protection from abnormal voltages consisting 



of protector blocks in the telephone plant and lightning arresters 

 in the supply system. 



2. Those which provide protection from abnormal currents consisting 



of heat coils and fuses in the telephone plant and fuses and cir- 

 cuit breakers in the supply systems. 



These protective devices are a secondary defense against abnormal 

 conditions which it is impracticable to avoid either by design or through 

 adherence to construction standards. 



P2ven when all practicable precautions with regard to clearances, 

 strength of construction and insulation have been taken, accidental 

 breaks occur in both power and telephone wires. In some cases there 

 are direct contacts between such wires. Higher than normal potentials 

 are also introduced into the telephone and power circuits by lightning 

 and other causes. 



It is because of the limitations of protective devices and other pro- 

 tective measures that joint use with certain types of circuits has been 

 in question. Considerable differences of opinion exist between engi- 

 neers as to the degree of hazard involved in joint use between telephone 

 plant and power circuits of various types, voltages, and connected 

 power. The problem has increased in importance as the use of higher 

 distribution voltages and greater generating capacity have been em- 

 ployed. 



This matter is under investigation by the Joint Subcommittee on 

 Development and Research. Studies are now in progress in one rural 

 area and in one suburban area to determine the over-all advantages and 

 disadvantages of the use of higher distribution voltages and of joint 

 use with these voltages under present conditions. 



The first experimental work done by the Joint Subcommittee in 

 connection with these problems was a detailed study of the character- 

 istics of various types of fuses. This study covered all of the well 

 known commercial types of telephone fuses and a number of experi- 

 mental models. The operating characteristics of these fuses were 

 obtained at voltages of 2300, 4000, 7500 and 13,200. The current 

 range was from 16 to 1000 amperes. These tests were carried on in a 



