238 BELL SYSTEM TECHNICAL JOURNAL 



mal conditions. It is necessary that the over-all protective equipment 

 be adequate and that the burden of overcoming weaknesses in the 

 protective equipment of one system be not thrown on the protective 

 equipment of the other. There are inherent limitations in both classes 

 of protective equipment that must be defined. 



Therefore, the next step in this investigation is a determination of 

 the over-all characteristics of power circuit and telephone circuit pro- 

 tection under typical conditions of contact between the two plants. 



While protective devices are an important element in connection 

 with joint use involving certain types of power circuits employing the 

 higher distribution voltages, there are also other important considera- 

 tions. The general insulation of the telephone plant must also be 

 considered, especially in connection with drop loops attached to and 

 entering subscribers' premises. These matters are also being studied 

 by the Joint Subcommittee. 



All of these problems, as is the case of others being studied by the 

 Joint General Committee, are being approached on the basis of deter- 

 mining the best over-all engineering solution such that both systems can 

 provide their services in the most convenient and economical manner. 



Inductive Coordination 



In the early history of joint use, noise induction problems involving 

 street lighting circuits appeared. Other interesting problems were 

 encountered such, for example, as the accidental grounding of one 

 corner of an isolated delta power system with its resulting unbalanced 

 voltage inductive effects on open-wire telephone circuits, which type 

 of telephone construction then predominated. 



As these problems arose they received careful study and with the 

 development and extended use of telephone cables and the use of 

 improved operating methods in power and telephone distribution 

 generally, inductive coordination of power and telephone distribution 

 systems in the urban communities became less troublesome and did 

 not for a time receive any large amount of consideration. 



However, during recent years the introduction and extended use of 

 various types of multi-grounded distribution systems described in the 

 paper by Messrs. Harrison and Silver and the existence of certain types 

 of signaling on local telephone circuits, have contributed toward mak- 

 ing important the consideration of noise inductive effects in connection 

 with joint use. This matter is discussed more fully in the paper by 

 Messrs. Harrison and Silver. 



The technical factors involved in inductive coordination problems 

 under joint use conditions are complicated. The details regarding 



