LOW-FREQUENCY INDUCTION 



589 



(c) Voltages between different wires on the telephone line. 



Telephone circuits are supplied with protective devices. The part 

 of the telephone protective system of most interest in connection with 

 low-frequency induction is the carbon-block protector. This device 

 provides a small air gap between carbon surfaces one of which is con- 

 nected to the telephone conductor and the other to ground or cable 

 sheath. When an excessive voltage is impressed on the telephone 



TO SOURCE 



CENTRAL 

 OFFICE 



ENTRANCE 

 / CABLE 



n .F 



;; 



POWER LINE 

 K ^^^ H- 



FAULT 



^^ 



A — EXPOSURE DIAGRAM 



CENTRAL 

 OFFICE 



CENTRAL 



OFFICE CABLE LINE 



EQUIPMENT IMPEDANCE IMPEDANCE 



\Wv-^r^M^ aDW(]D-W<]I>AAK2>A/V — ^AMrr \J 



INDUCED VOLTAGE 

 AND LINE IMPEDANCE 



CENTRAL 

 LINE OFFICE 



IMPEDANCE EQUIPMENT 



CAPACITY TO GROUND 

 l-^ OPEN-WIRE LINE 

 -CAPACITY TO GROUND 

 CABLE 



CAPACITY . 

 TO GROUND ■ 



B- DISTRIBUTION OF ADMITTANCES AND IMPEDANCES 



C — DISTRIBUTION OF VOLTAGE-TO-GROUND 

 Fig. 10 — Distribution of voltage to ground before protectors operate. 



conductor an arc is established in the air gap thereby grounding the 

 conductor. Protectors are located at central offices and at other 

 points, such as at junctions of open wire and cable where it is desired 

 to limit voltages on telephone wires due to lightning, contacts with 

 power systems, induction, and other extraneous voltages. 



In analyzing the distribution of induced voltage between a telephone 

 circuit and ground assume first that no protectors are operated. 

 Under this condition, the voltages to ground on the telephone wires at 

 various points are determined by the impedances between the wires and 

 ground along the line and at central offices where equipment is con- 



