588 BELL SYSTEM TECHNICAL JOURNAL 



to increase the difficulty of securing adequate selectivity in power 

 system relay operation and to make a more complex relay system 

 necessary. It also tends to increase over-voltages on the power 

 system and to reduce the factors of safety for lightning arrestors. 



In addition to the magnitude of the residual current, its duration is 

 of importance since the length of time that the induced voltage persists 

 on a telephone circuit has important reactions on its effects. For 

 example, the chance of permanently grounding telephone protectors, 

 with consequent interruption of service until the protector blocks are 

 replaced, depends not only on the amount of current through the 

 blocks but upon its duration. Likewise, many of the other effects, 

 which are described later, are materially affected by the duration of 

 the induced voltage. Since, except for self-clearing faults, the duration 

 of fault current is determined by the time of operation of the relaying 

 system, the reliability and speed of operation of the latter is an im- 

 portant factor. There are many types of relaying systems and it is 

 not practicable to go into a discussion of them here except to point 

 out that rapid and reliable relaying is usually simplest on a solidly 

 grounded neutral system. For systems with large impedances in 

 the neutrals, it may be difficult to secure rapid fault clearance, 

 particularly if the system layout is complicated. For isolated neutral 

 systems, rapid relaying on ground faults may be very difficult or 

 impracticable. 



Telephone Circuit Conditions 



The voltage due to magnetic induction accumulates along the 

 telephone circuit and can be represented as a voltage in series with 

 the telephone wires. Figure 10 shows schematically how this voltage 

 acts. The two sides of the metallic telephone circuit are assumed to 

 have the same induced voltage and impedance and are shown here 

 replaced by a single equivalent conductor. The total voltage which 

 is equal to the product of power line fault current and coupling is 

 represented by a number of generators connected in series through 

 impedances representing, in total, the line impedance inside the ex- 

 posure. At the ends of the exposure are connected impedances 

 representing those in the line and between line and ground outside of 

 the exposure. The longitudinal induced voltage acting through the 

 series and shunt impedances of the telephone line will produce the 

 following conditions of interest: 



(a) Voltages between the telephone wires and ground at various 



places along the telephone line. 

 {b) Current in the longitudinal telephone circuit. 



