598 BELL SYSTEM TECHNICAL JOURNAL 



operating protector blocks. Each outside trace shows the voltage 

 across one of the two blocks. It will be noted that the two traces are 

 not identical. The middle trace shows the resulting voltage across 

 the circuit. It is this voltage which may cause acoustic shocks. 

 The very jagged outline of this trace indicates that many frequencies 

 other than 60 cycles are present. 



The demonstration of Fig. 17 can be arranged to produce acoustic 



-VA- 



^ 



PROTECTOR 



LINE 

 IMPEDANCE 



^ 



AN\r 



OPERATORS 



TELEPHONE 



SET 



Fig. 17 — Demonstration of acoustic shock. 



shock. In this demonstration, sufficient voltage is impressed on the 

 circuit to operate the protector blocks at one end. An operator's 

 telephone set and a receiver are connected across the circuit at this 

 end. When the voltage rises high enough to operate the blocks a 

 relatively loud sound is emitted by the receiver. 



Other effects may accompany the unsymmetrical discharge of the 

 protector blocks. For example, the signals which are connected at 

 the ends of the telephone circuits may operate and give what is 

 commonly called a "false signal." 



Probability Factors 

 In the preceding discussion, a number of factors were mentioned 

 which may vary between different occurrences in the same inductive 

 exposures. Among these may be mentioned the following: 



(a) The impedances in the faulted circuit may vary between occur- 

 rences due to variations in the location of faults, variations in 

 the effective fault resistance, etc. The effect of the variation 

 in location of the fault, of course, is to change the line im- 

 pedance in the faulted circuit and hence the fault current. 



ib) The duration of the fault current may vary between occurrences 

 due to variations in conditions which affect the speed of opera- 



