214 BELL SYSTEM TECHNICAL JOURNAL 



Generally, the type of fault has little effect on the duration if there is 

 sufificient current to operate the relays. Conditions have been noted, 

 however, where the fault is of such high impedance that the current is 

 not adequate for the operation of the relays. Such high impedance 

 faults usually occur on wood pole lines and may result in burning of 

 pins, crossarms and poles. They may also occur on steel tower lines 

 as the result of branches of trees getting in contact with conductors. 



Effects Produced by Induced Voltages 



Low-frequency and transient voltages induced on telephone circuits 

 m^iy produce a variety of effects depending upon their magnitude and 

 duration. These effects include service interruption, false signals, tele- 

 graph signal distortion, damage to plant, electric shock, and acoustic 

 shock. 



Telephone circuits are very low energy circuits, the voltage for talk- 

 ing purposes rarely exceeding one or two volts, with maximum current 

 measured in milliamperes. For signaling purposes a maximum of 165 

 volts peak, is used with currents limited to about 0.10 ampere. For 

 telegraph service the voltages are limited to 135 volts between wire and 

 ground, while the current is limited to less than 0.10 ampere. By 

 contrast, the voltages due to induction, in some cases of exposure, may 

 be a thousand volts or more. 



Service Interriiption. — When the telephone protectors are operated 

 by induced voltage the behavior of the protector discharge gaps de- 

 pends upon the magnitude of the voltage and current and the length 

 of time the discharge lasts. In cases where the discharge is not 

 promptly extinguished or where the current is very high, the discharge 

 gaps may become permanently grounded. This causes interruption 

 to service until the affected protectors can be replaced, the time neces- 

 sary for such replacement depending, of course, upon the protector 

 locations. 



False Signals. — False switchboard signals are likely to be coincident 

 with protector operation. They produce a bad service reaction due 

 to operators answering false calling signals and cutting off connections 

 because of false disconnect indications. 



Distortion of Telegraph Signals. — The induced voltages appear in 

 just the same paths over the wires as the operating voltages of grounded 

 telegraph. The effect of such induced voltages depends on their 

 magnitude, character, and duration. Voltages much lower than those 

 sufficient to operate the protectors may cause detrimental effects 

 ranging from a slowing down of speed to complete failure. Where the 

 duration is short, the effect may be limited to distortion of signals, or, 

 if the voltages are high enough, to momentary interruptions. 



