220 BELL SYSTEM TECHNICAL JOURNAL 



graph instruments or by false signals. Improvement in balance of the 

 power line by transpositions will in some cases correct the difficulty. 



Protective Measures for Communication Systems. — In general, meas- 

 ures applicable to the communication system to prevent or reduce the 

 effects of induced voltages take the form of arrangements or devices 

 for removing or counteracting the voltages to ground or the currents in 

 the telephone circuits which might be produced by the induced voltages. 



Bell System Standard Protectors. — It is Bell System standard practise 

 to equip all telephone circuits which are exposed to the liability of 

 foreign voltages, with electrical protective devices. These devices are 

 made in various forms and combinations for different plant and ex- 

 posure conditions. The protector used at central offices and at sub- 

 scribers' stations includes a discharge gap which operates at approxi- 

 mately 350 volts and a fuse which opens the circuit at about 10 amperes. 

 Such devices are intended to offer a measure of protection against 

 lightning discharges and against the voltages and currents resulting 

 from accidental contacts with foreign wires or from low-frequency in- 

 duction. 



In order to protect telephone linemen or others working on open- 

 wire lines against electric shock from induced voltages, it is necessary 

 that the voltages between line wires, and between each line wire and 

 ground, be kept low. The use of protectors at central offices does not 

 so protect the linemen as the impedance drop on the line wires permits 

 high voltages between wires and ground at other points, such as the 

 terminals of the exposed section. 



It appeared however, that protectors of the Bell standard type 

 might be used on open- wire lines at locations immediately adjacent 

 to exposures to limit induced voltages to ground. A number of in- 

 stallations of this kind have been made but observations over a 

 period of time show that they introduce serious troubles as the pro- 

 tectors, being subjected to heavy discharges, often become permanently 

 grounded thus interrupting service. It also sometimes happens, as all 

 the line wires are not always equally exposed, that some of the protec- 

 tors operate and others do not, resulting in objectionable voltages 

 between line wires. 



Relay Protectors.- — In view of the inadequacy of existing forms of 

 protectors for such use, the subcommittee is experimenting with a 

 "relay protector." This device includes Bell standard protectors in 

 combination with a relay which operates to short-circuit them upon 

 the occurrence of a discharge, thus relieving the protectors of the duty 

 of carrying the large discharge current and greatly reducing their 

 tendency to become permanently grounded. In more recent types all 



