JOINT DE VELOPMENT A ND LO W-FREQ UENC Y IND UCTION 2 1 5 



Damage to Central Office or Other Telephone Plant. — The dielectric 

 strength of the telephone plant is adequate for the voltages used in 

 communication service, with appropriate factors of safety, but higher 

 voltages may sometimes, notwithstanding the protective devices, cause 

 dielectric failure, thus damaging the plant, particularly cables and wir- 

 ing or apparatus in telephone offices. 



Electric Shock. — Telephone linemen in the course of their work 

 upon wires at relatively close spacing, cannot avoid getting in contact 

 with the wires and if the wires were subject to sufficient induced voltage, 

 the men would be liable to receive electric shocks. On severely ex- 

 posed lines such voltages are liable to occur at any time, suddenly and 

 without warning. Electric shock might either inj ure a lineman directly 

 or startle him and cause him to lose his hold and fall from the pole. 

 Voltage to ground due to induction appears not only within the ex- 

 posed section of line but considerably beyond. A similar, and in some 

 respects worse, condition may exist with respect to employees working 

 on cable circuits which are either exposed or directly connected to 

 exposed circuits. In cables the wires on which the foreign voltage 

 appears are very close to the grounded metal sheath and usually also to 

 other wires at approximately earth potential, as well as to the earth 

 itself. This problem has become more difficult with the rapid growth 

 of the telephone and electric power systems and is engaging the sub- 

 committee's serious attention. 



Acoustic Shock. — Acoustic shocks are liable to occur with the break- 

 down of telephone protector discharge gaps, which temporarily un- 

 balances the circuit and causes a sudden and abnormally large current 

 in the receivers. This current gives rise to sudden and severe flexures 

 of the receiver diaphram, which produce loud sharp noises in the ear of 

 a person using the receiver. Telephone operators, due to the nature 

 of their work, are particularly liable to acoustic shocks, the effects of 

 which range from minor reactions to severe general disturbances of the 

 nervous system which may be painful and of long duration. In 

 addition, if danger of severe shocks exists, the operating force may be- 

 come fearful and the impaired morale seriously affect the service. 



Types of Protective Measures 



The foregoing effects of induction from paralleling power lines may 

 be reduced by: (1) measures in the power system to limit the influence, 

 (2) measures in the communication system to limit the susceptiveness 

 and (3) coordinated location of lines or other means to reduce the coup- 

 ling. As a solution in a specific situation, one measure may be suffi- 

 cient or two or more measures may be required, depending on the con- 



