622 ELECTRICAL EQUIPMENT 



tials to ground. On the other hand, where there are induced 

 potentials in the telephone line either between lines or from lines 

 to ground, that is either due to electro-magnetic or electrostatic 

 induction, a multi-gap arrester, using knurled cylinders for the 

 electrodes, is used between lines and ground. This is to avoid 

 continual grounding of the telephone lines through the low break- 

 down path of the vacuum arrester due to the induced potential to 

 ground which may be of quite high value. The vacuum gap is 

 put across the telephone lines where the induced potentials can 

 be controlled by careful transposition. Here the vacuum arrester 

 holds the voltage across the telephone apparatus to a value below 

 its breakdown. 



Where there is any possibility of induction troubles and this 

 may occur up to one-quarter or one-half mile away from the 

 power circuit under abnormal conditions, the telephone line insu- 

 lating transformer is of prime importance. This provides an 

 insulation barrier of 25,000 volts test between the telephone 

 instruments and the lines. On the line side of these transformers, 

 which should be used at every telephone station, are installed 

 the combined multi-gap and vacuum-gap unit which hold the 

 voltages to ground and between lines to moderate values. In 

 series with this in the telephone lines are fused switches for cut- 

 ting off the apparatus in case of heavy continued discharges 

 through the gaps, caused by induced potentials or crosses. They 

 can also be operated as straight switches to cut off the station in 

 any emergency. 



As a further protection in case of induced potentials particu- 

 larly for potentials to earth, the drainage coil or bleeding coil can 

 be used. These should be few in number, usually two, as too 

 many will seriously affect the operation of the telephone circuits. 

 These coils give a high impedance path across the telephone line 

 thus shunting the high-frequency talking currents, but provide at 

 the same time a low impedance path for the flow of equal currents 

 from both lines to ground at the center of the coil. These coils, 

 where used, should be protected by cut-outs to guard against 

 burn-out from heavy currents under abnormal conditions on the 

 power line. 



With the addition of possible crosses with the power line the 

 only additional feature to the above scheme is the double-pole 

 horn gap which serves as an auxiliary protection to the telephone 



