620 ELECTRICAL EQUIPMENT 



is of a permanent nature, such as caused by the puncture of an 

 insulator, the switch will immediately close a second time and be 

 locked in the closed position until opened by hand after the ground 

 has been removed. Should, however, the switch stay open for a 

 fraction of a second after the first stroke, the " second stroke 

 device " would become inoperative, as it will only come into action 

 when the switch starts to close the second time immediately after 

 the first time. To prevent the possible operation of the suppres- 

 sor in cases of short-circuits, an overload relay may be provided 

 which opens the control circuit of the suppressor. 



Fig. 395 shows an elementary diagram of an arcing ground 

 suppressor and Fig. 396 the phase selecting relay for the same. 



Short-circuit Suppressor. This device operates on the same 

 principle as the arcing ground suppressor, but it is intended for use 

 on grounded systems where any arc to ground would form a short- 

 circuit. The suppressor is connected between each line wire and 

 ground, and consists of a fuse in series with a gap which is instantly 

 closed when a short-circuit, caused by an arc-over or ground, 

 occurs. The arc is thus shunted until the fuse blows which gives 

 sufficient tune to allow the arc to extinguish itself. For a single- 

 phase short-circuit two of the fuses will blow and for a three-phase 

 short-circuit all three fuses. If the trouble does not clear itself 

 or if there is a dead ground, of course, the main oil circuit breaker 

 will finally disconnect the entire circuit as usual. 



Protection of Telephone Lines. Telephone lines paralleling 

 high-tension power transmission lines are subjected to influences 

 which may under certain conditions interfere with the proper 

 transmission of speech. This interfering influence is in all cases 

 due to the static induction from the high-tension transmission 

 line. Under normal operating conditions, that is, with fairly well- 

 balanced three-phase circuits, this influence will be slight, but with 

 abnormal operating conditions on the transmission line the effect 

 created on a telephone line may increase to such an extent as to 

 become destructive. In addition to these influences the tele- 

 phone line is subjected to disturbances occasioned by lightning 

 discharges, which, however, are very similar in character to the 

 effects created by abnormal conditions on the transmission line, 

 that is, during the time of switching with unbalanced phases or 

 arcing grounds, etc. 



Under normal operating conditions the effect of the static 



