OVER-VOLTAGE PROTECTION 



617 



assembled with a clock-operated drum in such a manner that a 

 continuous record is obtained, showing all discharges by means of 

 punctures in a moving roll of paper. This paper passes through 

 the gaps at a rate of about 1 inch per hour, which gives an accurate 

 record of the time and duration of each discharge. Besides being 

 valuable in recording discharges due to abnormal voltages on a 

 system, the discharge recorder is of value in indicating and 

 recording the daily charging of the lightning arresters. With such 

 a recorder it can be told whether the arresters are or are not 

 being properly charged by the station operator; and besides the 

 puncture gives some indication of the condition of the arrester. 



Except in underground cable systems, choke coils should 

 always be installed in the circuit between the lightning arrester 

 and the apparatus to be protected, thus holding back incoming 



FIG. 393. Strain-type Suspension Choke Coil for Station or Outdoor Service. 



impulse from the latter until the lightning arrester discharges to 

 earth. 



Choke coils are built either according to a stationary or sus- 

 pension design. Of the former, the hour-glass type (Fig. 392) 

 is the most satisfactory, in that it avoids the necessity of supports 

 between the turns, so that high-frequency disturbances in ground 

 are prevented from passing across the turns. The air insulation 

 between the turns is also preferable, so that in case of impulses 

 with extremely steep wave fronts, causing arcing between turns, 

 they will re-insulate themselves. 



Suspension choke coils (Fig. 393) can usually be incorporated 

 with the other high-tension wiring, thus saving a number of ex- 

 pensive insulators, for ..which reason they in many instances 

 may prove preferable. 



Fig. 394 shows a thunderstorm map for the years 1904-1913, 

 as prepared by the U. S. Weather Bureau. 



