614 ELECTRICAL EQUIPMENT 



It was previously stated that it is necessary to charge the cells 

 from time to time to prevent the dissolution and consequent rush 

 of dynamic current which would otherwise occur when the arrester 

 discharges. The charging operation consists simply in simul- 

 taneously closing the three horn gaps and holding them closed for a 

 period of five seconds, the full line potential thus applied across 

 the line cells causing a small charging current to flow and reform 

 the films to their normal condition. Thereafter, with the horn 

 gaps open in their normal position, the position of the transfer 

 device is reversed and the horn gaps again closed for five seconds 

 and returned to normal position. The complete charging opera- 

 tion takes but a few seconds and should be performed daily or 

 even oftener should conditions so demand. 



Most arresters are now provided with charging resistances so 

 as to minimize the oscillations set up by the charging and their 

 harmful effects on nearby telephone lines, at the same time also 

 greatly increasing the life of the cones and the electrolytes. An 

 auxiliary horn gap, fitted with a charging contact, and in series 

 with the resistance is installed above and in parallel with the main 

 gap (see Fig. 391). At the time of charging the contact bridges 

 the auxiliary gap and charges the cells through the resistance, 

 the current flow being limited to a moderate value. 



The charging current taken by an aluminum cell arrester is 

 the best means of indicating its condition, and the value may 

 readily be ascertained by a device known as a charging-current 

 indicator. An arrester in good condition has a charging current 

 of approximately 0.25 ampere on 25-cycle circuits, 0.30 ampere on 

 40-cycle, and 0.40 ampere on 60-cycle circuits. Should these 

 values be doubled, the arrester must be charged more frequently 

 and the current carefully measured until it comes down to normal. 

 It is only when this additional charging fails to reduce the charging 

 current that an inspection of the cells is necessary. The essential 

 parts of the charging-current indicator are an ammeter mounted 

 on a specially constructed switch stick and a set of jacks. These 

 jacks are so connected in the arrester circuit that when the amme- 

 ter switch stick is inserted in them and the horn gaps short-cir- 

 cuited, the charging current flows through the meter. 



Most modern arresters have their horn gaps provided with 

 spheres which greatly decrease the dielectric spark lag, especially 

 for voltages with steep wave fronts. The arrangement shown 



