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THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1954 



will operate. Any group relay operated will operate two guard time 

 relays gti and gt2 which will reoperate the end of cycle relays and 

 enable two timing circuits. The end of cycle relays operated release 

 the group restore (gr) and all busy (ab) relays restoring control of 

 the idle indicating lamps at all originating switchboards to the associated 

 trunks. Timing is obtained by means of cold cathode gas tubes in com- 

 bination with resistance-capacitance circuits. The control gap of the 

 gas tube (terminals 1 and 4) is connected across the capacitor which is 

 normally short circuited thru 510 ohms. To start timing, the gt- 

 relay remove the short circuit around the capacitor and connects ground 

 thru the tf- relay in series with the capacitor and the 0.556 megohm 

 resistor. The capacitor charges from the 130-volt battery. When the 

 voltage across the capacitor reaches the breakdown potential of the gas 

 tube the latter fires and operates the tf relay. The time to fire the tube 

 is determined from the following equation : 



T = 2.3i?CLog^-^, 



where E is the voltage of the battery, V is the voltage at which the 



OT-9 GR 



0T-- 



— I— ♦ 



TO CONTACTS 



OF GP- 



RELAYS OF 



OTHER GROUPS 



^^-AAAH|lh| [-" 



Fig. 11 — End of cycle and guard timing. 



