THE L3 SYSTEM — DESIGN 829 



the cable at the main stations is about 2,000 volts rms from center 

 conductor to ground. This potential diminishes about 100 volts per 

 repeater section in going out from the power feed point or as the power 

 section is shortened. (The maximum potential applied to the cable in LI 

 systems is about 800 volts rms between center conductor and ground). 

 Extensive tests on the installed cable showed that corona develops in 

 the cable at potentials varying in a random fashion between 1,200 and 

 1,600 volts rms. This would allow power feed points to be placed at a 

 maximum spacing of about 100 miles. By replacing the nitrogen, with 

 which the cables are normally filled, with a large molecule gas, sulfur- 

 hexafluoride (SFe), the corona potential of the cable is increased well 

 above the maximum operating potential. Only the cable sections ex- 

 posed to potentials greater than 1,200 volts will be filled with the new 

 gas. Elaborate and thorough tests have demonstrated that no deteriora- 

 tion of the cables will result from the use of this gas. Some additional 

 precautions are required in entering manholes and using high tempera- 

 ture torches for soldering when the SFe gas might be present. 



3.7 ALARM EQUIPMENT 



Since the auxiliary repeaters and certain of the main repeaters are 

 unattended it is necessary that arrangements be provided to indicate 

 at the attended stations when some piece of equipment fails to perform 

 satisfactorily. 



Auxiliary repeaters using pilot regulators are equipped with micro- 

 ammeter relays which monitor the operation of the regulator contin- 

 uously. These relays provide an indication of the operation of the regu- 

 lator and the power of the 7,266-kc pilot at the output of the repeater. 

 When conditions change from the nominal by a specified amount the 

 relay contacts close and are locked magnetically. This bridges an alarm 

 pair in the cable and operates an alarm at the nearest attended repeater. 

 By means of Wheatstone bridge measurements from this repeater over 

 the same alarm pair, the repeater in trouble can be located and a main- 

 tenance crew dispatched to make the necessary equipment replacements. 

 The relays can also be reset over the same alarm pair to aid in the 

 location process or to clear alarms which were initiated at unaffected 

 repeaters by deviations in the pilot due to troubles at preceding repeaters. 



At main repeaters, microammeter relays are provided on all six pilots 

 used to control the equalization of the system. Deviations in these pilots 

 operate the automatic switching equipment and initiate the usual office 

 alarms. Alarms are also provided to indicate fuse operation, transfers 



