140 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



tary high potentials increase the chances of corona formation and insu- 

 lation breakdown . 



4. To protect the cable repeaters from the excessive potentials or cur- 

 rents to which they might be subjected after an accidental open or short 

 circuit in the cable. 



5. To compensate for earth potentials up to 1,000 volts, of either po- 

 larity, that may develop between the grounds at Oban and Clarenville 

 during the magnetic storms accompanying the appearance of sun spots 

 and the aurora borealis. 



6. To provide adequate alarms and automatic safety features to en- 

 sure safe current and voltage conditions to both the cable and the 

 operating personnel. 



DESIGN REQUIREMENTS 



Reliable Cable Power 



The first basic problem of design was to select a reliable source of dc 

 power for energizing the cable repeaters. Although a string of batteries, 

 on continuous charge, is perhaps the most dependable source of direct 

 current, such an arrangement is not attractive here. A complex set of 

 high-potential switches would be required for removing sections of bat- 

 teries for maintenance and replacement purposes. Protection of the 

 repeater tubes from damage during a cable short circuit would be dif- 

 ficult. Facilities to accommodate changing earth potentials would be 

 cumbersome. Furthermore, the problem of hazards to personnel w^ould 

 be serious. 



The use of commercial ac power with transformers and rectifiers to 

 convert to high potential dc would expose the cable to power inter- 

 rviptions even with a standby diesel-driven alternator, because of the 

 time required to get the engine started. A diesel plant could be operated 

 on a continuous basis, but this prime power source would also present a 

 considerable failure hazard even with the best of maintenance care. 

 The two-motor alternator set, used so successfully in the Bell System's 

 type "L" carrier telephone system, was adopted as representing the most 

 reliable continuous power source available. This set normally operates 

 on commercial ac power, but when this fails, the directly-coupled 

 battery-operated dc motor quickly and automatically takes over the 

 drive from the induction motor, to prevent interruption of the alter- 

 nator output. Here the storage battery is still the foundation for con- 

 tinuity, but at a more reasonable voltage. 



As described later, the possibility of a system outage resulting from fail- 



