52 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



larity. Consequently, it may either aid or oppose the driving potential 

 applied to a particular cable. The power circuit regulation is such that 

 the presence of an aiding potential will be completely compensated, 

 while opposing potentials will be compensated only to the degree possible 

 with the maximum of 2,500 volts applied by the terminal plants. Beyond 

 this point, the line current will be allowed to drop, so as to avoid exces- 

 sive potential across the power filter capacitors in the shore end repeaters. 



Line faults caused by failures or misoperation of the power supplies 

 must be carefully guarded against. Such failures might result in surges 

 on the line, or excessive voltage or current. Protection against surges 

 takes the form of retardation coils in the terminal power separation fil- 

 ters, which limit the rate of current change to tolerable values. 



Failure of the capacitors in these filters could also create dangerous 

 surges. The protection here takes the form of a large voltage design mar- 

 gin. 



Faults in the HF Une of importance from the power feed standpoint 

 are short circuits to ground, and opens. The former tend to result in 

 excessive currents, the latter, excessive voltages. Very fast acting pro- 

 tection in the terminal power bays is required to cope with these, and 

 series electron tube regulators provide the means. 



Terminal Plan 



The plan of design of transmission terminal equipment, shown in Fig. 

 2, was based on the following objectives: 



(a) Use of standard, or modified standard equipment as far as possible. 



(b) To facilitate supply and maintenance of standard equipment, 

 use of Bell System equipment in North America and Post Office equip- 

 ment in the United Kingdom. 



(c) Provision of full duplicate equipment and means for quick shift 

 between regular and alternate. 



(d) Provision of special equipment to fit Canadian requirements. 



(e) Provision for ample order-wire (speaker and printer) equipment, 

 partly because there is no alternate undersea route. 



(f) Provision of no automatic loss regulation, because the loss changes 

 are so slow; provision of three link pilot frequencies as a basis for manual 

 regulation. 



Much of the equipment is standard. This includes group modems, 

 group connectors, pilot supply, frequency-shift telegraph equipment for 

 printer circuits, etc. Supergroup modulators are modified standard co- 

 axial carrier equipment. The channel modems for speaker and printer 

 circuits are a combination of Type-C open-wire carrier active equipment. 



