POWER-FEED SYSTEM — NEWFOUNDLAND-NOVA SCOTIA LINK 285 



Group (a) is treated as normal terminal transmission equipment and 

 is not interlocked. Groups (6) and (c), the power-change-over and cable- 

 terminating boxes respectively, both require that the local power equip- 

 ments are switched off before they can be made safe. The external panel 

 covers over both boxes have links disconnecting the ac supplies to both of 

 the local power equipments when the covers are removed (LK3-LK6). 

 This will not interrupt traffic, since single-end feeding will continue from 

 the distant station. 



The doorknob switch of the power-change-over box automatically 

 connects the live lead to ground (at the point B) when the door is opened. 

 This will not interrupt traffic since the impedance of L2 is high at carrier 

 frequencies. Changing over the power equipments and grounding arrange- 

 ments can therefore be performed without interrupting traffic. 



The doorknob switch of the cable-terminating box automatically 

 connects the center conductor of the cable to ground (at the point A) 

 when the door is opened. Traffic will therefore be interruped if main- 

 tenance work is necessary within this box. 



Access is gained to a power equipment by opening one or more of 

 four doors. Each of the doorknob switches disconnects the ac supply 

 (SWA) and short-circuits the output of the equipment (SWC) . 



It will be appreciated that if the correct procedure is adopted the local 

 power equipments will be switched off by SWB after the master/master 

 shut-down procedure (described in section on The Master-Slave Systems 

 of Operations) has been carried out and not by removing the panel 

 covers or opening the doors. 



Electrical Details of Power Equipment 

 Control Method. 



Fig. 5 is a functional simpHfied circuit diagram in which TRl, MRl 

 and CI represent a conventional unregulated power unit. 



The control circuit compares a signal proportional to the output cur- 

 rent, Fo , with a stable reference signal, Vr , the two signals being ap- 

 plied to the cathode and grid, respectively, of V5; the difference between 

 them is amplified and used to adjuste the voltage fed to the rectifier 

 circuit, MRl, to maintain the output current constant. 



Ignoring at this stage the auxiliaiy coil 1-2 of the magnetically con- 

 trolled diode V3, the output current flows through the coil 3-4 and con- 

 trols the voltage across R4 and hence, via the cathode-follower ^^4, the 

 potential Fo on the cathode of V5. Vr is a function of the output of a 

 conventional electronic constant- voltage power unit (reference power 



