POWER-FEED SYSTEM — NEWFOUNDLAND-NOVA SCOTIA LINK 287 



Alarms. 



The equipment trips and gives both aural and visual alarms for 

 -|-20 per cent current, +20 per cent full link voltage and for the failure 

 of certain auxiliary supplies that would damage the equipment. 



Aural and visual alarms are provided for ±1 per cent current and 

 ±3 per cent voltage and for equipment changes from slave to master 

 characteristic or vice versa. Visual indication is given if the ac supply 

 fails. 



Reliahilitij. 



Where possible, only components of proven integrity have been used. 

 High-power thermionic tubes have been excluded and the tube types 

 employed have been specially selected for long life. Electrolytic capaci- 

 tors have been excluded from all except one position, and in this case 

 the component has been divided into six units in parallel, the failure of 

 all but one of these units causing only a slight increase in the output 

 ripple. 



Particular attention has been given to the continuity of the output 

 circuit. A failure of a power equipment for any reason other than an open- 

 circuit in the output will not interrupt traffic, the link changing to 

 single-end feeding from the far end. A disconnection anywhere in the 

 dc feed path will disconnect all power from the line. Relatively short- 

 lived components in this part of the circuit have either been duplicated 

 in parallel or shunted by resistors capable of carrying the full line cur- 

 rent. 



Other facilities. 



Each equipment has facilities for checking its overall performance. A 

 \'ariable-ratio transformer can be introduced at SKI (Fig. 4) and with 

 the 4-position dummy load referred to earlier, the regulation against 

 alternating input voltage and output load can be measured. 



Provision is made for checking all the alarms, and the current can be 

 measured at strategic points in the control circuit either when the equip- 

 ment is normal or with the loop feedback disconnected. 



Separate large-size meters are provided for measuring the output volt- 

 age and current to an accuracy of ±1 per cent. A more accurate meas- 

 urement of current is obtained from potentiometric measurements made 

 across a standard resistor connected in series with the output. 



