150 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



which the voltage holds within 2,600 ± 200 volts as the turndown relays 

 operate and release to maintain the ceiling voltage. A fourth magnetic- 

 amplifier voltage detecting relay provides an alarm for ±5 per cent ex- 

 cursions in cable voltage from the normal value. Other alarms are pro- 

 vided to indicate low output in either of the two parallel regulating bays, 

 relay troubles, loss of magnetic amplifier ac control voltage, and fuse 

 failures. 



To limit the rate of change in the cable current under short circuit 

 conditions and to reduce the rise in voltage at the repeaters on open cir- 

 cuit failure, an inductance of about 36 henries is connected in series 

 with the cable circuit, and physically close to the cable termination, so 

 that any failure in the power supply would have the advantage of this 

 surge-limiting element. 



Metering 



Metering of the cable current is a very important part of the power 

 plant design. Not only are the cable current ammeters needed to set the 

 value of current desired, but their ability to indicate absolute current 

 values assists in obtaining stable regulation between the two ends of the 

 cable. The meters provided for this purpose are suppressed-zero, mag- 

 netically and statically shielded 150-300 milliampere, large scale amme- 

 ters, with 0.5 per cent accuracy. One of these meters is connected in the 

 ungrounded side and another in the grounded side of the cable supply 

 circuit to provide an accuracy check and to indicate any ground leakage 

 current in the supply circuit. They are connected to highly accurate 

 1-ohm four-terminal shielded resistors acting as shunts in the cable current 

 circuits with their shunt leads arranged for switching to a calibration 

 box for checking accuracy and for adjustment. This box employs a 

 Weston laboratory standard cell, essentially a single-point potentiometer 

 with the usual galvanometer, acting as a calibration standard at the 225- 

 milliampere point. Meters calibrated at Oban for 225 milliamperes were 

 expected to be within 0.2 per cent or 0.5 milliampere of those similarly 

 calibrated at Clarenville, and at present are within about 0.2 milliampere. 



The cable current is also indicated by a recording ammeter. Meters in 

 each regulating bay indicate the division of current between paralleled 

 regulators. These meters have only 1 per cent accuracy but are satis- 

 factory for adjusting load balance between parallel bays and also are 

 used in turnup of power on a particular bay. 



Cable voltage is read on a large scale voltmeter reading 0-3,000 volts 

 and having ±0.25 per cent accuracy. Since the accuracy is not critical. 



