640 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



When a line fault is discovered, the control circuit makes use of either 

 of two types of mechanism to record (a) the location of the line on the line 

 link switches, (b) the type of test being made, and (c) a rough approxi- 

 mation of the insulation resistance. Since a substantial time is required 

 to make this record, the line link frame is restored to service during this 

 interval, to reduce interference with service. One of the above two 

 devices is the trouble recorder, used only in the No. 5 crossbar offices. 

 The control circuit connects itself to this machine in much the same way 

 that a marker does, when it needs to record a trouble. Having made this 

 connection, a card is perforated showing the Une location and other data 

 pertinent to the trouble indicated. The other device consists of teletype- 

 writer equipment at the central office which transmits the required data 

 to a teletypewriter page printer located at the local test center. The 

 equipment at several central offices has common use of the same page 

 printer at the test center and several test circuits in one building use 

 the same teletypewriter transmitting equipment. This second recording 

 arrangement is the more convenient of the two, since it produces the 

 record at the place from which the activities of the outside plant repair 

 force are directed. This arrangement is available for both No. 1 and No. 5 

 crossbar offices. A typical teletype record is shown in Fig. 9. 



LINE-INSULATION TEST CIRCUIT 



The insulation resistance measuring device is required to respond, 

 not only to the very low current (five micro-amperes) obtainable with 

 insulation resistance up to ten megohms, but it must also give good 

 discrimination between resistance values in the order of 20,000 ohms. 

 This is accomplished by desensitizing the measuring device by means 

 of series and shunt resistors when a test using less than the maximum 

 range is desired. A second requirement is that the measuring device be 

 low in resistance so that the time constant of this resistance in combina- 

 tion with the line and filter capacity will be low enough to attain high 

 testing speeds. 



These leakage current amplitudes are so small that amphfication is 

 required in order to actuate the recording and control mechanisms of 

 the measuring system. These small direct currents could be amplified 

 by means of a dc amplifier. However, since it is easier to design and 

 construct an ac amplifier of suitable stability, it is desirable to use a 

 measuring device which has an alternating voltage output which varies 

 with the direct current input. 



A type of magnetic modulator, called a magnettor, which has these 



