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THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



"range" is chosen, the insulation fault detector will report all lines having 

 lower insulation resistance than the chosen value, and as already stated 

 it will further identify those lines having insulation resistance lower than 

 one-half and one-quarter of this resistance. 



This test circuit is connected to each of the lines in rapid succession 

 by the line-insulation test control circuit. As shown in the block diagram 

 of Fig. 8, the control circuit appropriates the control and testing wires 

 between one of the markers (usually marker No. 2) and each of the line 

 link frames. The arrangement shown is for a No. 5 crossbar office, but 

 the No. 1 crossbar arrangement is similar in principle. By means of 

 these appropriated connections the control circuit is able to select the 



LINE CONNECTED 

 I FOR TEST 



LINE INSULATION 



TO MARKERS 



Fig. 8 — Block diagram of line insulation 



LOCAL TEST CENTER 



test control circuit connections. 



^ 



