INSULATION TEST EQUIPMENT FOR LOCAL CROSSBAR SYSTEMS 639 



lines and if the selected line is not busy its tip and ring conductors are 

 connected to the insulation resistance measuring circuit (line-insulation 

 test circuit). To establish this connection, the control circuit chooses an 

 idle line link to connect the line vertical to the no test vertical and the 

 test path is completed through the no test connector to the line-insulation 

 test frame. 



The operation of the control circuit may be started in the central 

 office by operation of keys at the test frame, but it is more commonly 

 started by remote control from the local test center. This permits in- 

 sulation tests to be made even though the central office is unattended. 

 When the control circuit is started, it must also be directed to make one 

 of the three types of tests, and to choose one of the three test sensitivities 

 for each of these types as shown in Table I. This is accomphshed by 

 operating one of nine keys at the test frame or by selecting the test trunk 

 at the test center and dialing one of nine codes and then operating a key 

 at the test desk. Either action chooses the type of test and the sensitivity, 

 and causes the test circuit to start. A tenth key or a tenth code is used to 

 stop the test before the end of a complete cycle, when this is required, 

 so that the type of test or the sensitivity may be changed. 



A pre-arranged regular pattern is followed in testing the lines . A hne 

 link frame is selected, a particular five lines are tested, and then the 

 frame is released, and the next frame is selected, and another five lines 

 are tested, and so on. Lines found busy, dial PBX trunks and line link 

 frame terminations of intertoll trunks and similar circuits which would 

 cause false operation of the test circuit are passed by without test. The 

 line link frames are selected in order, beginning with the lowest numbered 

 frames and continuing to the highest. When one cycle through the 

 frames has been completed, another cycle will be started. However, on 

 the next cycle, different groups of five lines will be tested. On the first 

 cycle, the five lines in vertical group 0, horizontal group will be tested, 

 on the second cycle, the five lines of vertical group 1, horizontal group 

 will be tested, in each line link frame and so on until all lines in horizontal 

 group of all line link frames have been tested. On subsequent cycles 

 the lines of other horizontal groups are tested in regular order. 



Testing only five lines for each selection of a line link frame minimizes 

 interference with traffic. Since one to two seconds is required for testing 

 five lines, there will be only a slight delay to calls which require access to 

 the frame. In addition, a heavy traffic load will cause the control circuit 

 to stop insulation testing and restore to service the marker whose cabHng 

 it has been using. When the traffic has been reduced sufficiently, the 

 test will be restarted automatically. 



