INSULATION TEST EQUIPMENT FOR LOCAL CROSSBAR SYSTEMS 645 



which responds to the lowest insulation resistance value. Test resistors 

 of 320,000 ohms and 640,000 ohms are then substituted and the grid 

 bias of the other thyratrons adjusted so that they just conduct on the 

 proper value of resistance. This procedure cahbrates the device for one 

 range, using a shunt and series resistor which corresponds to this range. 

 Facihties are provided for substituting suitable test resistors for check- 

 ing the cahbration of other ranges, with the shunt and series resistors 

 for the range connected to the magnettor. 



EQUIPMENT FEATURES 



The apparatus components of the test control circuit and the line 

 insulation test circuit are assembled and wired in an 11 -foot bay, 23 

 inches wide. Fig. 12 is a front view of the test frame. Fig. 13 is an enlarged 

 view of the line insulation test circuit equipment and the control panel 

 which includes features for checking the accuracy of the test circuit 

 and for calibrating it. When the teletype method of recording failures 



MAGNETTOR 



Fig. 13 — Front view of control and test panel (at bottom) and line insula- 

 tion test unit (center) for No. 5 crossbar. 



