INSULATION TEST EQUIPMENT FOR LOCAL CROSSBAR SYSTEMS 643 



a resultant output which contains even harmonics of the input voltage. 

 The second harmonic is selected by means of a filter for use as an in- 

 dicator of the presence of leakage current. 



The second harmonic is amplified by a three stage negative feedback 

 vacuum tube amplifier, whose output is applied to the grids of three 

 thyratrons. Adjustments are made, as described in the following, so 

 that one of the thyratrons conducts when the insulation resistance is 

 less than the value selected for the test, an additional thyratron conducts 

 if the insulation resistance is one-half of this value or less, and all three 

 conduct if the resistance is one-fourth or less of the selected value. Three 

 relays, whose windings are connected in the anode circuits of the thyra- 

 trons, are actuated when the associated thyratrons conduct and cause a 

 record of the trouble to be made, or cause the control circuit to select 

 another line if the insulation resistance is above the range of interest. 



In order to calibrate the detecting circuit, a test resistor of 160,000 

 ohms is connected to the magnettor by operation of a key. The amplifier 

 gain is then adjusted so as to just cause conduction of the thyratron 



FLUX 



RESULTANT OUTPUT 



OF b & b' 

 EVEN HARMONICS ONLY 



Fig, 11 — Graphical representation of voltage and flux in the magnettor. 



