1188 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



Two commutator segments are required to make a dielectric strength 

 test. These segments are known as "initial charge" and "test". After 

 the unit under test has been charged at the test voltage for three sec- 

 onds on the "initial charge" segment, it passes to the "test" segment in' 

 which the unit is again connected to the test voltage through relay K2,' 

 current limiting and calibrating resistors R3 and R4 and the contacts; 

 on the preset terminal selecting relays KIO. ■. 



One of the two conditions (under heading A and B below) may be^ 

 encountered in making this test and the circuit operation for each will 

 be discussed separately. 



A. Circuit Operation for AcceptaUe Product. An acceptable product 

 retains the charge received on the "initial charge" segments and when! 

 this unit reaches the "test" segment, no further charging current of a- 

 magnitude great enough to operate relay K2 will flow through the unit. 

 Two seconds after the unit under test has been connected to the test 

 segment, a cammed timing switch S4 closes to operate discharge relay K9 

 to discharge the unit under test to ground through R7. The "self-check-" 

 ing" feature mentioned earlier in this article under "Design Considera- : 

 tions" functions as follows: After the unit under test has been on the 

 "test" segment for approximately 2% seconds, a cammed timing switch ; 

 (not shown) closes the memory test relay K6 which in turn closes the \ 

 "go" calibration indicator relay K5 and the "A" contacts on this relay; 

 grounds the high voltage test circuit through resistor R6. This resistor | 

 is of such a value as to permit sufficient current to operate relay K2. i 

 The contacts on relay K2 are not adequate to carry much current, so ' 

 an auxiliary relay K3 is closed through contacts "A" on relay K2. 

 Contacts "B" on relay K3 closes the indicator light circuit II and oper- 

 ates relay Kll and the acceptance solenoid K7. Contacts "A" on the 

 same relay lock relay Kll. The circuit is reset for the next vmit to be 

 tested by momentarily opening the reset cammed switch S2. Relay Kll 

 was added to the circuit to eliminate a "sneak circuit" that occurred 

 occasionally following the reset when relay K5 opened faster than relay 

 K3. This would result in relay K4 operating to reject the next unit tested. 

 Relay Kl is controlled by switch SI operated by the manual control 

 Tl on the test voltage power supply. The function of this relay is to add 

 calibrating resistor R4 to the test circuit for voltages above 1,000 volts. 

 Resistor R5, relay K8, and switch S3 control the manual calibrating 

 "No Go" circuit for breakdown indicating relay K2. 



B. Circuit Operation for Defective Product. Defective product will not 

 retain the charge it received on the "initial charge" segment and when 

 it reaches the "test" segment, current will flow through the breakdown 



