INSULATION TEST EQUIPMENT FOR LOCAL CROSSBAR SYSTEMS 635 



Table I — Crossbar Line-Insulation Tests 



* The equipment is arranged so that by cross connection changes the bands 

 of resistances in each range may be halved or doubled, if necessary, to meet 

 local conditions. 



in parallel. If the line-insulation resistance is more than the top limit 

 of the test range, the test equipment proceeds to the next Une. If, 

 however, the resistance measured is less than the top limit the test 

 equipment immediately retests the line with the central office ground 

 removed as shown in Fig. 5 (b). This re-test measures the leakage from 

 ring to tip. If leakage from ring to tip only is indicated the drop wire is 

 probably the cause. 



The No. 1 test is run under wet weather conditions to detect all lines 

 which are below the top limit of this test (normally 160,000 ohms). 

 If the maintenance force keeps these cases cleared out by running tests 

 during every rain the wire plant can be kept in good condition. During 

 long periods of dry weather all line insulation test indications pre- 

 viously recorded may have been investigated, then if a Ught rain occurs 

 it may be desirable to run the No. 2 test to provide a satisfactory number 

 of failure indications for subsequent investigation. The No. 2 test can 

 also be used to detect leakages in inside wiring which occur frequently 

 during the summer when houses are unheated and the inside humidity 



r 



TIP 



H'l 



IJ-400/XF 



LINE 



INSULATION 



TEST 



CIRCUIT 



CONTACT OPENED 

 DURING TEST 



(a) 



FIRST TEST CONDITION MEASURES 

 COMBINED SHORT AND RING TO 

 GROUND RESISTANCE 



(b) 



RETEST AFTER FAILURE ON 

 COMBINED TEST MEASURES 

 SHORT ONLY 



Fig. 5 — Short and ring ground test. 



