634 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



UJ Z 



p<0.2 



Fig. 4 

 used. 



INSULATION RESISTANCE IN OHMS 



— Wet weather tests — automatic line insulation testing equipment 



CROSSBAR TEST EQUIPMENT — TYPES OF TESTS AND SENSITIVITIES 



The crossbar line-insulation test equipment is arranged to make three 

 different kinds of tests from the standpoint of the way in which the 

 resistance measuring circuit (line-insulation test circuit) is connected 

 to the line and to battery or ground. Each kind of test may be made in 

 three different resistance ranges, hence there are nine different tests. 

 When the test equipment is performing one of these tests a record is 

 made of all lines which have an insulation resistance below the top limit 

 of the particular test. Each test range is divided into three bands of 

 resistance and the record indicates whether the insulation resistance 

 lies within the first quarter (low band), the second quarter (medium 

 band) or the upper half (high band) so that preference can be given to 

 the worst cases in clearing the trouble. The test numbers, ranges and 

 resistance bands are shown in the Table I. 



SHORT AND RING GROUND TEST 



The first kind of test is called "short and ring ground test." This 

 test measures the leak in the way in which it affects the line circuits, 

 pulsing circuits and supervisory circuits of the central office switching 

 system. The hne-insulation test circuit (LIT circuit) which is described 

 later on is connected to central office battery potential and to the fine 

 in the manner shown in Fig. 5 (a). As indicated in this illustration, 

 leakage resistance from ring to tip and from ring to ground are measured 



