632 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



patching work. Such a test center would handle at the current trouble 

 rate about 120 reports on subscriber lines per day. If the outside plant 

 is in a condition represented by the lower curve of Fig. 3 where 0.2 per 

 cent of the lines are below 10,000 ohm resistance, this represents 100 

 additional stations in the region where service reactions may be expected 

 from wet weather conditions. Consequently where these plant conditions 

 obtain, there is usually a noticeable increase in subscriber's reports in 

 wet weather and the repair load rises sharply. Without line insulation 

 test equipment to test the lines rapidly while the low insulation re- 

 sistance obtains, there is no way to pick out the worst lines from an 

 insulation resistance standpoint after the weather has cleared. Visual 

 inspections are costly and superficial indications do not always give 

 reliable evidence of low insulation resistance. Consequently the large 

 percentage of repairs to correct insulation defects are made as a result 

 of subscriber reports and only a small per cent by routine preventive 

 maintenance where rapid line-insulation testing equipment is not used. 



MAINTENANCE WITH RAPID LINE INSULATION TESTING 



The only experience to date with the fully automatic line insulation 

 test equipment for crossbar offices is in the Media, Pa., No. 5 crossbar 

 office. This test equipment has been in use for about one year and low 

 insulation resistance cases recorded on each test have been investigated 

 and repairs made. The condition of the outside plant in the exchange 

 area is represented in Fig. 4. The curves in this figure are based upon 

 the results of tests of the 4200 working lines in the summer of 1952 

 under very wet conditions of the outside plant. 



The small percentage of lines below 40,000 ohms indicates that the 

 poor insulation cases are being corrected well before reaching the stage 

 where the customers' service would be affected. This is done with a 

 minimum of maintenance effort as the test equipment spots the line 

 automatically. 



DETECTING SHEATH BREAKS 



While sheath breaks in aerial cable are brought to light by rainy 

 weather it is inadvisable to wait for rain to disclose them because of the 

 possible serious effects on service and the need then for repairs on an 

 emergency basis. Rapid line-insulation testing techniques have been 

 very successful in disclosing sheath breaks in aerial cable. During the 

 night the cable sheath cools and as the pressure inside decreases, outside 

 air with a high moisture content enters the sheath break. The paper 



