630 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



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106 



INSULATION RESISTANCE IN OHMS 



Fig. 1 — Wet weather versus dry weather tests — all types of Outside Plant 

 combined. 



test circuit of the local test desk which is used for testing subscriber 

 Unes reported in trouble. The speed at which lines can be tested from 

 the test desk is necessarily slow because each number must be dialed or 

 called individually therefore, only a small sample of hues was selected 

 from any one office. Rapid line insulation test equipment was not in use 

 in any area when this study was made. 



The upper curve of Fig. 1 shows that about 0.3 per cent of the Hues 

 were below 11,000 ohms in wet weather while in dry weather only about 

 0.3 per cent of these same lines were below 140,000 ohms. Similarly, in 

 wet weather two and one-quarter per cent of the lines were below 47,000 

 ohms but in dry weather only two and one-quarter per cent were below 

 900,000 ohms. The same wet weather test data summarized by types of 

 outside plant is shown in the curves of Fig. 2. Lines in underground cable 

 only, show the highest line insulation resistance. Those with aerial 

 cable are next and those with some open wire have the lowest insulation 

 resistance. 



Fig. 3 shows the wet weather line insulation resistance distribution 

 of 11,600 lines in an eastern city where the exchange outside plant had 

 been thoroughly reconditioned prior to conversion from manual to cross- 

 bar dial operation in 1940. The upper curve represents the approximate 

 distribution before reconditioning and the lower curve represents the 



