INSULATION TEST EQUIPMENT FOR LOCAL CROSSBAR SYSTEMS 629 



weather conditions. Trees grow up into the wires and during rainy- 

 weather the branches often drop across the wires. When this oc- 

 curs at many points on the open-wire run the combined leakage along 

 the pair may become very low. In some localities, moss growing on the 

 \\ires, salt spray or heavy fog conditions causing leakage at insulators 

 are additional causes of low insulation resistance on open-wire lines. 



Drop wire used to make the connection from the cable terminal to 

 the customer's premises is subject to damage by abrasion and the 

 insulation deteriorates from the effects of the weather. The old style 

 of drop wire, a large amount of which is still in use in the plant, is in- 

 sulated with a rubber compound and covered with a water proof cotton 

 braid. When this braid protection is lost due to abrasion or effects of 

 the weather after many years in service cracks form in the rubber in- 

 sulation. Moisture enters these cracks in wet weather causing low re- 

 sistance leakages. It is expected that the latest type of drop wire with 

 the tougher neoprene covering will withstand the hazards for a longer 

 period of time than does the old drop wire but undoubtedly the end of 

 its useful life wdll ultimately be determinable by measurement of the 

 insulation resistance under wet weather conditions. 



Inside wiring on the customer's premises often remains in service for 

 a long period of time and the insulation deteriorates over the years. If 

 the insulation is in poor condition, inside wiring will develop leakages 

 during periods of prolonged high humidity indoors which occur fre- 

 quently during the summer months. 



EFFECTS OF WEATHER ON LINE INSULATION RESISTANCE 



When weather conditions are favorable — clear weather with low 

 humidity — the insulation resistance of the line conductors in the out- 

 side plant is quite high compared to the design limit for central office 

 switching circuits. When the plant becomes thoroughly wet from a hard 

 rain the insulation resistance drops very considerably because of leakages 

 which are in parallel all along the line. Fig. 1 shows this very clearly. 

 The data for the curves in this figure were collected in a special study 

 conducted in 1931 to determine the insulation resistance distribution 

 of exchange lines in underground and aerial cable and open-wire plant 

 under different weather conditions — dry, humid and wet. About 6,000 

 dial lines selected at random in twelve large cities in different parts of 

 the United States were tested under different weather conditions and no 

 repairs were made throughout the study period except to correct un- 

 satisfactory service conditions. The tests were made with the voltmeter 



