1252 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1953 



noted that the polyethylene-insulated cables excel in dielectric strength. 

 The values shown in Table II are the test voltages that all the reels of 

 cable were required to withstand between conductors and from core to 

 sheath. These voltages are naturally made lower than the inherent di- 

 electric strength of the insulation to allow for minor manufacturing 

 irregularities and thus avoid excessive rejections. Some idea of the in- 

 herent dielectric strength of solid and expanded polyethylene insula- 

 tion can be obtained from tests on short samples of insulated conductor 

 immersed in water. In Fig. 6 the inherent dielectric strength obtained 

 in this manner for conductors with a 14-mil wall is plotted against 

 percent air in the insulation. It will be noted that the dielectric strength 

 falls off rapidly as the polyethylene is expanded. 



The capacitance unbalance to ground, or difference in direct capaci- 

 tances of the wires of a pair to ground, is a rough indication of one im- 

 portant factor in the susceptibility of cable circuits to noise and inter- 

 ference. The electrical disturbances which cause noise in the cable may 

 come from atmospheric static, radio stations, power lines, or from tele- 

 phone plant sources. The former energize the cable circuits via the sur- 

 face transfer impedance^ of the sheath, or by way of an open wire tap, 

 the latter via all of the conductors in the cable. For this reason the two 

 wires of a pair should have nearly equal capacitance to the surrounding 

 pairs and to the sheath. To achieve this condition the cylinders of in- 

 sulation on the two wires of a pair must be ahke in size and dielectric 



GRANDVILLE-ZEELAND (mICH.) PROJECT 

 51 PAIRS NO. 19 GAUGE - EXPANDED POLYETHYLENE INSULATION 



COOPERSTOWN- CHERRY VALLEY (n.Y.) PROJECT 

 26 PAIRS NO. 19 GAUGE - SOLID POLYETHYLENE INSULATION 



TROUT LAKE - ST. IGNACE (mICH.) PROJECT 

 51 PAIRS NO. 19 GAUGE - SOLID POLYETHYLENE INSULATION 



Fig. S—Polyethylene insulated multipair cables. 



