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BELL SYSTEM TECHNICAL JOURNAL 



insulated from ground, currents will not then enter the sheath, except 

 through the capacitance to the outside shield. The voltage across the core 

 insulation will then be so small that core insulation failures will not occur, 

 unless the voltage between the outside shield and the sheath is large enough 

 to puncture the coating. Thus, if the resistance of the outside shield were 

 1.1 ohms per mile (10-mil copper shield around 1.2" cable) and if the impulse 

 breakdown voltage of the coating were 20,000 volts, breakdown of the in- 

 sulation would not be expected except for currents in excess of 130 kilo- 

 amperes when the earth resistivity is as high as 4000 meter-ohms. For a 

 cable of 2" diameter with a 10-mil copper shield, breakdown of the thermo- 



SERVICE PAIRS 



COAXIALS 



SERVICE PAIRS 



PAPER CORE WRAP OVER COAXIALS AN£ 

 SERVICE PAIRS 



PAPER INSULATED QUADS 



PAPER CORE WRAP 



LEAD SHEATH 



- -- 2 LAYERS OF 45 MIL THERMOPLASTIC 



10 MIL CORRUGATED COPPER JACKET 

 LEND CLOTH TAPE 



Figure 14 — Thermoplastic covered, copper jacketed cable. 



plastic insulation would not be expected except for currents in excess of 200 

 kiloamperes when the breakdown voltage of the coating is 20 kv and the 

 earth resistivity is 4000 meter-ohms, or when the breakdown voltage is 

 10 kv and the earth resistivity 1000 meter-ohms. The above type of cable 

 is also advantageous in that low-frequency induced voltages and noise due 

 to static are reduced. 



A cable of the above type is now being installed for a distance of about 

 180 miles along the Atlanta-Meridian route, where the effective earth re- 

 sistivity varies between 1000 and 4000 meter-ohms and lightning storms 

 occur frequently. The diameter of the lead sheath of this cable is about 

 2" , and the sheath is covered with two layers of thermoplastic each 45 mils 



