the niobium/metal type. Resistive cryogenic submarine cables would 

 probably use aluminum, mainly because of the cost and weight advantages 

 of aluminum. 



Dielectric Materials (Summarized in Table III-3) 



The most important function of a dielectric material in an E-M 

 cable is the isolation of current-carrying elements. Most of the dielec- 

 tric materials used in cables in the ocean and their properties are listed 

 in Table III-3; they are either solid or liquid types, with most cables 

 using solid dielectrics of the plastic or elastomer types. The only sub- 

 marine cables which use liquid dielectrics are high-power seafloor cable 

 systems such as pipe-type systems and hollow-conductor cables. Actually, 

 the liquids in pipe and hollow-conductor systems are not used as dielec- 

 trics to isolate conductors since in both types of cable systems a solid 

 dielectric is used. The oil is used to improve the thermal characteris- 

 tics of the system and enhance the performance of the solid dielectric. 

 In recent years, compressed gas insulation has been used in underground 

 high-power cable systems-^ but, as yet, gas has not been used as the 

 dielectric in ocean cable systems. In a gas-insulated svstem solid 

 dielectric spacers are used to isolate the conductor from the pipe which 

 contains the gas. 



Polyethylene . Polyethylene has a good balance of electrical proper- 

 ties, mechanical properties, and cost. Its relatively low dielectric 

 constant combined with its suitability for thick-walled extrusions, make 

 it one of the best dielectrics yet developed for long-distance coaxial 

 telephone cables. 13,14 Natural (low-density) polyethylene has a specific 

 gravity around 0.9 and is suitable for virtually all types of E-M coaxial 

 cables. Natural polyethylene has a moderate dielectric strength so that 

 power up to above 5000 volts can be carried on most ocean telephone coaxial 

 cables. Cross-linked polyethylene^ » 15, 16 h as a high dielectric strength 

 and is used in high-voltage/high-power single-conductor cables to separate 

 the conductor cables from the electrical shield. 



The moisture resistance and low temperature performance of polyethy- 

 lene is superior to those of other cable insulating materials. The dis- 

 advantages of natural polyethylene are flammability , stiffness, and a 

 maximum operating temperature of 80°C. Cross-linking will raise the 

 continuous operating temperature to around 150°C. A recently tested 

 single-conductor power cable uses cross-linked polyethylene separated 

 from the conductor by a semi-conducting tape soaked in silicone oil. The 

 presence of the liquid allows smaller insulation thickness and hence 

 improved flexibility (smaller bending radius). 



Polypropylene . Polypropylene is superior to natural polyethylene 

 with respect to mechanical strength and dielectric strength; however, 

 polypropylene constructions are limited to relatively thin-walled extru- 

 sions. This plastic is also susceptible to copper poisoning which can 

 be prevented by proper tinning of the copper. 



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