Teflon (Polytetraf luoroethylene) . Teflon is the most thermally 

 stable and chemically resistant of all dielectrics. Its temperature 

 range is -90°C to 250°C and it is in wide use as a wire insulation where 

 there is a problem of overheating at solder terminations. Its abrasion 

 and cut-through resistance are not as good as those of cross- linked poly- 

 ethylene and polypropylene. Also, it is not suitable for high-voltage 

 applications — because of poor corona resistance. For these reasons, and 

 the fact that its cost is relatively high, teflon is used only in special 

 electro-mechanical coaxial or multi-conductor cables where chemical and 

 thermal stability is important. Its dielectric constant is lower than 

 polyethylene and most other commonly used dielectrics, so that it is par- 

 ticularly useful in ocean experiments where low signal attenuation is 

 vital. 



Silicone Rubber . Silicone rubber is a good, flexible, high-tempera- 

 ture elastomeric insulator and is used mainly in multi-conductor electro- 

 mechanical cables where a combination of relatively high voltage and 

 flexibility is desirable. Its dielectric constant is around 3.2 and, 

 hence, it is not particularly suitable for high frequency signal- 

 transmission cables. 



EPR (Ethylene Propylene Rubber) . EPR shows considerable promise as 

 a dielectric in high-power ocean cable systems because of its very high 

 resistance to corona. A number of EPR cables have been manufactured 

 for voltages up to 60 kV. It has superior thermal stability relative to 

 polyethylene and rubber. 



SF6. SFg (Sulfur Hexafluoride) is a gaseous dielectric which offers 

 advantages over other dielectrics used in high-voltage/high-ampacity power 

 cable systems. 19 xhe advantages of gas insulation are its excellent heat- 

 transfer characteristics, its low dielectric constant (unity at all fre- 

 quencies). Compressed-gas-insulated cables are being installed in under- 

 ground distribution systems, and it is expected that as pipe technology 

 improves, such cables will be utilized in ocean power distribution systems. 



Design of Conductor Systems 



Most cables used in the ocean have conductor systems which fall into 

 one of the three basic categories mentioned previously: coaxial, multi- 

 conductor, and single-conductor high-power cable. This classification 

 scheme is not complete since many E-M cables are hybrid types which may 

 have more than one coaxial cable together with single conductors or may 

 contain a large collection of insulated conductors which are grouped in 

 twisted pairs or quads. Also, an E-M cable which does not fall into the 

 basic classification scheme is the medium power three-phase system, con- 

 sisting of three or four heavy conductors in the same cable. 



Figure III-l shows schematics of the cross-sections of typical con- 

 ductor systems used in E-M and ocean cables. The two systems which lend 

 themselves to relatively simple mathematical analysis are the single- 

 conductor cable and the coaxial cable. It is extremely difficult, if not 

 impossible to mathematically construct the electromagnetic fields of the 



26 



