From: andrew@cucumber.demon.co.uk   
      
   In article ,   
   	"J.B. Wood"  writes:   
   > On 11/03/2014 10:10 AM, Andrew Gabriel wrote:   
   >> It's done that way so that any water leak into the cable bundles   
   >> or connections which results in electrolysis between the connections   
   >> and ground doesn't cause the metal conductors to be corroded away.   
   >>   
   >   
   > Hello, and thanks for the prompt reply.  That makes sense but I assume   
   > that it applies to cable insulation that is in physical contact with   
   > earth ground, not telephone cable mounted on utility poles.   
      
   Yes, indeed, although not just cable, but any street apparatus.   
      
   This technique probably doesn't protect the cable very much because   
   if that leaks, the water will likely contact both sides of the pair,   
   and electrolysis will corrode the grounded side away.   
      
   Early underground cables were what's commonly referred to as "paper   
   insulated", although a more correct description is "paper separated,   
   air insulated". These had a waterproof jacket, but predating modern   
   plastics, they were much less reliable than a modern cable sheath.   
   The cables were kept dry by pumping dry nitrogen into the cable   
   jackets at the exchange end to pressurise the cable jackets underground,   
   and that would help resist water penetration. There are probably still   
   some of these old cables in service (certainly were 20 years ago when   
   I last went into the sub's frame section of a telephone exchange and   
   the compressor kept kicking in to charge up the pressure storage vessel).   
      
   > BTW, I have   
   > an early 1950's Army publication, TM 11-678, "Fundamentals Of Telephony"   
   > that shows the central office battery terminal grounded but doesn't   
   > elaborate on whether it's a frame/chassis ground or connected to earth.   
      
   Both.   
      
   >   The TM is very well done (the Army & Navy knew how to write great   
   > training/technical manuals back then) albeit dated.  Sincerely,   
      
   --   
   Andrew Gabriel   
   [email address is not usable -- followup in the newsgroup]   
      
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