Off-topic informational anti-spam anecdotal - test-drb@ccmp.vtda.org

22 Jul 1998

Tony ended his long discourse on telephones with:

...
  In practice, you're not going to damage the
exchange no matter what you do 
 to the line at the subscriber end. The incoming lines are protected by an 
 array of spark gaps (probably gas filled), fuses and 'heat coils'. The old 
 electromechanical exchanges were certainly protected against lightening, 
 contact between telephone lines and (600V-ish) power lines, etc. Mains, at 
 the far end of the line, is trivial in comparison. 
ISTR that at our sites (electricity supply industry) we have to install 
special isolators on phone lines if the rise in local earth potential 
due to an earth fault exceeds 650V.  Such sites are known as "hot 
sites".  The implication is, anything less than 650V to ground the 
exchange can cope with.  Besides, I think the limiting factor is safety 
of engineers working at exchanges...

Philip.