"Jim Thompson" wrote in message
...
Two lines coming in...

How much risk is it to assume that both "Tips" are essentially earth
ground potential?

...Jim Thompson

Probably not a good idea to do that. Back in the days when the
subscriber phones had local batteries it was important. However
with the advent of no battery phones in the early 1900's it made
little difference which side was grounded. My father was an installer
and was taught in the 40's that ring/tip red/green were not critical.
Even if the tip were always ground that ground may not be the same
as the electric company's ground in your home/office as the phone
co often pounds in their own ground rod which can be many feet,
and volts, away from the electric ground rod.
Art