Alan Corey writes:


I've read all the answers so far looking for mentions of hum and this is
the first I've seen. As I recall, there is an earth ground rod at the
entrance, but the installer can tie it to either line of the balanced
pair to minimize hum.

No! If you ground tip or ring anywhere except via the line card, the
imbalance will destroy the line.

Both T & R do have a overvoltage protector that shunts to local
ground, and that is tied to the building/power ground (MGN -
Multi Ground Neutral).

Party line ringing disturbed this balance, but in theory the L-C
ringer circuit limited the imbalance.

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