POTS Battery Grounding

Hello, everyone. Historically (at least in the U.S.), the positive terminal of the 48-volt battery stack at the central office is
intentionally connected to an earth ground. I can't seem to find any info on why this is necessary or how it improves/facilitates the operation of a traditional analog common battery telephone system. Thanks for your time and comment. Sincerely,
--
J. B. Wood e-mail: arl snipped-for-privacy@hotmail.com

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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.
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Andrew Gabriel
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On 11/03/2014 10:10 AM, Andrew Gabriel wrote:

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. 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. The TM is very well done (the Army & Navy knew how to write great training/technical manuals back then) albeit dated. Sincerely,
--
J. B. Wood e-mail: arl snipped-for-privacy@hotmail.com

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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).

Both.

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Andrew Gabriel
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snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote:

It applies equally to all telephone cable, whether buried or aerial.

Neither side of the pair is directly grounded as such, as the resistence to ground from either side will be equal. But one side will have a negative voltage compared to the other side if the pair is used for a POTS line.

But in reference to grounding, all comm cables are grounded at regular intervals. Generally that would be every 3000 feet. The sheath from each connecting section is bonded to an earth ground.

Ultimately everything indicated as being "grounded" is connected to one single earth ground. The main significance though is that there will not normally be any current flow in the connections to that single earth ground. Hence in many of the diagrams in that text what is shown is a signal ground that is separate from chassis ground. But each chassis is normally connected to a single frame ground. And all frame grounds go to a single ground point for an area (probably that floor of the building) and that point is then connected via a single conductor to the building ground, and a single conductor connects that to the earth ground.

I'm not sure who wrote that particular manual, but almost certainly it was done by a major commercial telecom company. It might have been written by the Bell System, but it looks to me more like a product of General Telephone or maybe Automatic Electric. (I've long since forgotten what the easy to distinguish differences would be.)
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Floyd L. Davidson http://www.apaflo.com/
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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