Why -48V for telephone lines, and not positive?

[Arie de Muynck]

At least in Europe, most telephone exchanges use a negative voltage
referenced to good old mother Earth.

Why negative? Is it some 'cathodic protection' in case a pinhole leak
occurred?

 

[Guy Macon]

At least in Europe, most telephone exchanges use a negative voltage
referenced to good old mother Earth.

Why negative? Is it some 'cathodic protection' in case a pinhole
leak occurred?

Don't think pinhole leak. You are assuming modern insulation.
Think cotton insulation that may get damp - that's all they had
when this stuff was invented.

If negative was grounded, the electroplating effect would
take copper away from the wires and plate it on the grounding
rods/stakes. With positive grounded, the electroplating effect
takes copper away from the grounding rods and plates it on the
wires. Grounding rods are made to be much thicker and easier
to replace.

Extra credit question: why do most electronics have negative
ground? Hint: think about the physics of vacuum tubes.

BTW, in the old manual switchboards, tip was chosen to be
positive/ground and ring to be negative/power because the
tip is more likely to contact some exterior grounded surface.

 

[legg]

It is indeed to reduce galvanic corrosion in all hardware contacting
ground. In most cases, this will include chassis and grounding
hardware - those components which by-and-large inevitably make this
contact.

This is stated emphatically in NEBS GR1089 sect 72 and 73.

It is also a control method used, along with other types of cathodic
protection, in marine, civil and large industrial installations. The
intention is to polarize metalic structural hardware with respect to
its immediate environment, be it concrete, dirt, salt water or even
some kinds of air.

RL

 

[JeffM]

when this stuff was invented

Guy Macon

Yup. Stroger Switches were invented in 1884--by an undertaker.

 

[Winfield Hill]

JeffM wrote...

Yup. Stroger Switches were invented in 1884--by an undertaker.

Do you think they used -48V back then?

 

[Paul Burke]

JeffM wrote...



Do you think they used -48V back then?

I would imagine that the requirements for long(ish) distance
communication had been worked out long before, by the Morse telegraph
companies (you know, the clerk heroically tapping away with the arrow
stuck in his back, dies as the cavalry rides over the hill), though IIRC
they used single wire, earth return circuits. The polarisation to avoid
corrosion of the wires would have come out of that.

Paul Burke

 

[JeffM]

when this stuff was invented

Do you think they used -48V back then?
Winfield Hill

A lot of choices in this world are simply convienience--or just arbitrary.
It always seemed logical that the battery level was raised
as soon as distances became significant.
I have long believed--with no proof--
that the polarity was selected arbitrarily way back when.
That the galvanic thing worked out well seemed like serendipity.

 

[Frithiof Andreas Jensen]

Do you think they used -48V back then?

I would not be surprised if the -48V was what they arrived at by stringing
batteries together until "it worked" ;-)

 

[Allan Herriman]

I would not be surprised if the -48V was what they arrived at by stringing
batteries together until "it worked" ;-)

Some parts of the world use(d) -60V instead of -48V. That's 5 car
batteries instead of 4.



If anyone is designing equipment to run from '48V' they should be
aware that there are well defined tolerances and surge amplitudes,
etc.
See, e.g. ETSI EN 300 132-2 V2.1.2 (2003-09)

Regards,
Allan

 

[Tim Shoppa]

JeffM wrote...

Do you think they used -48V back then?

It was less standardized. Short loops were as little as -12V, long
loops could be more than -48V. It depended on who was selling the
switch and equipment too.

Large parts of the civilized world use -24V as the default on telco
loops.

Tim.

 

[Spehro Pefhany]

A lot of choices in this world are simply convienience--or just arbitrary.
It always seemed logical that the battery level was raised
as soon as distances became significant.
I have long believed--with no proof--
that the polarity was selected arbitrarily way back when.
That the galvanic thing worked out well seemed like serendipity.

I ran across a web page that says the negative polarity (positive side
grounded, IOW) only became standard in the US after WWII.


Best regards,
Spehro Pefhany

 

[BFoelsch]

JeffM wrote...

Do you think they used -48V back then?

I doubt that there was any standard in the days before electrical
amplification. My guess is that for a long line the voltage might be a good
deal more than 48 volts.

I looked in my old college text on Telephony (Albert, Fundamentals of
Telephony, Mc-Graw-Hill, 1943) and found no discussion on the polarity
issue, but he did offer that magneto (crank) telephone systems used 3 dry
cells per set, which would of course work out to 4.5 volts. However, these
sets coupled to the line through a transformer so the line carried purely
voice current an ringing current with no DC potential. He then goes on to
say that most central battery systems used 24 volts, except for dial and
toll offices, which used 48. So, I guess that as late as 1943 24 volts was
the standard.

He makes no mention of the polarity issue, but I picked through a lot of the
schematics of both dial and manual systems and they all show the
common-positive system.

 

[BFoelsch]

I would not be surprised if the -48V was what they arrived at by stringing
batteries together until "it worked" ;-)

I really doubt that. I would suspect that the telephone system was one of
the first real examples of engineered systems, definitely engineered
electrical systems. Of course, they could have "tweaked to suit" during the
developmental era, but I would guess that every nuance and implication of
the voltage chosen was carefully considered once they got to the
implementation stage.

 

[Guy Macon]

I really doubt that. I would suspect that the telephone system was one of
the first real examples of engineered systems, definitely engineered
electrical systems. Of course, they could have "tweaked to suit" during the
developmental era, but I would guess that every nuance and implication of
the voltage chosen was carefully considered once they got to the
implementation stage.

Indeed. Take a look here to see how much thought they were putting
into the system back in 1872...
http://sd.znet.com/~cdk14568/mpet/contents.html

Here is the section on voltage ang grounding. (note that this was
written before there was such a thing as a "volt"!)
http://sd.znet.com/~cdk14568/mpet/chap10.html#para169

BTW, here is an interview with an early radio pioneer who says
"normally in an arc transmitter the positive was grounded to
simplify cooling problems"...

http://www.ieee.org/organizations/history_center/oral_histories/transcripts/wh
ite19.html

Guy Macon <http://www.guymacon.com>

 

[Arie de Muynck]

"Guy Macon" ...

Don't think pinhole leak. You are assuming modern insulation.
Think cotton insulation that may get damp - that's all they had
when this stuff was invented.

I even think paper isolation, most older PTT cabling here in the
Netherlands.

If negative was grounded, the electroplating effect would
take copper away from the wires and plate it on the grounding
rods/stakes. With positive grounded, the electroplating effect
takes copper away from the grounding rods and plates it on the
wires. Grounding rods are made to be much thicker and easier
to replace.

Thanks, that's what I guestimated too. It came up as a car-pool discussion
and this was my hypothesis.

Extra credit question: why do most electronics have negative
ground? Hint: think about the physics of vacuum tubes.

Don't have to "think of the physics". It soaked into my reflexes by now ;-)
I started at 10 with 'modifying' my first tube radio, I'm now 52 and still
designing. And I still "ground the commons" of circuits (even if portable).

BTW, in the old manual switchboards, tip was chosen to be
positive/ground and ring to be negative/power because the
tip is more likely to contact some exterior grounded surface.

Makes sense - and DC adapters often prove it.

Regards,
Arie de Muynck

 

[Arie de Muynck]

"legg" ...

It is indeed to reduce galvanic corrosion in all hardware contacting
ground. In most cases, this will include chassis and grounding
hardware - those components which by-and-large inevitably make this
contact.

This is stated emphatically in NEBS GR1089 sect 72 and 73.

Thanks, this is the best proof for my hypothesis.

It is also a control method used, along with other types of cathodic
protection, in marine, civil and large industrial installations. The
intention is to polarize metalic structural hardware with respect to
its immediate environment, be it concrete, dirt, salt water or even
some kinds of air.

I knew, but was still speculating if it was also the reason for the negative
voltage on telephone lines. The dampness on the old telegraph isolators
would cause a small current - and galvanic corrosion. After an industry
switches to a certain polarity, they would stick to it.

regards,
Arie de Muynck

 

[Guy Macon]

I knew, but was still speculating if it was also the reason for the negative
voltage on telephone lines. The dampness on the old telegraph isolators
would cause a small current - and galvanic corrosion. After an industry
switches to a certain polarity, they would stick to it.

Most telegraphs had a different system: a battery on each end.
This resulted in negative ground at one end and positive ground
at the other end, and ground halfway between positive and negative
somewhere in-between.

 

[Arie de Muynck]

"Guy Macon" ...

Most telegraphs had a different system: a battery on each end.
This resulted in negative ground at one end and positive ground
at the other end, and ground halfway between positive and negative
somewhere in-between.

Only during signals - a low percentage of the time, I assume. And a floating
line in the meantime.

Regards,
Arie de Muynck

 

[Guy Macon]

Guy Macon <_see.web.page_@_www.guymacon.com_> ...

Only during signals - a low percentage of the time, I assume. And a floating
line in the meantime.

The line would be the voltage of the battery at the recieving end.
This wouldn't matter because telegraphs are current-operated.

 

[Arie de Muynck]

"Guy Macon" ...

The line would be the voltage of the battery at the recieving end.
This wouldn't matter because telegraphs are current-operated.

Got it - on unidirectional systems. I thought they were all half duplex to
save on wiring.

Regards,
Arie de Muynck