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

In Sweden -48V for POTS, and -96V for ISDN.

FWIW, 24V is still used in the US to power some cellular site and other telecom equipment. I think it was -24V.

The main reason for having "positive earth" is to prevent corrosion and oxidation. this article explains the reason reasonably although there is rather more to it;

In the 1946 book, The Telephone Handbook 8th edition by Joseph Poole AMIEE Wh. Sc. and later revised by N.V Knight B.Sc., AMIEE and W. Prickett, page 203 states;

"The positive terminal of the main battery is earthed at all exchanges so as to obtain uniformity in signalling over junctions, with earth and battery connections. With negative potential on the line conductors, earth faults are not masked by chemical deposits on the conductors, forming insulating films at the point of contact with the earthed object, as would be the case with positive polarity, and such faults are therefore rapidly brought to light."

Since oxidisation is a form of mild electrolytic action and this process makes itself evident at the anode or positive terminal, it means that exchange connection points such as relay and switch contacts etc which are positive with repect to ground, would become oxidised thus creating insulating deposits which would result in major problems for maintenance. Since both moisture and chemical impurities are present in the atmosphere which is in contact with the metal contact points and wiring joints (particularly underground cables), the opportunity for oxidisation to occur in a large network becomes enormous considering the number of such points which exist.

The reference to uniformity of signalling is not really a reason why positive earthing is used since it refers simply to a standard method for conveying the signals required for setting up and clearing connections, and this function would be necessary irrespective of which polarity was utilised. It perhaps refers to the fact that since other telephone engineering companies also used positive earthing that it would be wise to follow the same course for the sake of uniformity of signalling. One has to remember that this was in the days prior to automatic telephone exchanges where manual CB working was the norm and human operators carried out the connection and supervisory processes using dc line signalling in the main.

Some present day organisations have published information that the positive earthing system was started by telco's in the 1940's but this is plainly false and misleading. Positive earthing was around long before the 1940's.

"Ross Herbert" ...

Thanks, nice references!

All:

Cathodic protection has been used to protect metallic conductors in the environment for well over a hundred years!

Negative voltage applied to exposed metals provides "cathodic protection" for the metal conductors.

If conductors are positively charged and emersed in a solution (the ground surrounding them) then metallic ion migration will cause complete erosion and the total decay and breakdown of the metal in the conductors.

If kept at a postitive potential of 48 volts with respect to ground, number AWG 24 copper conductors would completely erode due to ionic migration and dissappear within about a month. That is why telco plant always is supplied with -48VDC with respect to ground, and never with a positive potential.

This is also why ships at sea are equipped with sacrificial zinc anodes which, due to the postion of zinc on the electrochemical potental scale with respect to salt water, preferentially corrode thus protecting a ship's metal hull from erosion.

Such sacrificial zinc anodes are a passive form of cathodic protection, whilst applying a negative voltage to exposed metals is an active form of protection.

All Navies and most merchant marine vessels use such active cathodic protection systems to prevent metal hull erosion and the telephone company copper plant applies -48 volts with respect to the environment to their copper cables for the same reason.

-- Pete

Professional Consultant Indialantic By-the-Sea, FL

If positive ground is so great for anticorrosion, why did the US auto industry eventually standardize on negative ground? Are there disadvantages?

-- Regards, Carl Ijames carl.ijames at verizon.net

Neither kind of ground is "anticorrosion." Whichever part of the system is negative loses metal and whichever part of the system is positive gains metal. With telephone systems, ground stakes are a lot easier to replace than wires are. With automotive systems, body and frame parts are harder to replace than wires are. Also, there was a time when cars had vacuum tube (valve) radios, which are happier with a negative ground. Finally, it is easier to explain a system if you can say "the current comes out of the positive battery terminal, goes through the light bulb, and returns to the negative battery terminal." (It really doesn't, as we all know, but it's too late to correct Ben Franklin's bad guess.)

The metal ions are postively charged, so they head for the cathode.

Best regards, Spehro Pefhany

For > 99.9% of the time, my phone is on-hook, so one of the wires is at -48V (actually it's about -52V) and the other is within 1V of earth.

Regards, Allan

Nice description, Guy. Just a short note on convention current flow.

IIRC, Michael Faraday established the direction of conventional current flow while he was working on electrolysis. At that time, nobody knew about electrons, or atoms, or ions. Faraday noticed the positive electrode got smaller and the negative electrode increased. Obviously, something had to be leaving the positive electrode and moving to the negative one.

Of course, he had no way of knowing electrons cannot flow in an electrolyte, and ions were the charge carriers in the solution. This was entirely understandable and forgiveable, given the lack of knowledge at the time.

Also, here's another clue showing Faraday's genius. Batteries of the time did not hold constant voltage, which messed up his calculations on mass transfer.

To overcome this problem, he put his cells in series, so each one would have exactly the same current. Then everything worked out as ratios.

Without our Fluke DVM's and Tek scopes, how many of us could figure this out?

Another advantage of being first is you get some constant named after you. And so now we have the Faraday constant, which is used anywhere you have electrolysis, as in aluminum refining, electroplating, and corrosion.

Best Wishes,

Mike Monett

Okay, now you've got my head spinning. Which is it? Does material leach from the more positive conductor and deposit on the more negative, or is it the other way round?

AAARGH!!! I wrote it BACKWARDS! Do'h! Whichever part of the system is negative gains metal and whichever part of the system is positive losess metal.

Profuse apologies!

I read in sci.electronics.design that Guy Macon wrote (in ) about 'Why -

At that time, European cars had positive earth battery systems.

Here is another article reprint from 1899 which, while not specifically relevant to the telephone system, explains the problem of electrolysis in underground lead pipes etc which were commonly used for gas and water supplies.

Note that in para 3 a simplified description of the electrolytic process is given;

"When electricity passes from place to place, a portion is absorbed in the bodies which form its path. In solids, this dissipated energy is transformed into heat and expended, sometimes disastrously, in raising the temperature of the conductor, while in liquids it is chiefly occupied in decomposing the fluid into its component chemical components. Thus, if water form a portion of the circuit, it will be separated into oxygen and hydrogen, the oxygen appearing at the positive pole, or anode, and the hydrogen at the negative pole, or cathode. The elemental gases only make their appearance at the surface of the conductors extending into the liquid, and, at the instant of decomposition, seem to be endowed with the extraordinary activity, readily corroding all but the most resisting substances."

This plainly indicates that corrosion occurs on the positive conductors in a dc electrical circuit where they are exposed to moisture.

Initially, the rail system DC supply had its positive pole earthed, and due to the lack of proper bonding in the return rails, earth leakage currents found their way into adjacent lead pipes (used for water and gas reticulation), thus making them electro-positive. The result was that severe corrosion of the pipes occurred where the current in the lead pipes emanated to return to the moist earth to find their way back to the rail line.

The article makes for interesting reading and those astute readers might care to investigate further on similar problems which occur to this day. The science of cathodic protection is big business today and involves federal, state and local governments and the providers of underground utility services such as telephone, gas, water and electricity. Strict standards of bonding and other means of protection are applied where electrified services impinge on underground services.

So did many US cars/tractors/trucks up until the 1950's. Not to mention 6V and 8V electrical systems.

Tim.

Since a car can build up static electricity, it must have a fairly high impedance to ground. Thus if it is fitted with an positive ion emitter (like the ion generators for home and office use, but for +ve ions), it would build up a -ve charge from the excess electrons. This is the stated operating principle of the auto anti-corrosion devices, which have been pretty controversial.

I don't see why they shouldn't work. Can anyone come up with a strong argument either way?

Clifford.

Some lands use another voltage levels... For analogous local telephone network in my city: not hold 60V hold 6..15V ring ... up to 200V! (be careful!) pair cable.

Maximum distance from the CO?

Best regards, Spehro Pefhany

Probably dates back to one of Stalin's 5 year plans- Inspired by the World's Greatest Engineer, our engineers, instructed to construct a 48V telephone network, not only completed installation 2 years ahead of plan, but achieved a 25% improvement in voltage...

Paul Burke