Why Telephones Use Negative Forty-Eight Volts DC11 Aug 2006

If you’ve been following the random stories throughout the site, you might just remember a mention of my

summer co-op job at Emerson Network Power (if not, you can refer to this post). The summer has gone well;

I’ve made some cash, learned some new tricks, and added a nice new section to my resume. However, there

was one little problem that arose…

All telephony equipment operates on negative forty-eight volt DC power systems. I was introduced to this “fact

of life” at the beginning of the summer, and I found it very odd. Not the fact that its forty-eight volts or that its

DC, but that its negative (the positive terminals of the batteries are “ground”). Why the heck was that voltage

negative? Anyway, summer continued, I got busy, and I forgot about this little question. Fast forward to the end

of summer…

I was sitting in on a division meeting listening to the engineers discuss various aspects of the projects they

were working on, when suddenly my supervisor turned to me and asked “So what have you learned this

summer?” Caught off guard, I immediately turn red and began to stammer something about “AutoCAD and

plant operations and Pro Engineer…” when he cuts me off and says, “No, what did you learn?” So then I

mumbled something about how telephones and telecommunications networks work, and in doing so,

mentioned the oddity of that negative voltage. Immediately he went “Ah ha!” And just like that I had a new

research assignment.

After a few days of poring over large communications theory books, searching the Internet, and scratching my

head, I knew why the power system was forty-eight volts DC, but no idea why it was negative. After about week

of searching, I finally broke down and asked; here is what I learned.

Telephony equipment uses forty-eight volts DC for very simple reasons: DC does not introduce noise on the

line and is easily produced from regular lead-acid (vehicle) batteries – which just happen to come in twelve-volt

increments (due to the chemical properties of the battery). Forty-eight volts is high enough to be efficient while

still being considered a “safe low voltage” and being a multiple of twelve (four batteries make up one “string”).

The negative polarity is much more elusive, but can be summed up in one word: corrosion. Thanks to a bit of

research performed by Sir Humphry Davy for the British Navy, we have a technology known as “cathodic

protection.” First developed to keep the copper hulls of British naval ships from corroding, this technology has

been applied to protecting everything from oilrigs to gas pipelines to telephony cabinets. By keeping the cabinet

frame at a more positive voltage than ground, corrosion is reduced and the life of the equipment is increased.

Who would have guessed?

Why -48V is preferred?(i) Positive voltages cause comparatively more corrosion in metal than negative voltages. 

(ii) Negative voltages are safer for human body while doing Telecom activities. 

(iii) Lightning may cause positive voltages in the equipment circuitry. In that case, negative voltages

(lack of electrons) neutralize positive charges and prevent excessive heat. 

(iv) Negative voltage is safer for long telephone line for transmitting power trough it. 

The reson for electing -48V in reference to ground is to avoid the nasty galvanic effects in telephone

cabling. +48V would have nastier effect on wet cabling.

Other answer:

You should also keep this in mind that all of USA uses 24 (+Ve) system not 48 (-Ve) system. Hence all

the arguments do not really hold much grounds. these are two different systems, that's all. the

voltages had to be in the multiples of basic 2 V hence -48 V or 24 V, actually it turns out to be -54 V

and +27V respectively. However there is one fact though, positive ground is more efficient then

negative ground.

Top 5 Reasons to keep Voltage to -48V DC in Telecom

Negative DC voltage causes of using & -48 V DC:(i) Positive voltage courses comparatively more corrosion in metal then Negative voltage.(ii) Negative voltage is safer for human body while doing Telecom activities.(iii) Thundering may cause positive voltage in the equipment circuit. In the case negative voltage (Lack of electrons) neutralizes positive charges and protects producing heat. (iv) Negative voltage is safer for long telephone line for transmitting power trough it.(v) A standard lead-acid battery provides 6V. This number corresponds to 8 of those, which is pretty standard. It's the highest, safest compromise voltage to run over long wires.Negative voltage is used so that leakage currents to ground caused by moisture do not electroplate away the copper in the wires.

Why -48v DC is used in Telecom

There are several aspects for using -48V in the telcom equipment some of them are as follows;

1)-Positive voltages cause comparatively more corrosion in metal than negative voltages. 2)- Negative voltages are safer for human body while doing Telecom activities. 3)- Lightning may cause positive voltages in the equipment circuitry. In that case, negative voltages (lack of electrons) neutralize positive charges and prevent excessive heat.

The -48V voltage was selected because it was enough to get through kilometers of thin telephone wire and still low enough to be safe (electrical safety regulations in many countries consider DC voltages lower than 50V to be safe low voltage circuits). 48V voltage is also easy to generate from normal lead acid batteries (4 x 12V car battery in series). 

Batteries are needed in telephone central to make sure that it operates also when mains voltage is cut and they also give very stable output voltage which is needed for reliable operation of all the circuit in the central office. 

The line feeding voltage was selected to be negative to make the electrochemical reactions on the wet telephone wiring to be less harmful. When the wires are at negative potential compared to the ground the metal ions go form the ground to the wire instead of the situation where positive voltage would cause metal from the wire to leave which causes quick corrosion.