Why are Negative Voltages Used in Telephones?

Negative to what?

48 volts was chosen due to it being a nice round number of lead acid batteries used for backup of the telephone network.

High enough not to cause problems with resistance of the wires and loads, but low enough not to cause a hazard for consumers.

John.

Electroman,

Your statement that the voltage is "low enough not to cause a hazard for consumers" is a relative one...

It is true that the supervisory voltage present on the phone line is 48v nominal D.C., but you are forgetting about the ringing signal. Here in the US, the ringing signal consists of an 88v 20Hz A.C. signal superimposed on the supervisory voltage.

While 88v may not sound like a lot, I vividly recall an unfortunate experience I had nearly 35 years ago, as a youth...

I was adding on an extension phone in my parent's house, and was without a mechanical wire stripper – I'd always used my front teeth to strip wires anywhere from 16 down to 30 AWG – certainly no problem with some bell wire. But I didn't think about the possibility that someone would call while I had the wire in my mouth.

All I can say is that 88v across the tongue and lips is not something you will soon forget...

Jman, I agree 88 volts is a bit much to swallow (pun intended) but in the UK the ringing voltage is 50 volts (ok 48volts but whose counting!) and the ringing signal is 48 volts to 0 volts, relative to the other wire.

Still not a good idea to have it in your mouth though!!!

I'm not sure whether any side of the twisted pair is earthed or floating, the isolation requirements and load reguirements are pretty stringent though...

John.

vi has a point. I do a lot with networking, and ISPs almost always want their equipment powered by 48 Volts instead of 110V or 220V. However, they always want it powered by -48 Volts. I have yet to get a good answer why.

Change the probes round on your circuit tester!!

Funny thing, someone asked me this same thing a few weeks ago!

48 volts is used for reasons stated - 50-60 volts is regarded as "touch potential". Having had a 55 volt dc shock while at University from a string of submarine batteries I can verify that this can be surprisingly painful!

The positive terminal is grounded because in the early days of the telegraph bare copper wires were used (beloved of Western movies, being shot from the hip at 50 yards by a 4" barrel Colt....Mythbusted anyone?). Making the wire negative prevented or at least reduced corrosion.

Actually in Exchanges the charging voltage is 54.1 volts. The extra margin allows for volt drop and battery regulation. It's quite unsettling seeing 4000amp dc busbars painted red and blue just running along naked and free, open to touch.

Woof!!! Both you and elnav just gave me great answers! Thanks! The best I can do is give both of you a + point on your posts.

Also, I read a story about a guy that go electrocuted by 50V AC and full line current. So, guess it doesn't take a lot of volts as long as the Amps are there!

Actually no. The interior of the body is an excellent conductor, and our nervous system works on microamps. A few milliamps is fatal. It's accepted that 30mA is the limit, but of course people with bad hearts or epilepsy etc may be more susceptible.

Our protection is our skin, body geometry and location. So if your skin is wet (or sweaty or cut) you're in trouble as the skin resistance gets very small. Which is why you never mix water and electricity. Or put wires in your mouth.... ;-)

Thus you can get a shock at 50 volts as I did. But I'm still here! If you get the shock across your chest, however (this is where geometry comes in), it could stall your heart (fibrilation). This does not normally work out well.

So experienced people (a) don't poke at things and (b) poke carefully with one hand in your pocket. If they survive, young engineers at University become experienced ;-)

The location factor is all about how well you are grounded. Rubber shoes, nylon carpet, tall buildings etc all work in your favour. But don't try and find out where the limit is!!

Because a.c. can conduct through a capacitance, a.c. can be more dangerous than d.c. But there's not much in it (Google "The Current Wars" between Edison and Westinghouse) - d.c. charges capacitance too. Note also that a.c. voltages are quoted as RMS so the peak is 1.4 times, nearly 50% more.

Amps can be dangerous for a different reason. If you get jewellery across the terminals of a 12 volt car battery for example, several hundred amps will flow and it will melt. Not good if it's around your neck at the time!

But isn't it the case that DC shocks immediately drop to ground voltage, while AC shocks do not, thus making them deadlier? Or is this just a myth/misunderstanding.

Depends on how you look at it. Most working electricians with experience in both DC and AC systems are more vary of DC because once it shocks you, the muscles contract and the "grip" doesn't let go. AC has zero crossing and as such goes through a zero potential every cycle. This typically means there is a better chance of gravity dragging your body point of contact away from the live conductor. ( Assuming your hand doesn't have a "death grip" on the live wire. )

Voltage potential is dangerous because it breaks down the insulation of protective gear, clothes, skin etc. But like someone else said, the killer is the current flow through the body because it induces cardiac arrest. And 30 - 50 mA will kill. With two pins and a D cell I could electrocute someone.

When I worked for a power utility company we had monthly safety meetings. Every so often the hipot tester was brought out to show how various current flow levels felt like. (never close to lethal levels)

The macho linemen bragged about how much current (in milliamps) they could tolerate before the pain got to be too much. I think the record was around 20mA.

My job routinely involved hipot testing 27.6kV distribution cable for underground use. My tester ran at 70,000 volts but a safety circuit tripped if the leakage current exceeded 10 milliamps.

One time I accidentally got too close to the test cable. The current went in my knee cap and out my steel toed boot planted firmly on the steel deck of the truck. Thankfully the current path never got anywhere close to my chest. Even so, it took an hour and a half before my leg muscles stopped cramping enough for me to stand up. It kept hurting like hell for a day afterwards.

When I am wiring 24V DC power circuits in steel hulled vessels on a hot summer day I see flashes of light in my eyes and and get a distinct headache from repeated contact with the battery posts. Doing the same work on a cool winter day with no sweat, I dont feel a thing. The difference being the lowered skin resistance as I sweat. Working on metal hulled vessels means you can't avoid being grounded. Same work in fibreglass hulls is never a problem. Its like doing bucket truck work on high voltage lines.

Personally I'm more vary of DC powe than AC. DC burns tend to go deeper into the skin and take much longer to heal. I have survived 45 years of this stuff but it still scares me. Maybe that's why I'm still alive. Too scared to get too close!

elnav.

I have been working in the electrical field for over thirty years and experienced three serious electrical contacts. One at 500VDC (+ to -), one at 600VAC (phase to phase), and the last at 13,800VAC (30 amps phase to ground with the entry at left hand and exit point at lower left leg). All of them induced a very high level of pain but nothing can aproach the level of pain and destruction that my body experienced during the 13,800VAC contact. Funny thing is; all three times I was assisting others on a consulting and/or technical application level and all were a result of unwisely trusting other people and existing safety programs.

The point?

ALL ELECTRICAL CONTACTS HAVE THE POTENTIAL FOR PAIN, CELL DESTRUCTION, AND/OR (IF YOU ARE LUCKY) DEATH.

LOWER VOLTAGE CONTACTS CAN DO DAMAGE THAT IS NOT VISIBLE BUT HAVE LASTING SIDE EFFECTS.

DO NOT TRUST ANYONE, ANYTIME, OR ANYWHERE.

VERIFY THERE IS NO POTENTIAL PRESENT AND MAKE SURE THAT IT IS DEAD AND GROUNDED BEFORE YOU TOUCH IT AND IF YOU CANNOT DO SO, DO NOT GET NEAR IT WITHOUT THE PROPER TRAINING AND PERSONAL PROTECTIVE EQUIPMENT AND SAFEGUARDS.

TAKE CARE, STAY SAFE AND BE HAPPY!

cordially,

SHOCKHISCAN - (My new Indian name-"he who danced with the devil and lived")

Arizona

WOW and thanks.

Wise words oh SHOCKHISCAN !

Your comments have resurfaced my not so fond memories of taking hits from floating ground avionics.

Most notable was the 450VDC hit I took as a young airman in the navy testing a gun control unit. I was testing the unit when the ship rolled and the unit slid across the work bench and headed for the deck. It was under power and when I used my hand to steady it I took all 450VDC and pretty much 50+ amps current through my palm to my wrist across some exposed filters. It vapor locked my arm gave me a case of stars and burst my skin on exit point - Needless to say when I get around avionics ( as I do in my profession now ) I always look to my wrist and remember. Many avionic controls are 28vdc but can pack a wallop of 50 or so amps. This has been burned in my mind forever as a result of my experience.

We had a kid in the lab on my last cruise repairing large display units. Some of us here will remember the old Simpson meters. well he took a pair of regular leads across a circuit on a display and it kicked him like a mule 35KV off the tube the Simpson never worked again ( they are really hard to kill ) I remember he was shaking as he came to me as I was the Senior tech asking me in halted speech why it happened. As I looked at the schematic I noted he was on the bleeder network for the tube. He was expecting 15vdc. he had poked at it on the wrong side of the circuit and it left a mark. He ended up fine but we had him in the sick bay for a few days while they watched his ticker. He had acquired the moniker of "35KV kid" and we framed and mounted his Simpson and gave it to him as a farewell gift at the end of the cruise.

Heads up, never underestimate a piece of equipment and understand what your poking at before you even move. Some of us remember the first time we reached into an old tube circuit TV without a grounding strap already applied. ( I did at the ripe old age of 8 as I wanted to help my dad)

more slower is often mo better . . .

the good news is after all the pops I have taken haven't affected me a bit .. bit bit.. bit.. uh yeah thats it.

RF burns are worst, they don't always heal. Just a black hole in the skin

Quite....

This is the origin of ANY Black hole, it starts small and never heals.....

You demonstrate clearly a proper and full understanding of electricity as regards to its dangers, few people do (even some engineers have areas they do not understand fully, though they think they do!).

My compliments.

I was once told by a very experienced field motion control engineer that DC is far more dangerous as it is similar to the kind of electricity your body works with in the nerves and the muscles. This did make sense as biologically your body does work with dc voltages to operate muscles and for nerves to signal. I never actually looked it up in detail so it could be wrong.

I have read this time and time again from real experts that DC is more dangerous than AC.

All I can say is that my experiences in the RN a great many years ago were the opposite!

Older ships (WWII and a bit later) were still around and were 220volt DC, with an open faced switchboard that you could (and occasionally did accidentally) touch all the bus bars and cabling. I am sure that a few people died over the years, but I never heard of anyone actually dying. I knew a few stupid people who got bad burns, but lived to talk about it afterwards....we even searched for earths with our fingers bridging where a fuse had been, got a jolt, that was the earth!!!!

Later Ring main carriers closed the DC off mostly, but not completely....

With NATO, RN ships went to 440 AC 3 phase, the first ship built with an open plan switchboard (early 50s at a guess!), people really started to get killed immediately, ship was removed from service and closed face switchboards only from that moment on.....

So my experiences were in total opposition to the Guru trends.....sorry....

Quite. In the absence of any test indications or test equipment a 48VDC power supply can be affirmed running by touching the terminals with a dry finger. Once is usually enough for this game; does anyone want this T-shirt?

Modern telephones don`t matter for -ve or +ve voltage as they have polarity corrector like bridge diode ckt to ringer ckt also, they were meant for those old antique telephones with vibrator bells for transient locks in the coils switching from ringing mode to online / offhook mode , in simple terms proper grounding path to DC , next electronics telephone continued with this .

On the issue of safety:

I understand that under the skin, the chest cavity has a resistance of approximately 200 ohms. So loss of skin resistance could results in lethal current levels at 48 volts (30-50 mA can kill, and it is possible at even lower levels).

I think we strayed away from the original question.

Why is telephone power -48V as opposed to +48V.

this potential is obviously measured relative to earth potential.

The reason for the negative potential is for corrosion protection when wet outdoor cable and or equipment sees a low resistance to earth. If the telephone wires were positive with respect to earth, far greater corrosion damage would take place.

Pipelines and other underground structures are also protected in a similar way by impressing a negative potential with respect to earth.

After all this negativity, I must reply.

"Why are Negative Voltages Used in Telephones?"

Because talking on the telephone is a negative thing to do!

You mean why Ground the Station Battery?

EletroChemical-- Cathodic Protection of cable sheaths.

I think your post would have been better if you had written instead:-

You mean why Ground the Station positive Battery terminal? (and not the negative)

Here in Pakistan it is 60VDC & 25Hz Pulsating DC; NOT Mounted on DC [Modulated] for Community Telephones & 48VDC for PABXs [Private Automatic Branch Exchange].Both the wires are well insulated from earth [ground].Lines are marked as "A" & "B" & when any of the wire leaks to Ground it is marked as Wire "A" or "B" Ground-fault.
For information its ZO [Characteristic Impedance is 260 Ohms [Normally talked about 300 Ohm line]Telephone cable, Fig "8" [Parallel] called "Drop-Wire" is a good 300 Ohms TV Feeder.

What in rest of the World?

case491 #20
" I was once told by a very experienced field motion control engineer that DC is far more dangerous as it is similar to the kind of electricity your body works with in the nerves and the muscles. This did make sense as biologically your body does work with dc voltages to operate muscles and for nerves to signal. "

Of course it is correct.
DC has no zero at all when in contact, while AC has a zero @ every zero-crossing
[twice every cycle].

We the Engineers & Techs are told & have HABBIT to sense a live line or surface by touching it with back of fingers.
Reason the skin at back is more sensitive than the front-skin

&

*** MORE IMPORTANT ***
The fingers close inside the Paw by sub-conscience

SO the fingers will naturally go out of contact of wire or surface
IF a leakage of LIVE line is present



I am a retired Bio-Medical Engineer had been looking after all type of
Electro-medical [we call it so] from Cardiac-Systems , X-Ray Units, Patient monitoring & Diagnostic [Pathology Labs], Dental-Units; phys-therapy-equipment etc etc

Except Cardiac-graphs [ECG] & Encephalographic which work on regular wave-forms specific to Heart & brain; almost all the "muscles and nerves " function on DC-Pulses; e.g. If you shift your eyes to left or right side Polarity of Voltage across your sides of eyes [left & right] changes.

In swallowing of diet the food-tract produces Dc signals progressively squeezing from top to bottom and all the valves in the tract function in harmony to shift the in-take smoothly to stomach.

Electrical-Signals from upper valve sent to lower valve to open & feed-back of the lower to the upper to squeeze downward till it reaches the stomach. A fool-proof sequence of operation.


Lower-Valves opening & upper valves squeezing to shift step-by-step to stomach smoothly.

etc etc.

What's all this talk about DC being used in the body?! My nervous system is composed of optical fiber! Isn't everyone's?