h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Voltage, voltage...

48 volts DC is the normal supply voltage across a telephone line a-b pair. In the UK and Europe it's supplied with enough current to drive a basic telephone, whereas in the US it's only supplied with "sealing current" to keep contacts clear.

ISDN lines (remember them?) are powered at 100-110 volts.

Ring signal is AC, about 90 volts. Do not suck the phone line while the phone is ringing, it's still supplied with enough juice to ring an electromechanical bell. You know, a real bell with a coil and dinger arrangement.

So yes, in Europe you could steal enough current to charge batteries, if you had a DC to DC battery charger. No, you couldn't defibrillate anyone with a 48V DC shock, or even a 90V AC shock. Neither lightning or real defibrillating shocks will travel far down a physical phoneline, since the equipment is all optimized for 48V DC and 0-4 KHz speech. ISDN goes up to about 80 KHz, and is iffy on the longest lines.

ADSL broadband is much higher frequency (from 90 KHz up to 1.odd MHz), but as a result is not possible on long lines or at its fastest on even moderately long lines. ("Offer is subject to compatability and availability.")

Any questions?

Zube

>> whereas in the US it's only supplied with "sealing current" to keep contacts clear. <<

Does that mean that in the US you have to plug your telephone into a seperate power supply?

I have to do that with my cordless rechargeable phone with integrated digital answering machine over here. A simple pulse dialling phone wouldn't need to be plugged in, even in America.

If you have a fancy phone that needs to be plugged in, it's a good idea to keep the simple phone you replaced in a drawer near the socket, since your fancy phone will stop working if there is a power cut, while your phone line will be usually powered from a battery room at the local exchange, and a simple phone will keep working.

Zube

Thats the same in the UK. I have a DECT cordless phone and a simple push button phone. So why do British Telecom shove 48 volts down the line? Seems a lot.

I also have a VoIP phone and a Skype (www.skype.com) phone which gives me the ability to carry on three telephone conversations at once all down the same piece of wire.

Donald

The voltage is so high because of the old need to power a physical bell. Since the system is still backwards-compatible for old 'phones, the voltage has never been reduced.

The left ear vs right ear debate seems to be more confused than I thought, so I'm not going to write anything on it.

That just leaves me to write a section on the voltages, power supplies and using your phone as a defibrillator.

The 90V AC ring signal packs such a wallop because it needed to power physical electromechanical bells, and there might be several on a given line.

I think I overstated the differences between the US and UK. I know that for ISDN lines, UK practice is to supply enough current to drive the users modem in a restricted state, whereas in the US only sealing current is supplied, but in POTS I think both use similar power arrangements.

The 48V DC feed is enough to power a simple phone (the microphone and earpiece), and is used for line signalling: when you off-hook you close a switch in your phone and the current goes away up from near zero. This current change is how the exchange knows you are off-hook.

DC current also helps keep contacts and connections in the wires out to your home clear ("sealing current").

When transmitting electricity over long distances, power is lost to resistance in the wires, and the lower the transmission voltage, the greater the losses. 48V is a compromise between a high enough voltage to avoid wasting power and a low enough voltage to avoid electrocuting people. Telephone company workers often have to work on lines without disconnecting the power. 48V DC will hurt, but won't kill anyone.

Probably.

Oh, and techically the line feed is -48V not +48V, for electrochemical reasons which I can't remember.

Zube

I thought I wwould pass on some ideas and information that may be of use to you when you go to expanding your article.

You went from the invention of the telephone to the Dial telephone. And while you gave a good reason for the numbering you left out how the telephone worked prior to the dial. Basically anyone old enough to have watched old movies would know about the telephones being strictly through outside help or an operator. You click the speaker holder a few clicks to get the operator's attention.

Later on you could ring-up the operator by turning a crank attached to the telephone box. You also could call some one on your party line by sending their ring. For Example my Grandmother's ring was two long and a short. Someone on her party line could crank the phone say two or three times around twice and then one quick crank all with pauses between.

Then came the dial telephone. The two lived side by side for a while. The more rural area kept their crank telephones for a good time while the more populated ares went to dial. But even dial telephone users had party lines.

Two incidents I thought may be of interest but not necesary to your article:

There was a story in the paper a good number of years ago about a Mother calling her daughter just as the reverse was happening. Both apparently dialed at the same time and there was no ring of course and they talked for awhile long distance. The weird part is neither party was charged for the telephone call. The Telephone company denied that this could have happened.

The other incident happened to me. I was working in a retail clothing store one day when there was a very bad electrical rain storm. Some one had called the store for some information. The clerk put the receiver on the counter and walked a few feet away. There was another person in the room about the same distance about 90 degrees from the telephone and the clerk who answered the telephone. I was approximately 180 degrees from the first clerk and maybe just a few more feet away. Suddenly we heard a lightening strike close by and at that same time a small streak of lightening flew from the telephone somewhere between the second clerk and myself. Unfortunately I can not tell you where that electrical charge went as it went on the other side of the wood bins that I was working on. I didn't think to ask if the other two saw where it went. Some one picked up the handset, looked at it, and hung it up. I can not honestly tell you that the telephone did or did not work after that. My best recollection the telephone was replaced.


I have never heard of anyone being killed but until the advent of the internet I suspect there are alot of things we never heard about unless it was local. Now if the President of the United States sneezes the whole world know about it within fifteen minutes.

Anyway, I hope I may have given you something you can look into that may enhance your paper. Good luck on your overall review. I think some of the criticisms regarding the dialing may have been mistaken as to the direction. I believe you are correct that you dialed clockwise and the return was counterclockwise. This would have accomodated the higher number of right hand dialers. Something to look up would be whether before tone dialing came into being could someone get a telephone for a left handed dialer.

And yes there are alot telephones that work on pulse in use today. Otherwise why when ever you call any where where you need to make choices do they say, "if you have a touch tone please pust 1 for sales, etc. or if you do not hava a touch tone please remain on the line and an operator will come on the line to handle your call." Now some places give you another option if you don't have touh tone, "you may speak or say 1 for etc." That part always confused me. Isn't speak or say the same thing?

MoFoLo

The only case I've ever heard of where somebody was killed by only 50V was a ship construction worker standing up to his chest in water and operating some sort of 50V electric screwdriver.

Humans can take all sorts of voltages without dying. It's the current that kills.

B

That's a commonly repeated and misleading platitude. It is the current through the human body that kills, but this is directly proportional to the applied voltage across the body.

Even so, a low current with extremely high voltage can be quite safe, but low voltage strong currents (such as a car battery) are really quite dangerous.

B

Yes, what happens is that if the current is flowing through a coil at, say 12 V, when you disconnect the lead the current continues to flow, pumping the voltage up to 20 or 30,000 volts in order for this to happen.

A static shock, from rubber shoes on a synthetic carpet, can be thousands of volts, but not enough current to hurt.

I think the point that I was building up to making is that the phone line current is pretty small - only about half an amp if I remember.

B

You can be killed by 100mA.

The optimal word being 'can'. Only happens if it passes directly across the heart via the bloodstream, and you're pretty frail.

B

It's still likely enough that 100mA residual current detectors are no longer allowed. They have been replaced by the 30mA type. A half amp through your body would probably kill anybody. The important thing is that the voltage in a telephone is not enough to induce anything like that sort of current in you. You see, resistance isn't futile.

It takes more than puns to finish me off.

B

But you got to admire its application.

Reminder to self: I still have to put something about electricity into this.