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I was reading this and I remembered getting some good belts off of phone lines.

Isn't the ring voltage greater than 70 V?

Bob

[This message has been edited by iwire (edited 05-28-2004).]
Idle line voltage is typically 48 to 50V DC, and the ring voltage is low-frequency AC superimposed on that. Voltages vary depending upon line lengths and the number of bells connected, but ringing voltage can actually exceed 90V.

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The electrical outlets in your home pack a whopping 15 amps; your humble telephone carries less than a tenth of a milliamp.
Well, I'll skip over the 15A angle as we all know that the current that actually flows depends upon the resistance.

But the second statement there is just untrue. The standing DC current when a phone is off-hook can be over 100mA. The ringing voltage can certainly pack a similarly dangerous level of current to someone immersed in water.

So let's put it this way: If I was soaking in the tub I wouldn't want anyone to come along a drop a phone in there with me..... [Linked Image]
Bob,a phone guy once told me years ago that it was 90v DC.


Russell
I have heard the same as Russell. One time years ago I had my helper hold onto the phone wires while I was testing a two line system. He got belted twice before he said 'no more'. [Linked Image]

Pierre
An alarm guy told me he used to strip the wires with his teeth becaue they were so small. He stripped an incoming phone line once when a call came in and got rapped in the mouth with 90V and it wasn't fun.
Agreed.... 90VDC on the ring. I've felt it, it's a good belt. Definitely could get electrocuted in a bath tub.
No, ringing supply is AC, usually at a low frequency of around 20Hz.
As a long time "phone guy" I have to agree with most of what's been said.

The battery voltage on an idle pots line (basically open circuit) should be 48 to 50 volts DC. It will drop when a phone goes off hook and the voltage then will depend on the length of the loop and size of the wire (as always)the particular phone and number of phones off hook at one time on that pair.

The ideal loop current is around 20-35ma. Less than that may result in transmission (low volume) problems and more than that can cause operational problems with some electronics particularly modems. I don't remember exactly but I think the TELCO guarantees a minimum of 20ma with no upper limit. So for short loop lengths you can see excessive loop current though 100ma would be extreme.

The ringing voltage is nomally 90 volts at 30Hz superimposed on the 48 volt DC battery voltage. Not sure what the ring current is though it's not that much but, yes you can get knocked on your a** if you are not careful. Something other than 30Hz may be used in some areas dependent on the ringing schemes used though the switch over to "standard" customer supplied phones pretty much eliminated party line and other selective ringing schemes.

The article alludes to using the phone during a storm. Even more than the above I would be concerned about lightning and other stray voltages entering the house on the phone lines. Most TELCO protectors clamp at a minimum of 300 volts or more and I wouldn't want to be in a bathtub talking on a corded phone at the time!

-Hal
Bob,

When I use to work for the alarm co. our telco lines use to carry 120 v DC. When you got hit by them you knew it. We use to have to solder each connection for the alarm co. and when you touch the solder to the hot wire and lean on a metal garage door, it really wakes you up. ( Take it from me. I did it.)I use to have several funny stories about Telco guys and our Burglar Alarm (BA) lines. There was the guy who was using his teeth to strip the wire, and we told him to watch out for the BA line. Well, when he found it, his head flipped back real fast. He then asked us what was in there. We told him that it was 120VDC. He alsmot broke his neck or at least get whiplash. There was another time a Telco guy told us about the lady who called into the CO ( Central Office) She was going to go on vacation, and she wanted the Telco people to come out and "Water" her Ground rod. It seems when the original installer went out there he couldn't get a good reading on the telco line. So he threw a bvuckey of water on her ground rod to get a better reading. It worked and he told her to throw a bucket of water on her ground rod once a week. She did but now she wanted to go on vacation and she wanted some to go out and water her ground rod. I have a few more.

Harold
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An alarm guy told me he used to strip the wires with his teeth becaue they were so small.
My sports teacher at school once did that with 220V wires. He moved into a new house or apartment and hung new light fixtures. Standing on the ladder he noticed he forgot the knife and decided to strip the wires in the ceiling with his teeth. Of course he had shut off the breaker before. Meanwhile his brother had noticed his radio didn't work. Went to the panel, "stupid, can't work without power" and on went the breaker...
He says he fell off the ladder from the jolt, luckily he didn't hurt himself too badly.
once on a construction site the phone was down and it was raining when i found the break in the wire and went to re-splice it i found out about ring voltage: ouch!
Use a wireless phone. anyone should stand 3V [Linked Image]
As to the voltage on phone lines, "nowadays" I think the voltage is even higher for digital lines (T1, DSL).

I've always heard that one should not use a sink or tub or shower during a lightning storm and to stay away from windows...

I do remember once during a storm there was a very bright flash of lightning outside. The phones briefly "chirped" and a light bulb in a lamp suddenly blew out (everything else was OK). That was a bit startling...
I've seen phone stuff blown by lightning twice. Once it was a big fancy phone and once a PABX. Both had huge black spots on the circuit board. pretty impressing.

The weirdest experience I had with lightning was in rural Italy. There was a real bad lightning storm and we were sitting inside. Suddenly I saw sparks flying from one hole of the receptacle to the other! Only lasted for an instant, then it was gone. Some electronics that had been plugged in were shot.
Per TIA/EIA/IS-968 section 4.4.4.2 (formerly FCC part 68) and UL60950/UL1950 (annex M), ring signals shall be less than 300v P-P, or 200 Vp to ground, across a termination of at least 1 megaohm. Of course that defines the high end you need to design to, not the norm. The NRTL's I have worked with use 125 Vp or 150 Vp when deciding how much spacing should be between a POTS line (TNV3) and other circuits.
i don,t know if your phone can hurt yoy,but just look at your bill every month,now thats a shock.
In the "old" days in Britain, the line came straight off the exchange batteries which were held at a nominal 50V DC. These days, it seems that the modern digital exchanges regulate it down from a slightly higher voltage. When a linecard goes bad (as seems to happen quite regularly in my local CO -- so much for modern reliability!) the voltage goes up to a little over 60V.

By the way, test voltages which exceed the usual nominal can also be applied to the line during routine tests as well.

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i don,t know if your phone can hurt yoy,but just look at your bill every month,now thats a shock
You gotta stop calling all those 900 numbers! [Linked Image] [Linked Image]


[This message has been edited by pauluk (edited 06-01-2004).]
As to the voltage on phone lines, "nowadays" I think the voltage is even higher for digital lines (T1, DSL).

No. T1's can power the terminal unit at the customer premises but it is still 48vdc. DSL is just piggy backed on a POTS line and so there would not be a difference.

Talking about lightning, we are primarily a telecom company and whenever we install a new key system or PBX we always provide secondary protectors on all CO lines. Secondary means after the TELCO provided protectors. As I mentioned earlier, most TELCO protectors will limit surge voltages above 300 volts, usually 400 to 600 depending on what they have on the truck that day. Our secondary protectors will clamp at 235 volts with a 150ma fuse that will open both sides of the line when it does. I can say that we have never lost a piece of equipment this way. We have lost plenty of protectors though.

One extreme case that comes to mind was an installation in a house for a home based business. This was in a rural area and every time there was a lightning storm the protectors would literally be blown across the basement. We would come pull out what was left and plug new ones in and they would be good until the next storm. We notified the TELCO because this should not be happening if their grounding and protectors were doing their job. Even though each time they lost their alarm system and other electronics but not the telephone system, this was our fault and they refused to pay for the service call. Can't win!

-Hal
hbiss, don't mix up yer DSLs! Residential ADSL is just POTS plus high frequency DSL, with the only hazard, under normal conditions, being ring voltage. But HDSL, HDSL2, and HDSL4 (all 1.544 Mb/s, like T1) are powered at -190 VDC constant. These will whack you pretty good - expletives and everything. A T1 remote unit when "line powered" and funtioning normally will usually not have any hazardous voltages present - not because the CO isn't generating hazardous voltages, but because it is a constant current system. The CO end will output whatever voltage it needs to, up to -140 VDC, or + and - 140 VDC depending on its settings, in order to drive 60 mA through the loop. The device at the end of the loop MAY (sometimes may not!) have < 50 VDC across it ,measured tip and ring to tip1 and ring1, but the repeaters along the way may have up to the max voltage to ground. Break the loop, or remove the remote unit, and one pair goes to the max output of the CO unit. "Locally powered" T1 systems not using line or span power will only have about a 3 Vrms signal present.
electech:

That's exactly what I was gonna say [Linked Image]
And that's exactly what I should'a said. [Linked Image] We don't do much with T1's and HDSL other than move or remove remote units for the TELCO or customer. Thanks for the info.

-Hal
Talk about a question becoming outdated!
The old-style phones did have limited power, and the threat from a shock is affected by the hz. And yes, it sometimes hurts.
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