Speaking of telephone electrocutions (http://www.straightdope.com/classics/a1_376.html), when I was a kid I was repeatedly told that it’s unsafe to talk on the phone during a thunderstorm because if lightning should strike the phone lines it could cause a potentially fatal power surge. Are there any documented cases of someone being electrocuted by talking on the phone during a thunderstorm? Are there any saftey devices in place (some sort of surge protector on the phone line, for instance) to prevent this from happening?
They try to protect the telephone system against lightning (after all, the equipment’s pretty expensive to replace), but lightning is very powerful.
The pulses that make a telephone ring are up to 90 volts, and can have a shocking ammount of power (sorry). One of these ring generators will bite ya.
http://www.powerdsine.com/Developers/Products/Telecom/RingGenerators/RingGen_30Watt.asp
While the base station is indeed connnected to the phone wires, the cordless handset should allow one trouble free communication while bathing or during lighting storms… not to mention relief from the hassle of the cord. 
Cecil is “all wet” when he tries to justify the lethality of outlets vs. the telephone system when he says:
Sorry, but the reason your outlets are dangerous is not because they can produce up to 15 amps of current. They’re dangerous for three reasons:
- The instantaneous voltage is high (around 170 V)
- This voltage is ground-referenced.
- Your outlet has the capability of sourcing over 20 mA with very a negligible decrease in voltage.
It is the combination of these three things that makes an outlet dangerous. Notice that none of them mention “15 amps.” This is because your outlets would be just as dangerous if they could source up to 1 amp, and they would be no more dangerous if they could produce up to 15,000 amps. As long as your outlet can source more than 20 mA while still maintaining a voltage greater than 50 V, the maximum current capability doesn’t matter.
If you find where the phone line enters your house, you will see it go thru a “lightning arrestor” which looks rather like an old cartridge fuse (at least, that’s what it looks like in phone lines installed years ago).
This is designed to burn out if lightning strikes the phone line coming into your house, and thus protect the phone equipment connected to the line (and avoid starting a fire in your house). If lightning strikes the phone line close enough to your house, I suppose it could blow the arrestor, and still leap right across this 6" gap and into your phone wiring. But the phone company puts these in, so apparently this arrestor works most of the time.
True story. As a child, growing up on a dairy farm (late 60’s), we had a telephone installed in the dairy barn. As the environment was usually quite damp from washing everything down, we discovered early on that if you picked up a ringing telephone with wet hands, you would get a very nasty shock. It may have been the rubber boots that kept us all alive, but I seem to recall being told that it was DC rather than AC, and was thus less lethal. I admit the possibility of being wrong, but I would definitely NOT answer a ring in the tub.
The talk battery is DC (-48VDC) but the ringing voltage is AC(75-90v).
If you’ve ever been working on a phone jack when the line happens to ring, it really hurts.
You can tell this is an old column, most non-wireless phones these days are useless without power. I still keep one or two corded phones around, just in case.
I’m an old Bell System employee - I started in Western Electric, just three years after this column. I’m pretty sure the surge protectors are for customers, not to protect expensive equipment. (I’d suspect the central office had better stuff.) Back when this column was written, remember, the telco owned the phone and all the wiring in the house - you just rented it.
Western made its own batteries. (We even had our own Bell System standard notebook paper.) A chemist I knew swore by them as a good steady power source for experiments.
Telephone current is actually 52.08v DC.
But 48v is easier for the average customer to get his/her brain around, so when explaining “how it works” to the layman, that’s the figure used.
Don’t believe me? Take a multimeter & check it yourself. Telco repeaters can zap out 160v DC.
Scopata Fuori
“Bad Cat!”
Couple notes about CO battery backup - I was a Bell System employee, too, a long, long time ago (Bell Labs). I remember getting a tour of a CO once, and there were a couple interesting things they told us:
1 - they tested their battery backup once a month by actually cutting power to the live central office during off peak hours, to verify that service didn’t get dropped, and the diesel generators they had started up.
2 - they had enough diesel fuel to run the central office for some absurd length of time without power, like a couple months.
No. 48 V is the figure quoted, because that is the nominal battery terminal voltage. Telco batteries use lead acid cells, which each have a nominal terminal voltage of 2 V. The batteries consist of 24 cells, thus have a nominal voltage of 48 V.
I work for British Telecom, the successor to the old Post Office Telephones which was a publicly-owned corporation. The battery room used to have big banks of open-top lead/acid cells with thermometers sticking out of the top to measure the temperature of the electrolyte. Only the battery room attendant was supposed to go in there and usually they were locked to prevent unauthorised access (although in some exchanges you had to go through there to get to somewhere else). These have all been recovered now and replaced by Power Equipment Racks on the end of most rows of transmission equipment containing banks of sealed 6V batteries looking very much like automobile ones. (-50V is the standard power level for tranmission equipment, except in Telephone Repeater Stations where it is -24V for some reason.) They are charged from rectifiers working off the mains power. Once a month they run the standby genny for half an hour or so, and cut over the essential services to check it all works the way it should. It’s supposed to be a seamless changeover (although it usually knocks out the PCs in my office and we have to restart).
If the genny fails each exchange has a connection point in the yard where they can bring in a mobile genny and plug it in.
this statement in no way represents the official views of British Telecommunications, plc
Lots of nits to pick here:
(1) You said yourself, the voltage is “nominal”, so there’s no sense arguing about small fractions of a volt. the actual voltage varies quite a bit because:
(a) The exact voltage of a lead-acid cell with no-load is not exactly 2 volts.
(b) The cells are not in a no-load condition, they have the whole phone system hanging on them!
© In order to keep the cells fully charged, there’s a contant charging current applied… In order to have a net current flow INTO the cells, it takes a bit more than the no-load voltage, something like 10% above.
(d) You’re not getting the full battery voltage at your end-- it first goes thru about a mile of wire, inside a pair of “off-hook” relays in the main office, then thru some fuses and varistors to protect against lightning surges, then thru a mile or four of wire getting to your house. That can add up to thousands of ohms of resistance, so the voltage at your end wil droop with even the slightest load.
So let’s not get off-track re the exact voltage… It’s enough to say that it’s around 50 volts, enough to get a bit of a jolt, but unlikely to hurt anyone under typical conditions.
With your theory why not just use 50 volts? Much easier to wrap the brain around.
Let me repeat my point in plain english, because it obviously missed you by a mile.
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Yes, I am well aware of all the points you made. They are all irrelevant to the point I was making.
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My point was discussing the reason the figure 48 volts was used in the article, and not some other number. I had no interest in addressing why you might measure something else at your terminal block.
in everyday work on the Main Distribution Frame you don’t get a shock off working circuits with -50V d.c. wetting on them. If, however, you’re sticking a sweaty armpit in there trying to trace a deeply buried pair of wires it’s quite possible to get a shock off the old-fashioned soldered termination blocks (the newer Line Jacks No.44A are less likely to do this as the jaws are recessed into the plastic). Also present are ISDN2 blocks with 120V d.c. on them, which you definitely want to avoid.
From lighting, (not the ringing supply) I’m sure several people have been killed but I can’t prove it. Just use common sense and don’t talk an a direct phone line during a lighting storm.
Yes, they have a lighting arrestor, a simple carbon block where the line enters the house to protect from lighting but takes a lot of volts to blow one.
It may not always do the job. Very common for modems to be damaged from lighting. My daughters house lost two modems from a lighting strike… neither had a surge protector. Good surge protectors protect not only protect 120, they also have inlets/outlets for phone lines and cable. And good surge protectors do work! That I know!
Nit to pick - I can’t speak to lethality, but DC tends to be more dangerous as it causes muscles to contract and stay contracted. In other words, you can’t let go of the wire.
Of course, “civilians” have far more opportunities to accidentally contact 120 volt AC than anything else, so the relative dangers of AC vs DC are largely moot unless you’re mucking around inside a battery room at the telco, or a large UPS with external batteries.