Telephone wires (landlines) and electric shock

[gkster]

The MPSIMS thread on the collapse of Champlain Towers has a post (link below) saying
“I would expect that the responders would do stuff like cut off electricity and gas for safety reasons, but they wouldn’t really need to worry about the telephone lines. They have such little power on them that there’s no real risk of getting shocked.”

This brings me back to 1988 when I was in grad school. After a miserable day, I decided that I needed a soak in the bathtub and a talk on the phone with a sympathetic friend. So I brought the phone (plain old black rotary dial with a 25 foot extension cord) into the bathroom and settled in. But when my friend realized that I was calling from the tub, he told me to hang up right away before I electrocuted myself.
So, was my friend right? Was there enough power in the line for a dangerous shock? And could rescue teams working in the rain or near standing water or puddles get shocked by downed phone lines?

Champlain Towers South in Miami has caved in {2021-06-24}

As was mentioned, POTS is Plain Old Telephone Service. For any youngsters out there, these are the older phones which had a telephone wire that plugged into a telephone port in the wall. They don’t have a normal electrical plug. They get their power from the telephone line. The telephone wires are run independently from electrical lines. If a telephone wire stayed intact during the collapse, it’s possible that an older style phone is still getting power. I would expect that the responders …

[DesertDog]

Nominal voltage for a POTS line is 48vdc and it’s pretty low current. With the receiver off the hook, a pretty safe bet in a collapsed building, the line voltage drops to around 9vdc.

Ringer voltage, on the other hand is around 90vdc but is applied to the line only to make the phone ring. I can tell you from personal experience, that stings when someone rings the line you’re working on.

In any event, all POTS lines lead to a central office somewhere. If the rescuers want the lines safed while they work the TelCo can disconnect them there. After all, the subscriber isn’t going to be using that line any time soon.

[Melbourne]

@DesertDog

Nominal voltage for a POTS line is 48vdc and it’s pretty low current. With the receiver off the hook, a pretty safe bet in a collapsed building, the line voltage drops to around 9vdc.

I think that’s the wrong way around. In a collapsed building, with all the receivers effectively “on hook”, hung up, which is to say “disconnected”, the line voltage will be around 48V DC. Where the wires have shorted out, the exchange will see those phones “off hook”: zero volts at the short, perhaps 68 mA short-circuit current.

I certainly wouldn’t want to be taking Ring Voltage while naked in a bath, and it’s enough to make someone fall off a ladder if it’s unexpected, but around here the danger the companies worried about most was that the phone lines might come into contact with a power line: the workers wouldn’t be expecting that.

I wouldn’t have worried about the small voltage across the microphone while talking to someone.

[Francis_Vaughan]

48volts can kill you. Especially if you are presenting a very low resistance, which sitting in a bath is about as good as it gets short of sitting in the sea.

This web page provides a really nice summary of everything you need to know.

There is an aphorism that “it isn’t the volts that kills you, its the amps” which is mostly true. Pass say 100 milli-amps through you and there is a good chance it can induce fibrillation. Being wet make it much easier for the apparently feeble 48 volts to push that sort of current through you. Indeed the page linked above sets the voltage at 30 volts. So yes, with a bit of bad luck, the phone could kill you when in the bath.

One of the oddities about the skin is that it provides a good resistance up to about 500 volts, and then the barrier breaks down. Skin may get up to 100,000Ω resistance, which is going to be hard to push current through. This is a best case. Wet skin drops to pretty much no protection at all, and your internal resistannce of about 300Ω dominates. That is a factor of 300 odd worse, and why 30 volts can drive a lethal 100mA through you if you are wet.

The lower currents needed to induce fibrillation than arrest is a serious issue, as CPR is pretty ineffectual at getting you going again when in fibrillation as opposed to a full arrest.

In the extreme, operating theatres must provide micro-current isolation for the patient for some procedures, as it is possible to get current into the wrong place altogether too easily, and whereas 100mA is considered the danger area through the skin, you get into micro or even nano-amps when talking current directly across the heart.

Electric shock kills for a range of reasons. Stopping your heart is only the start, and only needs a very small current flow at the right time and place. The up-side is that you can be restarted if got to in time. Serious electrocution is a different matter. Once actual physical damage has occurred your outlook is grim, even if you were not initially stopped heart dead.

Safeing a building, personally I would be thinking about ensuring the phone was cut off. I would doubt it would be a serious threat, unlike electrical power, but it provides an additional ignition source and the potential of highly unwelcome surprises for first responders and rescuers. Just getting a good zap digging about isn’t going to be a happy thing. If it was wet everywhere it would be worse. One of the common problems with mild electrocution is the sudden muscle spasm and subsequent falls and injuries. Which is not something you want when engaged in rescues.

[Melbourne]

@Francis_Vaughan

Especially if you are presenting a very low resistance,

But it comes to you through skinny little wires, which are not low resistance.

[Francis_Vaughan]

@Melbourne

But it comes to you through skinny little wires, which are not low resistance.

All a matter of degree. Skinny wires made of copper are still of a comparable resistance to you. The local loop is run with different gauges of wire (26, 24, 22ga) depending upon distance, but the rule seems to be that the round trip (ie both conductors, there and back) of the local loop should not exceed 1500Ω. This is the maximum, which will still give you enough of a belt to contract major muscles uncontrollably. If you live closer to the exchange the resistance is proportionately lower. If you lived within a km of the exchange it could still deliver a heart fibrillating shock. More than that you could live to rue the very unpleasant experience.

It is pretty unlikely to kill many people, the stars would need to align in a particularly nasty way, but it isn’t intrinsically impossible from the basic physics and physiology.

[Saint_Cad]

@gkster

They have such little power on them that there’s no real risk of getting shocked.

I was wiring a phone line at home when someone called. I can confirm this statement is incorrect.

[naita]

I would say the wires wouldn’t be the main resistance, the water would be. There’s also the question of where the power would go. A brief search only gives me the 48V DC fact, but not whether one of those legs is at ground potential. Regardless of that, unless you’re submerging only one side of the circuit the best return path will be through the other half.

[jnglmassiv]

Has anyone ever been directly killed by conventional POTS wiring? Lightning or short circuit to another service are excluded, things like falls from ladders, too.

[DesertDog]

@Francis_Vaughan

There is an aphorism that “it isn’t the volts that kills you, its the amps” which is mostly true.

Yes. High enough voltage – even DC – can burn you which, aside from any fibrillation effects, can make for a bad day. As I said, any voltage can be disconnected in the central office and most likely has been.

[Mr_Downtown]

@DesertDog

all POTS lines lead to a central office somewhere. If the rescuers want the lines safed while they work the TelCo can disconnect them there.

That’s probably not the easiest way to do it, as you’d need to track down all the subscriber loops serving that address. All the POTS lines for the building would be on a half dozen 25-pair cables coming in from a nearby pole, or (being coastal Florida) probably in a trench from a nearby outdoor large green junction box. Easy to find (just call JULIE) and easy to sever all the loops with one swipe of a backhoe.

[DesertDog]

Yeah, but they’re all labeled with the phone number at the central office. It’s not like if you move into a place and get phone service they sit there puzzling over which of the 10,000 wire pairs are the ones going to your house to give you your number.