Eli5: how does a cord landline work when the power is out?


Like the phones that have a special plug and everything, the cord one still work when the electricity is out and they don’t seem to have batteries in there, so how and why do they still work

In: 142

The phone company generates power at their central office with which they supply their network.

The line supplies it’s own power, separate from the mains power. IIRC, [edit: incorrectly – see downthread] the line sits at 3v, goes up to 24v on ring, and 9v when off hook.

I used to take advantage of this and use my landline to power various small electronics that I wanted to stay on if the power went out.

On the power poles, the telephone line is in the thicker bundle lower down the pole, so it’s less likely to get damaged.

A subscriber loop in a “plain old telephone service” (POTS) system carries its own 48 volt DC power, supplied by a bank of batteries in the central office.

I worked for AT&T and can vouch that the Central Office has giant batteries that supply power (and usually a generator as well).

The batteries sit in giant barrels…they’re huge! I mostly did residential installs so didn’t get a chance to go into a CO often, but all the servers and battery vats and everything were very cool.

Good old days. Now almost all the pots lines have been removed in favor of digital, no internet, no power no phone, also “voip” doesn’t fall under the same protections as a landline so a court order is not need to pull records or even calls.

I have not seen a POTS line in probably 20 years. Do they still install them? I guess they exist some places. I can only imagine the tel cos have about -100% interest in keeping this around.

Secondary question: Could you steal power from landlines?

In fiber to the home scenarios, the battery is in the demarcation box. In a fiber to the neighbourhood scenario, the batteries will be in a service box on the street. Otherwise, the batteries will be in the Central Office.

Batteries is the answer, i did a tour of am exchange about 20 years ago and they still used open wet cell batteries.

Lead acid chemistry batteries have a voltage of ~2V, car batteries have 6 of these in serial. Capacity comes from the lead plate service area so they are big round drums.

In the exchange 24 were lined up in series to provide 48V and multiple strings to provide. This filled an entire room and due to the nature of them they generate hydrogen which leads to a very flamable enviroment that needs to be well ventillated else they go boom. These days sealed cells are used in 4x12v battery strings.

The reason for the big strings is to provide the amperage, which while its low per line, its a lot of phone lines per exchange. The big amperage led to busbars that if you drop a spanner/screendriver on will flash vaporise/melt/arc. While ive not seen this i have had stories from guys that worked in exchanges in south africa that had a little less regulation than Australia.

48V is still used quite heavily in telecommunications big core router/switches as AC powersupplies dont go big enough

I’d love to see the submarine cable landing stations that provide several thousand volts to the submarine cables.

(I work in telecommunications)

Rather than being powered by the electricity in your home, power is provided to the phone network at the exchange end of things.

So instead of plugging your phone into a plug in your home, you are plugging it into the end of an extension lead that reaches all the way back to the phone company.

What this means is that if you have a localised power cut, it may not affect the location of the exchange providing per through your phone line, so your phone will still continue working as normal.

And for the times where a pretty cut does affect the exchange, they have backup systems in place to keep the phone network powered – using batteries or generators for example – so that it can still be used and provide a method of contact in emergencies (assuming of course the end user isn’t using personal devices that are reliant on local power).

They don’t take much power to run. The lines that run to the phone to connect them to the phone company also have a power wire that runs into them. The power is supplied by the phone company who have back up power in the event of an outage.

Plain old telephone services (POTS, for real) phones don’t require any power unless they’re ringing or somebody’s talking.

When you would receive a call, there’s literally a switch that closes a circuit, and from the telephone central office (CO, the closest facility serving your area) that circuit sends 105V to the phone, triggering the clapper to rattle back and forth against the bells, making it ring.

When you took the phone off the cradle, a different switch was triggered to start the lower voltage powering the ear and mouth (E&M) circuit.

Back at the CO, these switches were in two-post racks, and there were rows of batteries much like car batteries that provided the power. There was a regular power line to the CO, charging the batteries, but of course in a power outage, the batteries would keep working for some time.

If you want to keep going, https://en.m.wikipedia.org/wiki/Telephone_exchange

The old telephone system had batteries that sent power over the phone line. If the power went out, the phone would have power unless the phone line was cut. How long they lasted depended on how many lines there were connected to it, how old the battery was, and how many calls were made. They could last a few hours to several days.

Many now have landlines through cable service. They have battery back ups too, either in a box on a pole or along the street, or in a box inside the house. They typically provide power for up to a day or so, if the battery is new, but there are several added connection between the house and the telephone system, and if any of those has lost power with failed backup power you still won’t get a connection.

Canada here: we moved last year and experienced exactly this when a strong storm knocked out power for half the day yesterday. Our landline runs through our ISP’s modem, and we had no signal. The mobile networks were spotty at best; calls were choppy, texts barely went though, and data was non-existent unless you sent a km or two away

The simplest answer that is true is that the electricity is provided through the phone line. Phones did not need to be plugged into a power outlet until answering machines (voicemail for the ignorant) got built into the landline phones.

Those old phone lines carry (a little bit of) power, enough for the phone.
When you have a power outage you lose the power supplied by high voltage lines to your home, but that’s completely separate from the phone lines.

The telephone cable supplies a very small minimal amount of energy required by the landline to work. This is arranged by the telephone provider.

In a similar spirit we also have Power over Ethernet PoE. Which allows power to be transmitted over ethernet wires. So even when the lights go out, your internet access doesn’t.

*IF* your landline service is from the telephone company and they are still wired for POTS (literally “Plain Old Telephone Service”), their system has its own voltage over the wires, independent of the electric utility.

Increasingly, even so-called “landlines” are VOIP (“Voice Over Internet Protocol”) which runs via your internet router. so if your phone service is from a company like Comcast, or in some areas also AT&T, if the power goes out your router goes out, so your so-called “landline” is also dead.

# Because it was powered by the Telco, not your house. Unless the telco lost power, it still had it.

the telephone line itself supplies power. spikes and drops in voltage on the line determines if it’s busy calling or free.