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

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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

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24 Answers

Anonymous 0 Comments

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.

Anonymous 0 Comments

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.

Anonymous 0 Comments

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.

Anonymous 0 Comments

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

Anonymous 0 Comments

Secondary question: Could you steal power from landlines?

Anonymous 0 Comments

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.

Anonymous 0 Comments

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.

Anonymous 0 Comments

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.

Anonymous 0 Comments

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).

Anonymous 0 Comments

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)