I will say that this is a surprisingly deep and complex domain. The amount of flexibility, variety and scalability you see in DC architectures is mind-boogling. They can span from a 3kW system that fits in 2U all the way to multiples of 100kWs that span entire buildings and be powered through any combination of grid, solar and/or gas.
Honestly, that was pretty surprising to me when I had to work with some telco equipment a couple of decades ago. To this day, I don't think I've encountered anything else that requires negative voltage relative to ground.
[1] https://www.analogisnotdead.com/article26/what-is-going-on-w...
Automotive collectors can probably still relate to cars from the 1920s-50s having a "positive ground."
The crucial difference is the direction in which the current is flowing: is it going "in to", or "out of" a hot wire? This becomes rather important when those wires are leaving the building and are buried underground for miles, where they will inevitably develop minor faults.
With +48V corrosion will attack all those individual telephone wires, which will rapidly become a huge maintenance nightmare as you have to chase the precise location of each, dig it up, and patch it.
With -48V corrosion will attack the grounding rod at your exchange. Still not ideal, but monitoring it isn't too bad and replacing a corroded grounding rod isn't that difficult. Telephone wires will still develop minor faults, but it'll just cause some additional load rather than inevitably corroding away.
Yes, or something similar[1]:
A few of the more efficient grounding electrodes for buildings and structures are:
- Metal Underground Water Pipe
- Metal In-ground Support Structures
- Concrete-Encased Electrode (also known as “footer ground” or “Ufer ground”).
- Ground Ring
As mentioned this is particularly important for telecom and similar systems which have signal wires going literally through the ground.
[1]: https://www.nfpa.org/news-blogs-and-articles/blogs/2021/09/2...
edit: found it https://www.cnet.com/tech/tech-industry/google-uncloaks-once...
So the grid was always charging up the lead acid batteries, and the phone lines were always draining them? Or was there some kind of power switching going on where when the grid was available the batteries would just get "topped off" occasionally and were only drained when the power went out?
Actually, there was one. Even earlier phones had their own power. A dry-cell battery in each phone, and every 6 months, the phone company would come around with a cart and replace everyone's battery. Central battery was found to be more convenient, since phone company employees didn't have to go around to everyone's site. Central offices could economize scale and have actual generators feeding rechargeable batteries.
I was wiring in a phone extension for my grandma once as a boy and grabbed the live cable instead of the extension and stripped the wire with my teeth (as you do). I've been electrocuted a great number of times by the mains AC, but getting hit by that juicy DC was the best one yet. Jumped me 6ft across the room :D
The batteries, the grid/generator-supplied power supplies, and the telephone switch equipment are all connected in parallel -- as if the entire DC power infrastructure consists of only two wires, and everything involved with it connects only to those two wires.
1. In normal operation, the batteries are kept at a constant state of charge. The switches are powered from the same DC bus that keeps the batteries charged.
2. When the power grid goes down, the batteries slowly discharge and keep things running like nothing ever happened (for hours/days/weeks). There is no switchover for this; it's just the normal state, minus the ability to juice-up the batteries. (Remember: It's just one DC bus.)
3. When the grid comes back up (or the generators kick in), the batteries get recharged. There is no switchover for this either; nothing important even notices. (Still just one DC bus.)
4. If the grid stays up long enough, go to 1. Repeat as the external environment dictates. (And as you might guess, it's still one DC bus and there's also no switchover here. Things just continue to work.)
--
You can play with this at home with a capacitor (which loosely acts like a battery does), an LED+resistor combo (which acts as a load), and a small power supply that is appropriate for LED+resistor you've chosen (which acts as the AC-DC converting grid input).
Wire them all 3 parts up in parallel and the light comes on.
Disconnect the power supply, and the light stays on for a bit -- it successfully runs from power stored in the capacitor.
Reconnect the power supply, and the light comes on and the capacitor ("battery") recharges -- concurrently.
Improve staying power by adding more parallel capacitance. Reduce or eliminate it by reducing or eliminating capacitance. Goof around with it; it's fun. (Just don't wire the capacitor backwards. That's less fun.)
The batteries are floated at the line voltage nothing was really charging or discharging and there was no switchover.
This is similar to your cars 12v dc power system such the when the car is running the alternator is providing DC power and the batteries float doing nothing except buffering large fluctuations stabilizing voltage.
Another thing we lost in the age of VoIP landlines, but then again mobile towers also have batteries. Just don't be unlucky and have a power outage with 3% battery on your phone...
