48vdc was common in phone exchanges. They filled the basement with lead-acid batteries and to could run without the grid for a couple weeks. In turn the phone was 99.999% reliable for decades.
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.
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...
[1] https://www.analogisnotdead.com/article26/what-is-going-on-w...
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.
With DC systems you generally think about the issues - which is why modern cars are negative ground. However other than cars most people never encounter power systems of any size - inside a computer the voltages and distances are usually small enough that it doesn't matter what ground is. Not to mention most computers don't even have a chassis ground plane (there are circuit board ground planes but they conceptually different), and with non-conductive (plastic) cases ground doesn't even make sense.
With AC it's about where the ground is attached along the length of the transformer secondary. In the EU they ground one of the ends of the secondary, in the US we ground the center point.
I don't get to say this very often ... but the US way is objectively safer with no downside: 99% of human shocks are via ground, and it halves the voltage to ground (120V vs 240V). A neutral isn't required if there aren't 120V loads.
- uninsulated metal pins make contact with supply while partially exposed - much smaller distance between metal pins and the edge of the plug
But there's no inherent power tradeoff: you can have 240V outlets in the US, with the two prongs both 120V to ground. They're just really uncommon in residences.
Yes, but you only get the safety benefit on three phase equipment.
In the US there aren't a lot of 240V plugs, but if you get some installed you can get the safety benefits with plain old consumer goods.
The wye ground is beneficial even if there are no three phase loads, because the line-ground voltage is 1/sqrt(3) and you can use cheaper switchgear.
The 3-phase alternative to a wye ground is a corner ground (grounding a live phase of a delta secondary), which isn't done in modern installations because ground faults are full line-line voltage and you need more expensive switchgear.
Line-line faults are always interrupted by two breakers on a wye grounded 3 phase system. But with a corner ground, one breaker potentially has to break the full line voltage for a line-ground fault (you can't fuse the grounded phase).
My point is that in the context of a 230V single phase circuit, your ground is no longer in the middle. The ground is on one end of your single phase. If you want a safer single phase, you need to rebalance it to +115 and -115.
And because that problem of galvanic corrosion the GGP talked about, and the mirror one of material aggregation don't happen. And it also makes switches more reliable.
Both are less dangerous on telephone lines. But are very important on electricity ones.
1 - It won't break your posts, but can easily short small contacts.
Does that mean when you have electronics and use multiple dc-dc converters all the inputs and outputs share the same ground, it's not just the values for that pair of wires?
And if I want to use a telephone on an incorrectly wired 48dc circuit, I could switch the positive and negative wires, as long as the circuit in the telephone is isolated and never touches ground?
Thanks. Somehow I got in my head that all circuits were just about the delta from neutral and therefore nothing outside them mattered.
I think a circuit should mostly care about the deltas, but when you’re talking about things like phone lines, the earth becomes part of your circuit. You can’t influence its potential (it’s almost exactly neutral because any charge imbalance gets removed by interaction with the interplanetary medium) so everything else is going to end up being determined by what you need for their relative potential to that.
Objects don’t repel because they’re at the same potential. Electrostatic force comes from electric fields due to charge. If two objects truly have zero potential difference and no field between them, there’s no force.
You’re correct that circuits care about voltage differences. After all, all work requires a force gradient of some kind.
The interplanetary medium absolutely exists. I'm not talking about aether. I'm talking about the soup of dust, gas, and particles that fills space in the solar system. It contains a lot of charged particles, which is what keeps Earth extremely close to neutrally charged. Any deviation from neutral starts attracting positively charged particles and repelling negative, or vice versa, which equalizes the charge.
I didn't say objects repel because they're at the same potential. I said that objects at the same potential will still repel each other if that potential isn't zero.
Seriously, what is this reply? Aether? Objects repelling because they're at the same potential? You seem to have read a comment very different from what I wrote.
I think it all ties together. Ground potential as in sticking things in the actual ground is in practice pretty much the same as that theoretical potential at infinity, because space has enough charged particles flying around to equalize the imbalance. But it doesn't really matter from a practical perspective when making circuits.
No, it depends on the converter. There are converters that leave 160V on the DC power rail for a 110V AC input, and 155V on the DC "ground" rail.
They are economic and you could find then when galvanic isolation is at least in theory not important, but they're terribly unsafe when used on PCBs that people might muck with.
If you have some "normal" converters and some of this kind, sharing the ground would be quite dangerous.
I figured any happenstance from the multimeter that the grounds match was transitory and not to be trusted.
What's horrific converter performance in numbers?
An isolated flyback (to 12V) should be able to hit >92% and doesn't care if it's fed -48V or +48V or ±24V. TI webench gives me 95% though I'd only believe that if I'd built and measured it. What's the performance of your -48V → +48V?
[with the caveat that these frequently require custom transformers... not an issue with large runs, but finding something that can be done with an existing part for smaller runs is... meh]
Horrific performance by my definition would be 48v to say 1v. We only realistically use buck topologies for POL supplies. Such a ratio is really bad for current transients, not to mention issues like minimum on times for the controller.
(Thanks for the info!)
130A, 48V -> 1.2V @ 94% efficiency! Except:
- $100 ea.
- Fixed 1/40 voltage ratio, regulation done by upstream regulator.
- Look at the minimum specs for efficiency…
My guess is that their efficiency stats dont include losses in the upstream regulator.
100 usd per unit doesnt seem that excessive.
Automotive collectors can probably still relate to cars from the 1920s-50s having a "positive ground."
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 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.
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.)
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...
I don't own it personally, I say it in the same sense someone says 'my building' when they mean 'place my apartment is at'